In the figure, only one turn of wire is shown; however, in an actual meter movement, many turns of fine wire would be used, each turn adding more effective length to the coil. The coil is wound on an aluminum frame (bobbin) to which the pointer is attached. Oppositely wound hairsprings (only one is shown in the figure) are also attached to the bobbin, one at either end. The circuit to the coil is completed through the hairsprings. In addition to serving as conductors, the hairsprings serve as the restoring force that returns the pointer to the zero position when no current flows.
Q-5. What component of the D’Arsonval meter movement completes the circuit for current flow to the coil?
COIL MOVEMENT.—As we discussed previously, the deflecting (moving) force on the coil is proportional to the current flowing through the coil. This deflecting force tends to cause the coil to rotate against the restraining force of the hairsprings. When the deflecting force and the restraining force are equal, the coil and the pointer stop moving. As we have just stated, the deflecting force is proportional to the current in the coil, the angle (amount) of rotation is proportional to the deflecting force; therefore, the angle of rotation is proportional to the current through the coil. When current stops flowing through the coil, the deflecting force stops, and the restoring force of the springs returns the pointer to the zero position.
Q-6. What component supplies restoring force to the coil of the D’Arsonval meter movement? DIRECTION OF FORCE.—The current through the single turn of wire is in the direction
indicated in the figure (away from you on the right-hand side and toward you on the left-hand side). If we apply the right-hand motor rule, the direction of force is upward on the left-hand side and downward on the right-hand side; therefore, the direction of motion of the coil and pointer is clockwise. If the current were reversed in the wire, the direction of motion of the coil and pointer would be reversed. For a review of the right-hand rule for motors, refer to NEETS, Module 5, Introduction to Generators and Motors.
PRINCIPLE OF OPERATION.—A more detailed view of the basic D'Arsonval movement, as it is used in ammeters and voltmeters, is shown in figure 3-3. The principle of operation is the same as that discussed in the simplified version. The iron core is rigidly supported between the pole pieces; it serves to
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concentrate the flux in the narrow space between the iron core and the pole piece. Current flows into one hairspring, through the coil, and out the other hairspring. The restoring forces of the spiral springs return the pointer to the normal zero position when the current through the coil is interrupted. Conductors connect the hairsprings with the outside terminals of the meter. If the instrument is not DAMPED to absorb the energy of the moving element, the pointer will oscillate (vibrate) for a period of time before coming to a stop in its final position. Damping is an energy-absorbing system that prevents this.
DAMPING.—This is accomplished in many D'Arsonval movements by means of the motion of the aluminum bobbin on which the coil is wound. As the bobbin rotates in the magnetic field, an electromotive force is induced into it as it cuts through the lines of force. Induced currents flow in the bobbin in a direction opposite to the motion; this causes the bobbin to go beyond its final position only once before stopping. The overall sensitivity of the meter can be increased by the use of a lightweight rotating assembly (bobbin, coil, and pointer) and by the use of jewel bearings, as shown in figure 3-3.
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POLE CONSTRUCTION.—Note that the pole pieces in figures 3-2 and 3-3 have curved faces. You can see the advantage of this type of construction if you remember that lines of force enter and leave a magnetic field in the air gap at right angles to the coil, regardless of the angular position of the coil. Because of this type of construction, a more linear scale is possible than if the pole faces were flat.
Q-7. What advantage is gained by using pole pieces with curved faces in the D’Arsonval meter movement?
The movable coil of the D'Arsonval meter movement we have been discussing up to now uses small- size wire in its windings. This small-size wire places limits on the amount of current that can be safely passed through the coil. Therefore, the basic D'Arsonval movement discussed can be used to indicate or measure only very small currents. Certain circuit changes must be made to the basic D'Arsonval meter movement for it to be practical in everyday use. To measure large currents, you must use a SHUNT with the meter.
A shunt is a physically large, low-resistance conductor connected in parallel (shunt) with the meter terminals. It is used to carry the majority of the load current. Such a shunt is designed with the correct amount of resistance so that only a small portion of the total current flows through the meter coil. The meter current is proportional to the total load current. If the shunt is of such a value that the meter is calibrated in milliamperes, the instrument is called a MILLIAMMETER. If the shunt has such a value that the meter must be calibrated in terms of amperes, it is called an AMMETER.
Q-8. What structurally large, low-resistance conductor is connected in parallel with the meter movement to prevent damage?
SHUNT RESISTANCE.—A single, standardized meter movement is normally used in all ammeters, no matter what the range is for a particular meter. For example, meters with working ranges of 0 to 10 amperes, 0 to 5 amperes, or 0 to 1 ampere all use the same meter movement. The various ranges are achieved through the use of different values of shunt resistance with the same meter movement. The designer of the ammeter simply calculates the correct shunt resistance required to extend the range of the meter movement to measure any desired value of current. This shunt is then connected across the meter terminals. Shunts may be located inside the meter case (internal shunts) with the proper switching arrangements for changing them. They may also be located outside the meter case (external shunts) with the necessary leads to connect them to the meter.
EXTERNAL SHUNTS.—An external-shunt circuit is shown in figure 3-4, view A. Typical external shunts are shown in view B. View C shows a meter movement mounted within the case. The case provides protection against breakage, magnetic shielding in some cases, and portability.
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SHUNT CONSTRUCTION.—The shunt strips (view B of figure 3-4) are usually made of the alloy Manganin. Manganin has a temperature coefficient of almost zero. The zero-temperature coefficient property is desirable because of the heavy currents that often flow through shunts producing heat. A zero- temperature coefficient material is not affected by this heat; therefore, it remains stable in temperature. Most other materials increase their resistance as they are heated. If shunts were made of these materials, they would carry less current. More and more current would flow through the meter movement, and the chances of damage would increase. Using shunts constructed with zero-temperature coefficient materials eliminates this problem.
Q-9. What type of temperature coefficient material does not produce increased heat in response to increased current flow?
The ends of the shunt strips are embedded in heavy copper blocks. The blocks are attached to the meter coil leads and the line terminals. To ensure accurate readings, you should not interchangeably use the meter leads for a particular ammeter with those for a meter of a different range. Slight changes in lead length and size may vary the resistance of the meter circuit. If this happens, current will also change and cause incorrect meter readings. External shunts are generally used where currents greater than 50 amperes must be measured.
SHUNT SELECTION.—When using an external-shunt ammeter, you should select a suitable shunt so that the scale deflection can be easily read. For example, if the scale has 150 divisions and the load current you want to measure is known to be between 50 and 100 amperes, a 150-ampere shunt would be the correct choice. Under these conditions, each division of the scale represents 1 ampere. In other words,
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a full-scale deflection of the pointer would rest on the 150th division mark, indicating that 150 amperes of load current is flowing. At half-scale deflection, the pointer would rest on the 75th division mark, indicating that 75 amperes of load current is flowing.
A shunt having exactly the same current rating as the expected normal load current should never be selected. If you were to select such a shunt, higher than normal load currents could possibly drive the pointer off scale and damage the meter movement. A good choice of shunt values will place the indicating needle somewhere near the midscale indication when the load current you are reading is normal. For example, assume that the meter scale is divided into 100 equal divisions and you want to measure a current of 60 amperes. The shunt to use would be a 100-ampere shunt. This would make each division of the scale equal to 1 ampere. The meter indication would fall on the 60th division showing that 60 amperes of load current is flowing. Therefore, an allowance (40 amperes) remains for unexpected surge currents.
Q-10. A good choice of shunt resistance will place the indicating pointer near what part of the meter scale with a normal load?
INTERNAL SHUNTS FOR METERS IN THE 0- TO 50-AMPERE RANGE.—When measuring current ranges below 50 amperes, you will most often use internal shunts (Rshunt). In this way, you can easily change the range of the meter by means of a switching arrangement. A switch will select the correct internal shunt with the necessary current rating and resistance. Before you can calculate the required resistance of the shunt for each range, the total resistance of the meter movement must be known. For example, suppose you desire to use a 100-microampere D'Arsonval meter with an internal coil resistance of 100 ohms to measure line currents up to 1 ampere. The meter will deflect to its full-scale position when the current through the deflection coil is 100 microamperes.
Since the coil resistance is 100 ohms, you can calculate the coil's voltage (Ecoil) by using Ohm's law, as follows:
When the pointer is deflected to full scale, 100 microamperes of current flows through the coil and 0.01 volt drops across it. Remember, 100 microamperes is the maximum safe current for this meter movement. Exceeding this value will damage the meter. The shunt must carry any additional load current.
The meter coil has a 0.01 volt drop across it, and, because the shunt and coil are in parallel, the shunt also has a voltage drop of 0.01 volt. The current that flows through the shunt is the difference between the full-scale meter current and the line current being fed into the shunt. In this case, meter current is 100 microamperes. Full-scale deflection is desired only when the total current is 1 ampere. Therefore, the shunt current must equal 1 ampere minus 100 microamperes, or 0.9999 ampere. Ohm's law is again used to provide the approximate value of required shunt resistance (Rshunt), as follows:
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To increase the range of the 100-microampere meter to 1 ampere (full-scale deflection), place a 0.01- ohm shunt in parallel with the meter movement.
You can convert the 100-microampere instrument to a 10-ampere meter by using a proper shunt. The voltage drop for a full-scale deflection is still 0.01 volt across the coil and the shunt. The meter current is still 100 microamperes. The shunt current must therefore be 9.9999 amperes under full-scale deflection. Again, this is an approximate figure found by the application of Ohm’s law.
You can also convert the same instrument to a 50-ampere meter by using the proper shunt resistance, as follows:
INTERNAL SHUNTS FOR METERS IN THE MILLIAMPERE RANGE.—The above method of computing the shunt resistance is satisfactory in most cases; however, it can only be used when the line current is in the ampere range and the meter current is relatively small compared to the load current. In such cases, you can use an approximate value of resistance for the shunt, as was done above. However, when the line current is in the milliampere range and the coil current becomes an appreciable percentage of the line current, a more accurate calculation must be made. For example, suppose you desire to use a meter movement that has a full-scale deflection of 1 milliampere and a coil resistance of 50 ohms to measure currents up to 10 milliamperes. Using Ohm's law, you can figure the voltage (Ecoil) across the meter coil (and the shunt) at full-scale deflection, as follows:
The current that flows through the shunt (Ishunt) is the difference between the line current and the meter current, as figured below:
The shunt resistance (Rshunt) may then be figured, as follows:
Notice that, in this case, the exact value of shunt resistance has been used rather than an approximation.
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The formula for determining the resistance of the shunt is given by Rs = Im /Is times Rm, where Rs is the shunt resistance in ohms; Im is the meter current at full-scale deflection; Is is the shunt current at full- scale deflection; and Rm is the resistance of the meter coil. If the values given in the previous example are used in this equation, it will yield 5.55 ohms, the value previously calculated.
SWITCHING SHUNT VALUES.—Various values of shunt resistance can be used, by means of a suitable switching arrangement, to increase the number of current ranges that can be covered by the meter. Two switching arrangements are shown in figure 3-5. View A is the simpler of the two arrangements when a number of shunts are used to calculate the values of the shunt resistors. However, it has two disadvantages:
1. When the switch is moved from one shunt resistor to another, the shunt is momentarily removed from the meter. The line current then flows through the meter coil. Even a momentary surge of current could easily damage the coil.
2. The contact resistance (resistance between the blades of the switch when they are in contact) is in series with the shunt, but not with the meter coil. In shunts that must pass high currents, this contact resistance becomes an appreciable part of the total shunt resistance. Because the contact resistance is of a variable nature, the ammeter indication may not be accurate.
The generally preferred method of range switching is shown in (figure 3-5, view B). Although only two ranges are shown, as many ranges as needed can be used. In this type of circuit, the contact resistance of the range-selector switch is external to the shunt and meter in each range position. The contact resistance in this case has no effect on the accuracy of the current measurement.
When you are using ammeters, a primary rule of safety is that such current-measuring instruments must always be connected in series with a circuit, never in parallel with it. When an ammeter is connected across a constant-potential source of appreciable voltage, the low internal resistance of the meter bypasses the circuit resistance. This results in the application of the source voltage (or a good
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portion of it) directly to the meter terminals. The resulting excessive current burns up the meter coil and renders the meter useless until repaired.
Q-11. In what manner are current-measuring instruments connected to a circuit? If you do not know the approximate value of current in the circuit, you should take a reading at the
highest range of the ammeter; then you should switch progressively to lower ranges until a suitable reading is obtained. Most ammeter scales indicate the current being measured in increasing values from left to right. If you connect the meter without observing proper polarity, the pointer may be deflected backwards (from right to left). This action often damages the meter movement. You should ensure that the ammeter is always connected so that the current will flow into the negative terminal and out the positive terminal. Figure 3-6 shows various circuit arrangements and the proper ammeter connection methods to measure current in various portions of the circuit.
Q-12. An ammeter should always be connected so that current will flow into what terminal and out of what terminal?
Ammeter sensitivity is determined by the amount of current required by the meter coil to produce full-scale deflection of the pointer. The smaller the amount of current required to produce this deflection, the greater the sensitivity of the meter. A meter movement that requires only 100 microamperes for full- scale deflection has a greater sensitivity than a meter movement that requires 1 milliampere for the same deflection.
Q-13. (True or False) The larger the current required to produce full-scale deflection of the meter coil, the better the sensitivity of the meter.
Good sensitivity is especially important in ammeters to be used in circuits in which small currents flow. As the meter is connected in series with the load, the current flows through the meter. If the internal resistance of the meter is a large portion of the load resistance, an effect known as METER-LOADING will occur. Meter-loading is the condition that exists when the insertion of a meter into a circuit changes the operation of that circuit. This condition is not desirable. The purpose of inserting a meter into a circuit is to allow the measurement of circuit current in the normal operating condition. If the meter changes the
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circuit operation and changes the amount of current flow, the reading you obtain will be in error. An example of this is shown in figure 3-7.
Q-14. What condition exists when the insertion of a meter into a circuit changes the operation of the circuit?
In view A of figure 3-7, the circuit to be tested has an applied voltage of 100 millivolts and a resistance of 100 ohms. The current normally flowing in this circuit is 1 milliampere. In view B, an ammeter that requires 1 milliampere for full-scale deflection and that has an internal resistance of 100 ohms has been inserted. Since 1 milliampere of current flow is shown in view A, you might naturally assume that with the meter inserted into the circuit, a full-scale deflection will occur. You might also assume that the 1 milliampere of circuit current will be measured. However, neither of these assumptions is correct. With the ammeter inserted into the circuit, as shown in view B, the total resistance of the circuit is 200 ohms. With an applied voltage of 100 millivolts, applying Ohm’s law shows the actual current (Icircuit) to be 0.5 milliampere.
Since the meter reads 0.5 milliampere instead of the normal value of current, the meter reveals that a definite loading effect has taken place. In cases such as this, the use of ammeters, which have a lower internal resistance and a better current sensitivity, is desirable.
Up to this point, we have been discussing the 100-microampere D’Arsonval movement and its use as an ammeter. However, it can also be used to measure voltage if a MULTIPLIER (high resistance) is placed in series with the moving coil of the meter. For low-voltage instruments, this resistance is physically mounted inside the meter case with the D’Arsonval movement. The series resistance is constructed of a wire-wound resistance that has a low temperature coefficient wound on either a spool or
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card frame. For high-voltage ranges, the series resistance can be connected externally. A simplified diagram of a voltmeter is shown in figure 3-8.
Q-15. What modification is made to the D’Arsonval meter movement to enable the meter to measure voltage?
Keep in mind that the D’Arsonval meter movement uses current flow to produce a magnetic field that is proportional to the current. The meter movement is, therefore, an indicator of current flow rather than voltage. The addition of the series resistance is what allows the meter to be calibrated in terms of voltage; that is, the meter movement of a voltmeter operates because of the current flow through the meter, but the scale is marked in volts. For example, the meter movement shown in figure 3-9 has an internal resistance of 100 ohms, requires 100 microamperes for full-scale deflection, and has a voltage drop of 10 millivolts when full-scale deflection is reached. If you were to place this meter directly across a 10-volt source, an excessive current (in milliamperes) would flow. The meter would be destroyed because of the excessive current flowing through the meter movement. This can be seen in the following Ohm’s law application:
Using this equation, you can see that a current through the meter of 100 milliamperes is excessive and will cause damage.
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Since the normal voltage drop for the meter is 10 millivolts at full-scale deflection, some means must be supplied to drop the extra 9.99 volts without applying it directly to the meter. This is done by the addition of a multiplier resistor, as shown in figure 3-9.
The value of series resistance is determined by the current required for full-scale deflection and by the range of the voltages to be measured. Since the current through the meter circuit is directly proportional to the applied voltage, the meter scale can be calibrated directly in volts for a fixed value of series resistance. For example, let’s assume that the basic meter is to be made into a voltmeter with a full- scale deflection of 1 volt. The coil resistance of the basic meter is 100 ohms, and 100 microamperes of current causes full-scale deflection. The resistance (R
meter) required to limit the total current in the circuit to 100 microamperes can be found as follows:
Because the meter coil already measures 100 ohms, the series resistance required is equal to 10 kilohms minus 100 ohms, or 9.9 kilohms.
Q-16. What factors determine the value of the multiplier resistor? Multirange voltmeters use one meter movement. The required resistances are connected in series
with the meter by a switching arrangement. A schematic diagram of a multirange voltmeter with three ranges is shown in figure 3-10. The total meter resistance (R
meter) for each of the three ranges, beginning with the 1-volt range, is figured by the application of Ohm’s law, as follows:
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The actual value of the multiplying series resistor (Rseries) for each of these circuits is 100 ohms less than the total resistance. This allows for the resistance of the meter coil (Rcoil). Voltmeter Circuit Connections
When voltmeters are used, a primary rule of safety is that such voltage-measuring instruments must always be connected in parallel with (across) a circuit. If you are unsure of the level of the voltage to be measured, take a reading at the highest range of the voltmeter and progressively (step by step) lower the range until a suitable reading is obtained. In many cases, the voltmeter you will be using will not be a
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center-zero- (0 reading is in the center) indicating instrument. Observing the correct polarity is important when connecting the instrument to the circuit. Voltmeter polarity is the same as for the dc ammeter; that is, current flows from negative to positive.
Q-17. In what manner are voltage-measuring instruments connected to the circuit to be measured? Influence of a Voltmeter in a Circuit
The purpose of a voltmeter is to indicate the potential difference between two points in a circuit. When a voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has a low resistance, it will draw a substantial amount of current. This action lowers the effective resistance of the circuit and changes the voltage reading. When you are making voltage measurements in high-resistance circuits, use a HIGH-RESISTANCE VOLTMETER to prevent the shunting action of the voltmeter. The effect is less noticeable in low-resistance circuits because the shunting effect is less. The problem of voltmeter shunting (sometimes called circuit loading) is illustrated in figure 3-11.
Q-18. When making voltage measurements in a high-resistance circuit, you should always use a voltmeter with what relative value of resistance?
In view A of figure 3-11, a source of 150 volts is applied to a series circuit consisting of two 10- kilohm resistors. View A shows the voltage drop across each resistor to be 75 volts. In the 150-volt range, the voltmeter to be used has a total internal resistance of 10 kilohms. View B shows the voltmeter connected across the circuit. The parallel combination of R2 and the meter now present a total resistance of 5 kilohms. Because of the addition of the voltmeter, the voltage drops change to 100 volts across R1 and 50 volts across R2. Notice that this is not the normal voltage drop across R2. Actual circuit conditions have been altered because of the voltmeter.
The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (Rmeter), plus the series resistance (Rseries), by the full-scale reading in volts. In equation form, sensitivity is expressed as follows:
This is the same as saying the sensitivity is equal to the reciprocal of the full-scale deflection current. In equation form, this is expressed as follows:
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Therefore, the sensitivity of a 100-microampere movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.
Q-19. What term is used to express the sensitivity of a voltmeter? METERS USED FOR MEASURING RESISTANCE
The two instruments you will use most often to check continuity, or to measure the resistance of a circuit or circuit component, are the OHMMETER and the MEGGER (MEGOHMMETER). The ohmmeter is widely used to measure resistance and to check the continuity of electrical circuits and devices. Its range usually extends to only a few megohms. The megger is widely used for measuring insulation resistance, such as that between a wire and the outer surface of its insulation, and the insulation resistance of cables and insulators. The range of a megger can be extended to more than 1,000 megohms.
Q-20. What instrument is used for measuring the insulation resistance of cables? The Ohmmeter
A simple ohmmeter circuit is shown in figure 3-12. The ohmmeter consists of the dc milliammeter, discussed earlier in this chapter, and the added features shown below:
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• One or more resistors (one of which is variable). Q-21. What added features enable a dc milliammeter to function as an ohmmeter?
The deflection of the pointer of an ohmmeter is controlled by the amount of battery current passing through the moving coil. Before you can measure the resistance of an unknown resistor or electrical circuit, you must calibrate the ohmmeter to be used. If the value of resistance to be measured can be estimated within reasonable limits, select a range on the ohmmeter that will give approximately half-scale deflection when the resistance is inserted between the probes. If you cannot estimate the resistance to be measured, then set the range switch on the highest scale. Whatever range you select, the meter must be calibrated to read zero before the unknown resistance is measured.
To calibrate the meter, you first short the test leads together, as shown in figure 3-12. With the test leads shorted, a complete series circuit exists. The complete series circuit consists of the 3-volt source, the resistance of the meter coil (R
meter), the resistance of the zero-adjust rheostat, and the series multiplying resistor (Rseries). The shorted test leads cause current to flow and the meter pointer to deflect.
Notice that the zero point on the ohmmeter scale (as opposed to the zero points for voltage and current) is located at the extreme right side of the scale. With the test leads shorted, the zero-adjust potentiometer is set so that the pointer rests on the zero mark. Therefore, a full-scale deflection indicates zero resistance between the leads.
Q-22. A full-scale deflection on an ohmmeter scale indicates what resistance between the leads? If you change the range on the meter, you must "zero" (calibrate) the meter again to obtain an
accurate reading. When you separate the test leads, the pointer of the meter will return to the left side of the scale. This action, as explained earlier, is caused by the restoring force of the spring tension acting on the movable coil assembly. The reading at the left side of the scale indicates an infinite resistance.
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After you have adjusted the ohmmeter for zero reading, it is ready to be connected to a circuit to measure resistance. A typical circuit and ohmmeter arrangement is shown in figure 3-13. You must ensure that the power switch of the circuit to be measured is in the de-energized (OFF) position. This prevents the source voltage of the circuit from being applied to the meter, a condition that could cause severe damage to the meter movement.
Remember that the ohmmeter is an open circuit when the test leads are separated. To take a resistance reading with a meter, you must provide a path for current flow produced by the meter’s battery. In view A of figure 3-13, the meter is connected at points A and B to produce this path. Connecting these test leads places resistors R1 and R2 in series with the resistance of the meter coil, the zero-adjust potentiometer, and the series multiplying resistor. Since you previously calibrated the meter, the amount of coil movement now depends only on the resistances of R1 and R2.
The addition of R1 and R2 into the meter circuit raises the total series resistance and decreases the current. This decreases the amount of pointer deflection. The pointer comes to rest at a scale reading that indicates the combined resistance of R1 and R2. If you were to replace either R1 or R2, or both, with a resistor having a larger ohmic value, the current flow in the moving coil of the meter would be decreased even more. This would further decrease the pointer deflection, and the scale indication would read a still higher circuit resistance. View B is a simplified version of the circuitry in view A.
From our ohmmeter discussion, two facts should be apparent: (1) Movement of the moving coil is proportional to the amount of current flow, and (2) the scale reading of the ohmmeter is inversely proportional to current flow in the moving coil.
The amount of circuit resistance to be measured may vary over a wide range. In some cases, it may only be a few ohms; in other cases, it may be as great as 1 megohm. Scale multiplication features are built into most ohmmeters so that they will indicate any ohmic value being measured and offer the least amount of error. Most ohmmeters are equipped with a selector switch for selecting the multiplication scale desired. For example, view A of figure 3-14 shows a typical meter that has a six-position switch. The positions are marked on the meter in multiples of 10, from R · 1 through R · 100K.
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The range used to measure any particular unknown resistance (Rx in view A of figure 3-14) depends on the approximate ohmic value of the unknown resistance. For instance, the ohmmeter scale of the figure is calibrated in divisions from 0 to infinity. Note that the divisions are easier to read on the right-hand portion of the scale than on the left. For this reason, if Rx is greater than 1,000 ohms and if you are using the R · 1 range, you will be unable to accurately read the indicated resistance. This happens because the combined series resistance of resistors Rx is too large for range R · 1 to allow enough battery current to flow to deflect the pointer away from infinity. You need to turn the range switch to the R · 10 position to obtain the 1,000-ohm reading.
Let’s assume that you have changed the range switch to the R · 10 position and the pointer now deflects to a reading of 375 ohms, as shown in view B of figure 3-14. This would indicate to you that unknown resistance Rx has 3,750 (375 times 10) ohms of resistance. The change of range caused the deflection because resistor R · 10 has only 1/10 the resistance of resistor R · 1. Therefore, selecting the smaller series resistance allowed a battery current of sufficient value to cause a readable pointer deflection. If the R · 100 range were used to measure the same 3,750 ohm resistor, the pointer would deflect still further to the 37.5-ohm position, as shown in view C. This increased deflection would occur because resistor R · 100 has only 1/10 the resistance of resistor R · 10.
Q-23. The R · 100 resistance selection on an ohmmeter has what amount of resistance compared to the R · 10 selection?
The circuit arrangement in view A of figure 3-14 allows the same amount of current to flow through the moving meter coil. The same amount is allowed to flow whether the meter measures 10,000 ohms on the R · 1 scale, 100,000 ohms on the R · 10 scale, or 1,000,000 ohms on the R · 100 scale.
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The same amount of current must always be used to deflect the pointer to a certain position on the scale (midscale position, for example), regardless of the multiplication factor being used. Since the multiplier resistors are of different values, you must always "zero" the meter for each multiplication scale selected. When selecting a range on the ohmmeter, select the one that will result in the pointer coming to rest as close to the midpoint of the scale as possible. This will enable you to read the resistance more accurately because scale readings are more easily interpreted at or near midpoint.
An ordinary ohmmeter cannot be used for measuring multimillion ohm values of resistances, such as those in conductor insulation. To test for such insulation breakdown, you need to use a much higher potential than that supplied by the battery of an ohmmeter. This potential is placed between the conductor and the outside of the insulation. A megger (megohmmeter) is used for these tests. The megger, shown in figure 3-15, is a portable instrument consisting of two main elements: (1) a hand-driven dc generator, which supplies the necessary voltage for making the measurement, and (2) the instrument portion, which indicates the value of the resistance you are measuring. The instrument portion is of the opposed-coil type, as shown in view A. Coils a and b are mounted on movable member c. A fixed angular relationship exists between coils, and they are free to turn as a unit in a magnetic field. Coil b tends to move the pointer counterclockwise, and coil a tends to move it clockwise.
Coil a is connected in series with R3 and unknown resistance Rx. The combination of coil a, R3, and Rx forms a direct series path between the + and - brushes of the dc generator. Coil b is connected in series with R2, and this combination is also connected across the generator. Notice that the movable member (pointer) of the instrument portion of the megger has no restoring springs. Therefore, when the generator is not being operated, the pointer will float freely and may come to rest at any position on the scale.
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The guard ring, shown in view A of figure 3-15, shunts any leakage currents to the negative side of the generator. This prevents such current from flowing through coil a and affecting the meter reading.
Q-24. What is the purpose of the guard ring in a megohmmeter? If the test leads are open, no current will flow in coil a. However, current will flow internally through
coil b and deflect the pointer to infinity. This reading indicates a resistance too large to measure. When a resistance, such as Rx, is connected between the test leads, current also flows in coil a; the pointer tends to move clockwise. At the same time, coil b still tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position in which the two forces are balanced. This position depends upon the value of Rx, which controls the amount of the current in coil a. Because changes in voltage affect both coils in the same proportion, the position of the moving element is independent of the voltage. If you short the test leads together, the pointer will come to rest at zero because the current in coil a is relatively large. Since R3 limits the current, the instrument will not be damaged under these circumstances. The external appearance of one type of megger is shown in view B of figure 3-15.
Most meggers you will use are rated at 500 volts; however, there are other types. Meggers are usually equipped with friction clutches, which are designed to slip if the generator is cranked faster than its rated speed. This prevents the generator speed and output voltage from exceeding rated values. A 1,000-volt generator is available for extended ranges. When an extremely high resistance, such as 10,000 megohms or more, is to be measured, a high voltage is needed to cause enough current flow to actuate the meter movement.
When using a megger, you can easily be injured or damage equipment if you do not observe the following MINIMUM safety precautions:
• Use meggers on high-resistance measurements only (such as insulation measurements or to check two separate conductors on a cable).
• Never touch the test leads while the handle is being cranked.
• De-energize and discharge the circuit completely before connecting a megger.
Q-25. Most meggers you will use are rated at what voltage? Q-26. The development of excessive test voltages is avoided by the use of meggers equipped with what
device?
The electrodynamometer-type meter differs from the galvanometer types we have just studied in that two fixed coils are used to produce the magnetic field instead of a permanent magnet. Two movable coils are also used in the electrodynamometer meter. The electrodynamometer meter is most commonly found in various types of power meters.
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Q-27. What components in an electrodynamometer-type meter movement produce the magnetic field? As shown in figure 3-16, the fixed coils are connected in series and positioned coaxially (in line)
with a space between them. The two movable coils are also positioned coaxially and are connected in series. The two pairs of coils (fixed pair and movable pair) are also connected in series with each other. The movable coil is pivot-mounted between the fixed coils. The main shaft on which the movable coils are mounted is restrained by spiral springs that restore the pointer to zero when no current is flowing through the coil. These springs also act as conductors for delivering current to the movable coils. Since these conducting springs are very small, the meter cannot carry a high value of current.
Q-28. What is the limiting factor as to the amount of current an electrodynamometer meter movement can handle?
The meter is mechanically damped by means of aluminum vanes that move in enclosed air chambers. Although very accurate, electrodynamometer-type meters do not have the sensitivity of the D’Arsonval-type meter movement. For this reason, you will not find them used outside of the laboratory environment to a large extent.
The primary advantage of the electrodynamometer-type meter movement is that it can be used to measure alternating as well as direct current. If you apply alternating current to the standard galvanometer-type meter, it will not produce a usable reading. Instead, the meter will vibrate at or near the zero reading. On one-half cycle of the ac, the meter is deflected to the left and on the other half cycle
3-24
to the right. Since the frequencies you will be measuring are 60 hertz or greater, the meter is incapable of mechanically responding at this speed. The result is simply a vibration near the zero point; in addition, no useful reading of voltage or current is obtained. This problem does not exist with the electrodynamometer-type movement. Current flow through the stationary (fixed) coils sets up a magnetic field. Current flow through the moving coils sets up an opposing magnetic field. With two magnetic fields opposing, the pointer deflects to the right. If the current reverses direction, the magnetic fields of both sets of coils will be reversed. With both fields reversed, the coils still oppose each other, and the pointer still deflects to the right. Therefore, no rectifying devices are required to enable the electrodynamometer meter movement to read both ac and dc. Rectifying devices are required for the D’Arsonval-type meter movement to enable it to be used for measuring ac voltages and currents.
Q-29. What is the primary advantage of the electrodynamometer-type meter over the D’Arsonval-type meter?
When an electrodynamometer is used as a voltmeter, no problems in construction are encountered because the current required is not more than 0.1 ampere. This amount of current can be handled easily by the spiral springs. When the electrodynamometer is used as a voltmeter, its internal connections and construction are as shown in view A of figure 3-17. Fixed coils a and b are wound of fine wire since the current flow through them will not exceed 0.1 ampere. They are connected directly in series with movable coil c and the series current-limiting resistor.
3-25
When the electrodynamometer is used as an ammeter, a special type of construction must be used. This is because the large currents that flow through the meter cannot be carried through the moving coils. In the ammeter in view B of figure 3-17, stationary coils a and b are wound of heavier wire to carry up to 5.0 amperes. An inductive shunt (XL) is wired in parallel with the moving coils and permits only a small part of the total current to flow through the moving coil. The current flowing through the moving coil is directly proportional to the total current flowing through the instrument. The shunt has the same ratio of reactance to resistance as the moving coil does. Therefore, the instrument will be reasonably correct at frequencies at which it is used if ac currents are to be measured.
Electric power is measured by means of a wattmeter. This instrument is of the electrodynamometer type. As shown in figure 3-18, it consists of a pair of fixed coils, known as current coils, and a moving coil, called the voltage (potential) coil. The fixed current coils are wound with a few turns of a relatively large conductor. The voltage coil is wound with many turns of fine wire. It is mounted on a shaft that is supported in jeweled bearings so that it can turn inside the stationary coils. The movable coil carries a needle (pointer) that moves over a suitably graduated scale. Coil springs hold the needle at the zero position in the absence of a signal.
The current coil of the wattmeter is connected in series with the circuit (load), and the voltage coil is connected across the line. When line current flows through the current coil of a wattmeter, a field is set up around the coil. The strength of this field is in phase with and proportional to the line current. The voltage coil of the wattmeter generally has a high-resistance resistor connected in series with it. The purpose for this connection is to make the voltage-coil circuit of the meter as purely resistive as possible. As a result, current in the voltage circuit is practically in phase with line voltage. Therefore, when voltage is
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impressed on the voltage circuit, current is proportional to and in phase with the line voltage. Figure 3-19 shows the proper way to connect a wattmeter into a circuit.
Electrodynamic wattmeters are subject to errors arising from such factors as temperature and frequency. For example, heat through the coils eventually causes the small springs attached to the pointer to lengthen and lose tension, which produces deflection errors. Large currents through the wattmeter also produce a noticeable deflection error. These errors are caused by the heat (I2R) loss through coils from the application of high currents. Because of this, the maximum current range of electrodynamic wattmeters is normally restricted to approximately 20 amperes. The voltage range of wattmeters is usually limited to several hundred volts because of heat dissipation within the voltage circuit. However, the voltage range can be extended by the use of voltage multipliers.
Good-quality, portable wattmeters usually have an accuracy of 0.2 to 0.25 percent. You must remember, though, that electrodynamic wattmeter errors increase with frequency. For the higher frequency and power ranges, special types of wattmeters are made specifically for those ranges. We will discuss two such wattmeters in chapter 5 of this module.
The wattmeter consists of two circuits, either of which will be damaged if too much current passes through them. You should be especially aware of this fact because the reading on the instrument will not tell you whether or not the coils are being overheated. If an ammeter or voltmeter is overloaded, the pointer will indicate beyond the upper limit of its scale. In the wattmeter, both the current and potential circuit may carry such an overload that their insulations burn; yet the pointer may be only part of the way up the scale. This is because the position of the pointer depends upon the power factor of the circuit as well as upon the voltage and current. Therefore, a low power-factor circuit will provide a very low reading on the wattmeter. The reading will be low, even when the current and voltage circuits are loaded to the maximum safe limit. The safe rating for each wattmeter is always distinctly rated, not in watts, but in volts and amperes.
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in their use. The techniques suggested here are not all-inclusive. You will find, as you develop your technical skills, other variations and techniques in use. Consider the techniques for measuring current in a circuit. You can accomplish this by placing an ammeter in series with the circuit or by measuring the voltage across a resistor of known value and using Ohm’s law to figure current. This last technique has the advantage of eliminating the necessity of opening the circuit to connect the ammeter.
Open circuits are those in which the flow of current is interrupted by a broken wire, defective switch, or any means by which the current cannot flow. The test used to detect open circuits (or to see if the circuit is complete or continuous) is continuity testing.
An ohmmeter (which contains its own batteries) is excellent for use in a continuity test. Normally, continuity tests are performed in circuits where the resistance is very low, such as the resistance of a copper conductor. An open is indicated in these circuits by a very high or infinite resistance between two continuously connected points.
Figure 3-20 shows a continuity test of a cable that connects two electronic units. Notice that both plugs are disconnected and the ohmmeter is in series with conductor D under test. The power should be off. When checking conductors A, B, and C (connection of ohmmeter to conductors not shown), the current from the ohmmeter flows through plug 2 (female) through conductor A, B, or C to plug 1 (female). From plug 1, current passes through the jumper to the chassis, which is "grounded" to the ship’s structure. The metal structure serves as the return path to the chassis of unit 2 and completes the circuit through the series-connected ohmmeter. The ohmmeter indicates a low resistance because no break exists in conductors A, B, or C. However, checking conductor D reveals an open. The ohmmeter is shown indicating maximum resistance because current cannot flow in an open circuit. With an open circuit, the ohmmeter needle is all the way to the left since it is a series-type ohmmeter (reads right to left).
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Where conditions are such that the ship’s structure cannot be used as the return path, one of the other conductors (known to be good) may be used. For example, to check D, you can connect a jumper from pin D to pin A of plug 1 (female) and the ohmmeter leads to pins D and A of plug 2 (female). This technique will also reveal the open in the circuit.
Grounded circuits are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship. Grounds can have many causes. The two most common are the fraying of insulation from a wire and moisture-soaked insulation. The fraying of insulation from a wire allows bare wire to come into contact with the metal ground. Moisture-soaked insulation causes reduced insulation resistance (also classified as a ground).
Grounds are usually indicated by blown fuses or tripped circuit breakers. Blown fuses or tripped circuit breakers, however, can also result from a short circuit other than a ground. A high-resistance ground can also occur when current is increased significantly but not enough to rupture the fuse or trip the circuit breaker.
Before testing any circuit, ensure the circuit under test has been de-energized and checked with a safety shorting probe.
In testing for grounds, you may use a megger or an ohmmeter. Measuring the resistance to ground from points in a circuit determines if the point is grounded. Referring again to figure 3-20, you can see one possible means of testing a cable for grounds. If the jumper is removed from pin D of plug 1 (female), a test for ground can be made for each conductor in the cable. You can do this by connecting one meter lead to ground and the other to each of the pins of either of the plugs. A low resistance indicates that some part of that conductor or one of the plug assemblies is grounded. Both plugs must be removed from their units; if only one plug is removed, a false indication is possible because a conductor may be grounded through the unit.
A short circuit, other than a grounded one, is one where two conductors touch each other directly or through another conducting element. Two conductors with frayed insulation may touch and cause a short. Too much solder on the pin of a connector may short to the adjacent pin. In a short circuit, enough current may or may not flow to blow a fuse or open a circuit breaker. A short may occur between two cables carrying signals but might not be indicated by a blown fuse.
Shorts occur in many components, such as transformers, motor windings, and capacitors. The major test method used to detect shorts in such components is to measure resistance. The indicated resistance is then compared with the resistance given on schematics or in the equipment technical manuals to determine whether the measured value is within specifications.
An ohmmeter is the device used to check for shorts. You can use the ohmmeter to detect a short between two conductors by measuring the resistance between them (be sure electrical power has been disconnected). A low resistance reading indicates a short. You can test the circuit in figure 3-20 for a short by first removing the jumper and disconnecting both plugs; you then measure the resistance between the two suspended conductors.
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The following section discusses voltage measurements on live circuits. BE SURE YOU ALWAYS FOLLOW PRESCRIBED SAFETY RULES WHEN MEASURING VOLTAGES.
Voltage tests must be made with the power applied; therefore, the prescribed safety precautions must be followed to prevent injury to personnel and damage to the equipment. You will find in your maintenance work that the voltage test is of utmost importance. It is used not only in isolating casualties to major components but also in the maintenance of subassemblies, units, and circuits. Before checking a circuit voltage, you should check the voltage of the power source to be sure that the normal voltage is being applied to the circuit.
The voltmeter is used for voltage tests. In using the voltmeter, make certain that the meter used is designed for the type of current (ac or dc) to be tested and has a scale with a suitable range. Since defective parts in a circuit can cause higher than normal voltages to be present at the point of test, the highest voltmeter range available should be used first. Once you have obtained a reading, determine if a lower scale can be used that will cause no damage to the meter movement. If so, use the lower scale. This provides a more accurate reading.
Another consideration in the circuit voltage test is the resistance and current in the circuit. A low resistance in a high-current circuit could result in considerable voltage drop, whereas the same resistance in a low-current circuit may be minimal. Abnormal resistance in part of a circuit can be checked with either an ohmmeter or a voltmeter. Where practical, an ohmmeter should be used because the test is then carried out with a "dead" circuit.
The majority of the electronic circuits you will encounter in equipment will be low-current circuits, and most voltage readings will be direct current. Also, many of the schematics will indicate the voltages at various test points. Therefore, if you suspect that a certain stage is defective, you can check the voltage by connecting a voltmeter from the test point to ground. If the suspected stage is not defective, the voltmeter readings should match the voltages given on the schematic.
Some technical manuals also contain voltage charts on which all the voltage measurements are tabulated. These charts usually indicate the sensitivity of the meter (for example, 20,000 ohms/volt) used to obtain the voltage readings for the chart. To obtain comparable results, you must use a voltmeter of the same sensitivity (or greater) as that specified. Make certain that the voltmeter is not "loading" the circuit while taking a measurement. If the meter resistance is not considerably higher than the circuit resistance, the reading will be markedly lower than the true circuit voltage because of the voltmeter’s loading effect. (To calculate meter resistance, multiply the rated ohms-per-volt sensitivity value of the meter by the scale in use. For example, a 1,000-ohms-per-volt meter set to the 300-volt scale will have a resistance of 300,000 ohms.) RESISTANCE TESTS
Before checking the resistance of a circuit or of a part, make certain that the power has been turned off. Also make sure capacitors in the associated circuit are fully discharged. To check continuity, always use the lowest ohmmeter range. If the highest range is used, the meter may indicate zero, even though appreciable resistance is present in the circuit. Conversely, to check a high resistance, use the highest scale since the lower range scale may indicate infinity, even though the resistance is less than a megohm. In making resistance tests, you must remember that even though the external ohmmeter leads are
3-30
connected in parallel with the circuit to be measured, the internal meter circuitry is electrically connected in series.
In making resistance tests, take into account that other circuits containing resistances and capacitances may be in parallel with the circuit to be measured. Erroneous conclusions may be drawn from readings obtained in such cases. Remember, a capacitor blocks the dc flow from the ohmmeter. To obtain an accurate reading when other parts are connected across the suspected circuit, disconnect one end of the circuit to be measured from the equipment. For example, many of the resistors in major components and subassemblies are connected across transformer windings. To obtain a valid resistance measurement, you must isolate the resistors to be measured from the shunt resistances of the coils of the transformers.
Resistance tests are also used to check a component for grounds. In these tests, the component to be tested should be disconnected from the rest of the circuit so that no normal circuit ground will exist. Dismounting the component to be checked is not necessary. The ohmmeter is set for a high-resistance range. Then the ohmmeter is connected between ground and each electrically separate circuit of the component being tested. Any resistance reading less than infinity indicates at least a partial ground. You can also check capacitors suspected of being short-circuited by measuring the resistance. To check a capacitor suspected of being open, temporarily shunt a known good capacitor then recheck the performance of the circuit.
To avoid possible damage to equipment during resistance tests, observe the following precautions:
• Always connect an ammeter in series—never in parallel.
• Connect a voltmeter in parallel.
• Never connect an ohmmeter to a live circuit.
• Observe polarity when using a dc ammeter or a dc voltmeter.
• View meters directly from the front. When viewed from an angle off to the side, an incorrect reading will result because of OPTICAL PARALLAX. (Parallax was covered in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.)
• Always choose an instrument suitable for the measurement desired.
• Select the highest range first and then switch to the proper range.
• In using a meter, choose a scale that will result in an indication as near midscale as possible.
• Do not mount or use instruments in the presence of a strong magnetic field.
• Remember, a low internal resistance voltmeter (low sensitivity) may shunt the circuit being measured and result in incorrect readings.
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The important points of this chapter are summarized in the following paragraphs. You should be familiar with these points before continuing with your study of test equipment.
A permanent-magnet, moving-coil meter movement (D’ARSONVAL movement) uses the interaction of magnetic fields to produce movement.
DAMPING is used to smooth out the vibration and to help prevent overshooting of the meter pointer.
ELECTRODYNAMOMETER movements are usually used in wattmeters. They operate much like the D’Arsonval meter movement, except field coils are used instead of a permanent magnet. Electrodynamometer movements measure either ac or dc without the use of a rectifier.
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A SHUNT is a physically large, low-resistance conductor connected in parallel with the meter terminals. It carries the majority of the load current so that only a small portion of the total current will flow through the meter coil.
An AMMETER measures current and is always connected in series with the circuit being measured. An ammeter should have a low resistance so that the effect of the ammeter on the circuit will be kept to a minimum.
VOLTMETERS are used to measure voltage and are always connected in parallel with the circuit being measured. A voltmeter should have a high resistance compared to the circuit being measured to minimize the loading effect. Voltmeter sensitivity is expressed in ohms per volt.
OHMMETERS are used to measure resistance and to check continuity. An ohmmeter is electrically connected in series with the resistance being measured. The ohmmeter range, which allows a midscale deflection, should be used.
A MEGOHMMETER (MEGGER) is used to measure very high resistance, such as the insulation of wiring.
A WATTMETER is usually an electrodynamometer and is used to measure power.
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A CONTINUITY TEST is accomplished with an ohmmeter. This test is used to check for opens (or to see if the circuit is complete or continuous).
GROUNDED CIRCUITS are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship or chassis. In testing for grounds, you may use either an ohmmeter or a megger.
A SHORT CIRCUIT, other than a grounded one, is where two conductors touch each other directly or through another conducting element. An ohmmeter is used to test for shorts.
ANSWERS TO QUESTIONS Q1. THROUGH Q29. A-1. Self-excited. A-2. Phosphor bronze ribbons.
A-3. The pointer arrangement and the light and mirror arrangement.
A-4. Coil balance.
A-5. Hairspring.
A-6. Hairspring.
A-7. Makes it possible to have a more linear scale than if the poles were flat. A-8. Shunt.
A-9. Zero-temperature coefficient. A-10. Midscale.
A-11. In series.
A-12. Negative, positive.
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A-13. False.
A-14. Meter-loading.
A-15. A multimeter (high resistance) is placed in series with the coil of the meter. A-16. The current required for full-scale deflection, and the range of the voltage to be measured. A-17. In parallel.
A-18. High.
A-19. Ohms per volt.
A-20. Megohmmeter (megger). A-21. 1. A source of dc potential. 2. One or more resistors (one of which is variable). A-22. Zero.
A-23. 1/10.
A-24. Shunts leakage current, which prevents false readings. A-25. 500.
A-26. Friction clutches.
A-27. Fixed coils.
A-28. Size of spiral conducting. A-29. The electrodynamometer-type meter can be used to measure both ac and dc currents.
4-1
Upon completing this chapter, you should be able to:
1. Describe the proper operating procedures for using the multimeter.
2. Describe the proper operating procedures for using the digital multimeter.
3. Describe the proper operating procedures for using the differential voltmeter.
4. Describe the proper operation of the transistor tester.
5. Describe the proper procedure for using the RCL bridge to measure resistance, capacitance, and inductance.
In the previous chapters, you have learned how to use some basic and miscellaneous measuring instruments to perform required maintenance and upkeep of electronic systems and components. You were also introduced to the construction and operation of basic meter movements in test equipment. This chapter will introduce you to some of the testing instruments commonly used in the Navy today.
During troubleshooting, you will often be required to measure voltage, current, and resistance. Rather than using three or more separate meters for these measurements, you can use the MULTIMETER. The multimeter contains circuitry that allows it to be used as a voltmeter, an ammeter, or an ohmmeter. A multimeter is often called a VOLT-OHM-MILLIAMMETER (VOM).
One of the greatest advantages of a VOM is that no external power source is required for its operation; therefore, no warm-up is necessary. Other advantages are its portability, versatility, and freedom from calibration errors caused by aging tubes, line voltage variations, and so forth.
Q-1. What is one of the greatest advantages of a VOM? Two disadvantages are that (1) the VOM tends to "load" the circuit under test, and (2) the meter
movement is easily damaged as a result of improper testing procedures.
Never press down on or place any object on the glass face of any multimeter. This can disable the meter movement from operating properly or cause damage.
4-2
In the discussion that follows, you will become familiar with the operation and use of the multimeter in measuring resistance, voltage, and current.
The meter selected for this discussion is the Simpson 260 multimeter, as shown in figure 4-1. The Simpson 260 is a typical VOM used in the Navy today.
4-3
The multimeter has two selector switches. The switch on the lower left is the function switch, and the one in the lower center is the range switch. The function switch selects the type of current you will be measuring (+dc, - dc, or ac). The range switch is a 12-position switch that selects the range of ohmmeter, voltmeter, or milliammeter measurements you will make.
The multimeter is equipped with a pair of test leads; red is the positive lead and black is the negative, or common, lead. Eight jacks are located on the lower part of the front panel. To prepare the meter for use, simply insert the test leads into the proper jacks to obtain the circuit and range desired for each application. In most applications, the black lead will be inserted into the jack marked at the lower left with a negative sign ( - ) or with the word COMMON. Measuring Resistance
Before proceeding, you should be aware of the following important safety precaution that must be observed when using the ohmmeter function of a VOM:
Never connect an ohmmeter to a "hot" (energized) circuit. Be sure that no power is applied and that all capacitors are discharged.
Q-2. Before you connect a VOM in a circuit for an ohmmeter reading, in what condition must the circuit be?
The internal components of the multimeter use very little current and are protected from damage by an overload protection circuit (fuse or circuit breaker). However, damage may still occur if you neglect the safety precaution in the CAUTION instructions above.
Because no external power is applied to the component being tested in a resistance check, a logical question you may ask is, Where does the power for deflection of the ohmmeter come from? The multimeter contains its own two-battery power supply inside the case. The resistive components inside the multimeter are of such values that when the leads are connected together (no resistance), the meter indicates a full-scale deflection. Because there is no resistance between the shorted leads, full-scale deflection represents zero resistance.
Before making a measurement, you must zero the ohmmeter to ensure accurate readings. This is accomplished by shorting the leads together and adjusting the OHMS ADJ control so that the pointer is pointing directly at the zero mark on the OHMS scale. The ZERO OHMS control is continuously variable and is used to adjust the meter circuit sensitivity to compensate for battery aging in the ohmmeter circuits.
An important point to remember when you are making an accurate resistance measurement is to "zero" the meter each time you select a new range. If this is not done, the readings you obtain will probably be incorrect.
When making a resistance measurement on a resistor, you must give the following considerations to the resistor being tested:
• The resistor must be electrically isolated. In some instances, a soldered connection will have to be disconnected to isolate the resistor. Generally, isolating one side of the resistor is satisfactory for you to make an accurate reading.
4-4
• The meter leads must make good electrical contact with the resistor leads. Points of contact should be checked for dirt, grease, varnish, paint or any other material that may affect current flow.
• Touch only the insulated portions of the test leads. Your body has a certain amount of resistance, which the ohmmeter will measure if you touch the uninsulated portions of the leads.
Figure 4-2 is a functional block diagram of the ohmmeter circuit in a VOM. The proper method of checking a resistor is to connect the red lead to one end of the resistor and the black lead to the other end of the resistor.
Because zero resistance causes full-scale deflection, you should realize that the deflection of the meter is inversely proportional to the resistance being tested; that is, for a small resistance value, the deflection will be nearly full scale; and for a large resistance value, the deflection will be considerably less. This means that the left portion of the OHMS scale represents high resistance; the right side of the scale represents low resistance. Zero resistance (a short circuit) is indicated on the extreme right side of the scale; infinite resistance (an open circuit) is located on the extreme left side of the scale.
Notice that you read the OHMS scale on the multimeter from RIGHT to LEFT. For example, the pointer of the multimeter in figure 4-3 indicates 8.0 ohms. To determine the actual value of a resistor, multiply the reading on the meter scale by the range switch setting (R · 1, R · 100, or R · 10,000).
4-5
Notice that the scale marks are crowded on the left side of the OHMS scale, which makes them difficult to read. Therefore, the best range to select is one in which the pointer will fall in the space from midscale to slightly to the right side of midscale. The divisions in this area of the scale are evenly spaced and provide for easier reading and greater accuracy.
Q-3. When taking resistance readings with a VOM, you will obtain the most accurate readings at or near what part of the scale?
To explain the relationship between the meter readings and the range switch setting, let’s use an example. Suppose you have a 2,400-ohm resistor, which you have identified by the resistor color code. With the range switch in the R · 1 position, you connect the meter across the resistor. The meter point then deflects between 200 and the point labeled with the infinity symbol ( ¥ ) on the extreme left side of the scale. Because the R · 1 range is selected, you multiply the reading by 1. Obviously, the scale reading is not accurate enough. Therefore, you move the range selector switch to the next higher scale position (R · 100) to obtain a more easily read value.
In the R · 100 position, you again zero the meter. This time, the pointer moves to the 24 mark on the scale. Because the R · 100 scale is selected, the reading is multiplied by 100. This gives a more accurate reading of 2,400 ohms (24 times 100).
If you position the range switch to the R · 10,000 scale, accuracy decreases. The most accurate readings are obtained at or near midscale. Other VOM instruments have ranges with other settings, such as R · 10, R · 100, or R · 1,000, to make it easier to make such readings.
Another thing to remember when you are measuring resistance is the tolerance of the resistor. If the tolerance of the resistor in the preceding example is 10 percent, we would expect a reading between approximately 2,160 and 2,640 ohms. If the reading is not within these limits, the resistor has probably changed value and should be discarded.
An open resistor will indicate no deflection on the meter. A shorted resistor causes full-scale deflection to the right on the lowest range scale, such as if the leads were shorted together.
4-6
You set the multimeter to operate as a dc voltmeter by placing the function switch in either of two positions: +DC or - DC. The meter leads, as in the case of the ohmmeter function, must be connected to the proper meter jacks. When you measure dc voltages, be sure the red lead is the positive lead and the black lead is the negative, or common, lead. View A of figure 4-4 is a functional block diagram of dc voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc voltages.
When the meter is connected in a circuit, it becomes a circuit component. Because all meters have some resistance, they alter the circuit by changing the current. The resistance presented by the voltmeter depends on the amount of voltage being measured and the position of the function switch.
Some multimeters use a 20,000 ohms-per-volt meter sensitivity for measuring dc voltage and a 5,000 ohms-per-volt sensitivity for measuring ac voltage. The higher the meter resistance, the less it will load the circuit. The idea is to keep circuit loading to an absolute minimum so that the circuit under test is unaffected by the meter. In this way, you can get a clearer picture of what the circuit malfunction is, not the effect of the meter on the circuit.
Again, refer to figure 4-4. With the function switch set to either +DC or - DC, let’s consider the effect of the range switch on the meter scale to be used. When measuring dc voltages, you have eight voltage ranges available: .25V, 2.5V, 10V, 50V, 250V, and 500V (1- and 1,000-volt special application plug-ins are also available). The setting of the range switch determines the maximum value represented on the meter. When measuring dc voltages, use the scale marked DC (figure 4-3). The last number at the extreme right side of the DC scale indicates the maximum value of the range being used. When the range switch is in the 2.5V position, the scale represents a maximum of 2.5 volts.
To simplify the relationship between the digits on the meter scale and the setting of the range switch, always use the multiple of the full-scale-deflection digits on the meter face that correspond to the numbers of the range switch. For example, use the 250 scale for the 250MV jack, 2.5V, and 250V ranges; the 50 on the scale for 50V and 500V ranges; and the 10 on the scale for the 10V and 1,000V ranges.
4-7
For explanation purposes, let’s assume you wish to measure 30 volts dc. In this case, select the next higher range position, 50V. When you place the range switch to the 50V position (as shown in view B of figure 4-4), the meter pointer should rise from a little more than midscale to 30, which represents 30 volts dc.
When measuring a known dc voltage, position the range switch to a setting that will cause approximately midscale deflection. Readings taken near the center of the scale are the most accurate. When measuring an unknown dc voltage, always begin on the highest voltage range. Using the range switch, work down to an appropriate range. If the meter pointer moves to the left, you should reverse the polarity of the function switch.
Always check the polarity before connecting the meter.
Q-4. Besides setting up the meter for expected voltage ranges, what must be strictly observed when taking dc voltage readings?
Now let’s discuss how you take a voltage measurement on a component within a circuit. As an example, let’s measure the voltage drop across the resistor shown in figure 4-5.
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When measuring a dc voltage drop across a component in a circuit, you must connect the voltmeter in parallel with the component. As you can see in figure 4-5, the positive (red) lead is connected to the positive side of the resistor, and the negative (black) lead is connected to the negative side. A voltage reading is obtained on the meter when current flows through the resistor.
Some voltmeter readings will require the use of a ground as a reference point. Under these conditions, one voltmeter lead is connected to the equipment ground, and the other lead is connected to the test point where voltage is to be measured. Be sure to observe polarity.
To measure ac voltages, you must set the function switch to the AC position. The same procedure used to measure dc voltages applies, except that in reading the voltage, you use the AC volts scale (the polarity of the test leads is not important). When measuring very low or high frequencies of ac voltages, you should be aware that the multimeter has a tendency to be inaccurate. View A of figure 4-6 is a
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functional block diagram of the ac and output voltage circuits in the multimeter. View B shows the jacks and switch positions used to measure ac voltages.
You will often measure the ac component of an output voltage where both ac and dc voltage levels exist. This occurs primarily in amplifier circuits.
The multimeter has a 0.1-microfarad, 400-volt blocking capacitor in series with the OUTPUT jack. The capacitor blocks the dc component of the current in the circuit under test, but allows the ac component to pass on to the indicating circuits.
When using OUTPUT, do not attempt to use the meter in a circuit in which the dc voltage component exceeds the 400-volt rating of the blocking capacitor.
To use the multimeter to measure output voltage, you must follow these steps:
1. Set the function switch to AC.
2. Plug the black test lead into the COMMON jack and the red test lead into the OUTPUT jack. 3. Set the range switch at the appropriate range position, marked as 2.5V, 10V, 50V, or 250V.
4. Connect the test leads to the component being measured with the black test lead to the negative side of the component.
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5. Turn on the power in the test circuit. Read the output voltage on the appropriate ac voltage scale. For the 2.5V range, read the value directly on the scale marked 2.5. For the 10V, 50V, or 250V range, use the red scale marked AC and read the black figures immediately above the scale.
The multimeter can function as an ammeter to measure current flow.
When using the multimeter as a current-indicating instrument, NEVER connect the test leads directly across a voltage. ALWAYS connect the instrument in series with the load.
To use the multimeter as an ammeter, you must take the following steps:
1. Set the function switch at +DC (assuming the current to be positive). 2. Plug the black test lead in the COMMON jack and the red test lead into the + jack. 3. Set the range switch at one of the five ampere-range positions.
4. Ensure the equipment is OFF and then physically open the circuit in which the current is being measured.
5. Connect the VOM in series with the circuit, ensuring that proper polarity is observed when making this connection.
6. Turn the equipment ON and then read the current on the DC scale. (This is the same scale used to measure dc voltages.)
The setting of the range switch determines the maximum value represented by the DC scale. Always use the range scale that corresponds to the range switch setting.
Never attempt to measure currents greater than the setting of the range switch. Increase the range with a shunt, if necessary, but do not exceed the marked current.
When measuring unknown currents, follow the same procedures as when measuring voltages. Always start with the highest range available and work down. Use the range that gives approximately half-scale deflection. If this procedure isn’t followed, the meter could be burned out.
Figure 4-7 is a functional block diagram of the dc current circuits in a multimeter.
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A dc high-voltage probe is available for use with the multimeter. The probe extends the range of the multimeter in a safe and convenient manner. It is primarily used to measure high-voltage, low-power, dc- current sources, such as the anode supplies in television receivers and other cathode-ray tube circuitry.
Do not use this probe on electrical equipment that can deliver high power under short-circuit conditions, such as from a large dc motor-generator set.
Also available is an ac high-voltage probe. The 10,000-volt ac probe is similar to the high-voltage dc probe with the following exceptions:
• The ac high-voltage probe is designed primarily to extend the range of a 5,000-ohms-per-volt VOM.
• The probe is used with the VOM in the 10V AC position.
• You take readings on the 0-10V AC scale and multiply by 1,000.
As you studied in chapter 3 (externally excited meters), placing a meter into a circuit causes energy to be taken from the circuit. The amount of energy taken depends on the sensitivity of the meter. In some cases, this energy loss cannot be tolerated. For example, in extremely sensitive circuits, such as oscillator grid circuits and automatic volume control circuits, degradation of normal circuit operation will occur. This often results in failure to obtain a usable indication of the fault. The use of electronic multimeters is practical in these sensitive electronic circuits. The higher the input impedance of a meter, the less the loading effect and the more accurate the measurements taken. Electronic multimeters have considerably greater input impedances than do nonelectronic multimeters.
One example of a typical electronic multimeter in use within the Navy is the electronic Model 8000A Digital Multimeter. Most electronic digital multimeters overcome the disadvantage of requiring a continuous external power source by combining an external ac source with an internal rechargeable battery. Another advantage of this meter is that it can be read directly and does not use a scale. Figure 4-8 shows the model 8000A multimeter.
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The locations of all controls, connectors, and indicators are shown in figure 4-8. The INPUT terminals, located on the left-hand side of the meter face, provide input connections for voltage or resistance (V-?) and milliampere current (MA) measurements with respect to the common terminal. The readout section, located across the upper half of the meter face, contains light-emitting diode (LED) indicators. They display the measured input and polarity signs for dc measurements. The POWER switch, located on the lower right-hand side of the meter face, is a push-button switch used to energize the instrument. The RANGE switches, located on the lower, middle, right-hand portion of the meter face, select the voltage (200 millivolts, 2, 20, 200, or 1,200 volts), current (200 microamperes, 2, 200, or 2,000 milliamperes), and resistance (200 ohms, 2, 20, 200, or 2,000 kilohms) ranges. The FUNCTION switches, located on the lower, middle, left-hand portion of the meter face, select the voltage, current, or resistance modes. The MA input terminal is also a fuse holder for the current protection fuse.
Power is supplied by internal rechargeable batteries that allow the instrument to operate for at least 8 hours. Recharging the batteries is accomplished by switching the POWER switch to OFF and connecting the instrument to an ac power line. You can use the instrument when recharging the batteries on ac power, but the recharging time will be extended.
Q-5. Power for the electronic digital multimeter is normally supplied by what internal power source? Overload Protection
An overload condition is indicated by the simultaneous flashing of the display readouts. The dc voltage function can withstand up to 1,200 volts dc or 1,200 volts root-mean-square (rms) on any range.
Q-6. How is an overload condition indicated by the electronic digital multimeter? The ac voltage function can sustain up to 1,200 volts rms on the 20-, 200-, and 1,200-volt ranges and
500 volts rms on the 200-millivolt and 2-volt ranges. The current input fuse is protected above 2 amperes rms. Protection for the resistance function is to 130 volts rms in the 200-ohm and 2-kilohm ranges, and 250 volts rms in the 20-kilohm through 20-megohm ranges.
Table 4-1 lists the proper function push buttons, range push buttons, and input terminal connections for performing specific measurements with the model 8000A.
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Table 4-1.—Basic Measurement Instructions
1200V rms (sinusoidal), not to exceed 107 V-Hz on 20, 200, 1200V ranges. 500V rms (sinusoidal) on 200mV and 2V ranges.
Kilohms K ��� �������� 200, or ����.
Figure 4-9 is a block diagram of an electronic digital multimeter. Note that the block diagram divides the instrument into three major sections: the SIGNAL CONDITIONING section, the ANALOG-TO- DIGITAL CONVERTER section, and the DISPLAY section.
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The signal conditioning section provides a dc analog voltage, characteristic of the applied input, to the analog-to-digital converter section. This task is accomplished by the input voltage divider, current shunts, ac converter, active filter, and associated switching.
The analog-to-digital (a/d) converter section changes the dc output voltage from the signal conditioning section to digital information. The a/d converter uses a voltage-to-frequency conversion technique. A dc voltage at the input of the a/d converter is changed to a frequency by the analog integrated circuit (ic). This frequency is characteristic of the magnitude and polarity of the dc input voltage. Counting of the output frequency from the analog ic is accomplished by the digital ic. The resulting count is transferred in binary format to the display section. (Binary number systems are covered in NEETS, Module 13, Introduction to Number Systems, Boolean Algebra, and Logic Circuits.)
The display section takes the digital (binary) information from the a/d converter section, decodes it, and visually displays it. The decoded digital information is displayed on numerical LED readouts.
Q-7. In an electronic digital multimeter, the digital information is displayed by what type of numerical readouts?
Several accessories are available for use with the electronic digital multimeter. One accessory is the test lead kit, shown in figure 4-10. The kit contains two color-coded test leads with threaded adapters. These adapters attach to banana plugs, pin tips, test prod tips, alligator clips, and binding post lugs.
Figure 4-11 shows a high-current probe. This probe extends the ac current measurement capability from 2 to 600 amperes at frequencies up to 400 hertz.
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Figure 4-12 shows a high-voltage probe. The probe extends the dc voltage range to 30 kilovolts.
Figure 4-13 shows a high-frequency probe, which allows measurements over a frequency range of 10 kilohertz to 500 megahertz.
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The DIFFERENTIAL VOLTMETER provides extremely accurate voltage measurements and is a highly reliable piece of precision test equipment. Its general function is to compare an unknown voltage with a known internal reference voltage and to indicate the difference in their values. The differential voltmeter in common use in the Navy today is the model 893A (figure 4-14).
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Q-8. What is the general function of the differential voltmeter? The differential voltmeter can be used as a conventional TRANSISTORIZED ELECTRONIC
VOLTMETER (TVM) and a DIFFERENTIAL NULL VOLTMETER. It can also be used to measure variations of a voltage near some known value (NULL DETECTOR), high resistance values (MEGGOMETER), and for dBm measurements. METER DESIGN CHARACTERISTICS
The differential voltmeter is a solid-state instrument that provides the capability of making dc voltage measurements from +/ - 10 microvolts to +/ - 1,100 volts. Ac voltages from 0.001 to 1,100 volts can be measured over a frequency range from 5 hertz to 100 kilohertz. Both of these measurements can be made without concern for loading the circuit. The differential voltmeter has four voltage readout dials that vary the resistance of the divider assembly as described above.
The differential voltmeter uses a built-in NULL DETECTOR to measure an unknown voltage. The meter circuitry compares the unknown voltage to a known, adjustable reference voltage supplied by the meter. The reference voltage is provided by a high-voltage dc power supply and decade resistor divider assembly strings that are set by voltage readout dials. In this way, the output from the high-voltage power supply can be precisely divided into increments as small as 10 microvolts. The readout dials are used to adjust the meter pointer to 0 and the unknown voltage is then read from the voltage dials.
A primary feature of the differential voltmeter is that it does not draw current from the unknown source for dc measurements when the measurement is obtained. Therefore, the determination of the unknown dc potential is independent of its source.
The front panel of a typical differential voltmeter is shown in figure 4-15. With a few differences, the controls and terminals are similar to those used on other differential voltmeters. The NULL SENSE
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switch selects the conventional TVM mode of operation and the various full-scale null detector sensitivity ranges when the instrument is operated in the differential mode of operation. The RANGE switch allows selection of the desired input voltage range, positions the readout dial decimal point, and selects the various ranges of the NULL SENSE switch. The readout dials provide a digital readout of the measured voltage when the instrument is in the differential mode.
There are two primary modes of operation: the conventional transistorized voltmeter mode and the differential null mode. These modes are described in the next paragraphs.
as shown in figure 4-16. The null detector drives the front panel meter and provides a full-scale meter deflection for any full-scale input. Positive or negative voltage measurements are made by reversing the meter terminals through the contacts of the MODE switch.
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Figure 4-16 is a functional block diagram of a differential voltmeter.  The circuitry is interconnected by various switching arrangements when you perform the desired ac or dc conventional or differential voltage measurements.
Placing the MODE switch in figure 4-16 to the AC position connects the instrument circuitry as a conventional transistorized ac voltmeter. A full-scale input voltage at the input terminals of the instrument results in a voltage being applied to the input of the null detector. The null detector drives the front panel meter that indicates the value of the measured ac voltage.
When the instrument is used as a dc differential voltmeter, the MODE and NULL SENS switches in figure 4-16 are placed to their respective +/ - dc and desired full-scale meter sensitivity positions. In this mode of operation, the NULL SENS switch selects a suitable resistance value to determine the full-scale sensitivity of the meter. The dc input voltage applied to the instrument is then compared with the null detector, and any resulting difference is used to drive the meter. The meter terminals can be reversed through the contacts of the MODE switch for +/ - dc voltage measurements.
Laboratory transistor test sets are used in experimental work to test characteristics of transistors. For maintenance and repair, however, checking all transistor parameters is not necessary. A check of two or three performance characteristics is usually sufficient to determine whether a transistor needs to be replaced.
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Two of the most important parameters used for transistor testing are the transistor CURRENT GAIN (BETA) and the COLLECTOR LEAKAGE or REVERSE CURRENT (ICO). Two other tests that can be accomplished include the electrode resistance and diode measurements. You may want to review NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, for a review of transistors before continuing this section.
The Semiconductor Test Set AN/USM-206A (figure 4-17) is a rugged, field-type tester designed to test transistors and semiconductor diodes. The set will measure the beta of a transistor, the resistance appearing at the electrodes, and the reverse current of a transistor or semiconductor diode. It will also measure a shorted or open condition of a diode, the forward transconductance of a field-effect transistor, and the condition of its own batteries.
To assure that accurate and useful information is gained from the transistor tester, you should make the following preliminary checks of the tester before testing any transistors:
1. With the POLARITY switch in the OFF position, the meter pointer should indicate exactly zero. (When required, rotate the meter ZERO ADJUST KNOB on the front of the meter to fulfill this requirement.) To prevent battery drain, be sure to leave the POLARITY switch in the OFF position when measurements are not actually being made.
2. Always check the condition of the test set batteries. To make this check, disconnect the test set power cord, place the polarity switch in the PNP position, and place the function switch first to BAT. 1and then to BAT. 2. In both BAT positions, the meter pointer should move so as to indicate within the red BAT box.
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If the transistor is to be tested out of the circuit, plug it into the test jack located on the right-hand side below the meter. If the transistor is to be tested in the circuit, at least 300 ohms must exist between E- B (emitter to base), C-B (collector to base), and C-E (collector to emitter) for accurate measurement. Initial setting of the test set controls is performed as follows:
1. Set the function switch to BETA.
2. Set the POLARITY switch to PNP or NPN (depending on the type of transistor under test). 3. Set the RANGE switch to X10.
4. Adjust METER ZERO for zero meter indication (transistor disconnected). 5. The POLARITY switch should remain OFF while the transistor is connected to or disconnected
from the test set; it should then be set to PNP or NPN, as in step 2 above.
If the beta reading is less than 10, perform the following steps:
1. Reset the RANGE switch to X1 and reset the meter to zero.
2. After connecting the yellow test lead to the emitter, the green test lead to the base, and the blue test lead to the collector, plug the test probe (not shown) into the jack located at the lower right- hand corner of the test set.
3. When testing grounded equipment, unplug the 115-volt line cord and use battery operation. A beta reading is attained by multiplying the meter reading times the RANGE switch setting. Refer to the transistor characteristics book provided with the tester to determine if the reading is normal for the type of transistor under test.
Adjust the METER ZERO control for a zero meter indication. Plug the transistor to be tested into the jack, or connect the test leads to the device. Set the PNP/NPN switch to correspond with the type of transistor under test. Set the function switch to ICO and the RANGE switch to X0.1, X1.0, or X10, as specified by the transistor data book for allowable leakage. Read leakage on the bottom scale and multiply by the range setting figure as required.
Connect the in-circuit probe test leads to the transistor with the yellow lead to the emitter, the green lead to the base, and the blue lead to the collector. Set the function switch to the OHMS E-B position and read the resistance between the emitter and base electrode on the center scale of the meter marked OHMS.
To read the resistance between the collector and base and the collector and emitter, set the function switch to OHMS C-B and OHMS C-E, respectively. These in-circuit electrode resistance measurements are used to correctly interpret the in-circuit beta measurements. The accuracy of beta times 1 and 10 range is – 15 percent only when the emitter-to-base load is equal to or greater than 300 ohms.
Diode in-circuit quality measurements are made by connecting the green test lead to the cathode and the yellow test lead to the anode. Set the function switch to DIODE IN/CKT and the RANGE switch to
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times 1 position. Ensure that the meter has been properly zeroed on this scale. If the meter reads down- scale, reverse the polarity switch. If the meter reads less than midscale, the diode under test is either open or shorted. The related circuit impedance of this test is less than 25 ohms.
current bridges. These bridges are composed of capacitors, inductors, and resistors in a wide variety of combinations. These bridges operate on the principle of the Wheatstone bridge; that is, an unknown resistance is balanced against known resistances and, after the bridge has been balanced, the unknown resistance is calculated in terms of the known resistance.
The universal Impedance Bridge, Model 250DE (shown in figure 4-18) is used to measure resistance, capacitance, and inductance (RCL) values. It is also used to make other special tests, such as determining the turns ratio of transformers and capacitor quality tests. This instrument is self-contained, except for a source of line power, and has an approximate 500-hour battery life expectancy. It has its own source of 1,000-hertz bridge current with a sensitive bridge balance indicator and an adjustable source of direct current for electrolytic capacitor and resistance testing. The bridge also contains a meter with suitable ranges to test for current leakage on electrolytic capacitors.
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Figure 4-18 is a panel view of the model 250DE bridge switches, dials, controls, and connections. Refer to the figure as we briefly discuss some of the switches and dials below.
• The FUNCTION switch selects the type of bridge circuit that will measure resistance, capacitance, or inductance.
• The RANGE switch selects the multiplier for each function.
• L-R-C decade dials are a DEKASTAT decade resistor that is the main balancing element of the bridge. The setting of the dials after the bridge is balanced indicates the value of inductance, resistance, or capacitance.
• The D-Q dial is used to balance the phase of the capacitance or inductance of the bridge. The setting of the dial after the bridge is balanced indicates the value of dissipation factor (D) or storage factor (Q).
• The GEN-DET switch selects bridge generator and detector connections, ac or dc, internal or external generator. The switch also connects the internal batteries to the battery test circuit.
• The DET GAIN control adjusts the sensitivity of the ac-dc detector and turns on power to the generator.
L, R, and C terminals 1, 2, and 3 are used to connect unknown resistors, inductors, and capacitors to the bridge. Resistors and inductors are connected between terminals 1 and 2, and capacitors are connected between terminals 2 and 3. EXT BIAS terminals are normally connected with a shorting lug. They allow insertion of a dc voltage or current to bias capacitors or inductors. EXT DET connector is a BNC coaxial socket that allows an external detector to be used with the instrument. It is connected to the bridge at ALL TIMES.
EXT D-Q terminals are normally connected with a shorting lug. They allow an external rheostat to extend the range of the D-Q dial. EXT GEN terminals provide a connection to the bridge for an external generator. When the GEN-DET switch is in the AC EXT GEN position, the terminals connect an isolation transformer so that a grounded external generator can be used. When the GEN-DET switch is in the DC EXT GEN position, the terminals are connected directly to the bridge.
The model 250DE bridge has a battery supply consisting of four 1.5 V dc batteries with an expected life of 500 hours. The battery power supply should be checked before each day’s operation. Turn DET GAIN control to 1 and set GEN-DET switch to BATT. TEST (battery test). If the meter deflects beyond the BAT OK mark, the battery is good.
Resistance is usually measured with direct current for maximum accuracy. The model 250DE bridge can be used to measure resistance with alternating current, but external reactance compensation is usually required. On high-resistance ranges, care should be taken to avoid leakage across a resistor under test. Insulation with a resistance of 109 ohms, which is adequate for most purposes, will cause a measurement
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error of 1 percent if it shunts a 10-megohm resistor. Using the following steps, you will be able to measure dc resistance ONLY:
1. Turn the DET GAIN control to 2.
2. Set the FUNCTION switch to R · 1 or R · 10.
3. Set L-R-C decade dials to 3.000.
4. Connect the unknown resistor to R-L terminals 1 and 2.
5. Set the GEN-DET switch to INT DC.
6. Adjust the RANGE switch for minimum detector deflection. 7. Adjust L-R-C decade dials for null, turning the DET GAIN control clockwise to increase
sensitivity as necessary.
8. The measured resistance is the product of the L-R-C decade dial setting times the RANGE and FUNCTION switch settings.
Capacitance is measured in terms of a two-element equivalent circuit consisting of a capacitor in series with a resistor. The internal ac generator and detector of the model 250DE bridge are tuned to 1 kilohertz. Other frequencies can be used, but an external generator and detector are required. The D and Q ranges of the bridge can be extended by use of an external rheostat connected to the terminals provided. The measured capacitance is the product of the L-R-C dial setting times the setting of the RANGE switch. Using the following steps, you can make a standard capacitance measurement:
1. Turn the DET GAIN control to 1.
2. Set the FUNCTION switch to C, D · 0.1 or D · 0.01 SERIES.
3. Set L-R-C decade dials to 3.000 and D-Q dial to 0. 4. Connect the unknown capacitor to C terminals 2 and 3.
5. Set the GEN DET switch to INT 1 kHz.
6. Adjust the RANGE SWITCH for minimum detector deflection. 7. Adjust L-R-C decade dials and D-Q dial alternately for a minimum meter deflection, turning the
DET GAIN control clockwise to increase sensitivity as necessary.
8. The measured capacitance is the product of the L-R-C decade dial settings.
9. The measured dissipation factor (D) is the product of the D-Q setting times the FUNCTION switch setting.
Inductance is measured in terms of a two-element equivalent circuit consisting of an inductance either in series or in parallel with a resistance. The internal ac generator and detector of the model 250DE bridge are tuned to 1 kHz. Other frequencies can be used, but like capacitance measurements, an external
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generator and detector are required. When inductance is being measured in ac or dc, it should be realized that iron-core inductors are sensitive to current variations. Quantitative measurements of dc effects can be made by supplying current to the unknown inductor through the EXT BIAS terminal. Use the following steps to make inductance measurements:
1. Turn the DET GAIN control to 1.
2. Set the FUNCTION switch to L PARALLEL if Q is greater than 10 to L SERIES if Q is less than 10.
3. Set L-R-C decade dials to 3.000 and D-Q dial to maximum. 4. Connect the unknown inductor to the R-L terminals 1 and 2
5. Set the GEN DET switch to INT 1 kHz.
6. Adjust the RANGE switch for minimum detector deflection. 7. Adjust the L-R-C decade dials and D-Q dial alternately for a minimum meter deflection, turning
the DET GAIN control clockwise to increase sensitivity as necessary.
8. The measured inductance is the product of the L-R-C decade dial setting times the RANGE switch setting.
9. The measured storage factor (Q) is read directly from the D-Q dial, inner scale for parallel and outer scale for series inductance.
The important points of this chapter are summarized in the following paragraphs. You should be familiar with these points before continuing with your studies of test equipment.
 Observe the following safety precautions when using a multimeter:
• De-energize and discharge the circuit completely before connecting a multimeter.
• Never apply power to the circuit while you are measuring resistance with an ohmmeter.
• Connect the ammeter in series for current measurements and in parallel for voltage measurements.
• Be certain the multimeter is switched to ac before attempting to measure ac circuits.
• Observe proper dc polarity when measuring dc circuits.
• Always start with the highest voltage or current range.
• Select a final range that allows a reading near the middle of the scale.
• Adjust the "0 ohms" reading after changing resistance ranges and before making a resistance measurement.
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An ELECTRONIC DIGITAL MULTIMETER is used in sensitive electronic circuits where only extremely small amounts of energy can be extracted without disturbing the circuits under test, or causing them to be inoperative.
The DIFFERENTIAL VOLTMETER is a precision piece of test equipment used to compare an unknown voltage with an internal reference voltage and to indicate the difference in their values.
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A SEMICONDUCTOR TEST SET is used to measure the beta of a transistor, the resistance appearing at the electrodes, and the reverse current of a transistor or semiconductor diode. It also measures a shorted or open condition of a diode, the forward transconductance of a field-effect transistor, and the condition of its own batteries.
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Resistance, capacitance, and inductance are measured for precise accuracy by RCL BRIDGES. They are composed of capacitors, inductors, and resistors and operate on the principle of the Wheatstone bridge.
ANSWERS TO QUESTIONS Q1. THROUGH Q8. A-1. No external power source is required.
A-2. De-energized.
A-3. Midscale.
A-4. Polarity.
A-5. Rechargeable batteries.
A-6. Simultaneous flashing of display readouts. A-7. Light-emitting diodes.
A-8. To compare an unknown voltage with a known reference voltage and indicate the difference in their values.
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Upon completing this chapter, you should be able to:
1. Explain the theory of operation of two types of power meters.
2. Describe the purpose of the controls and indicators found on power meters.
3. Describe the proper procedure for taking power measurements for incident and reflected energy.
4. Describe the uses and purposes of the controls and indicators found on the signal generator.
5. Explain the theory of operation of a typical frequency counter.
6. Describe the uses and purposes of the controls and indicators found on the frequency counter.
7. Explain the uses and purposes of the controls and indicators found on the Huntron Tracker 2000.
8. Describe the proper procedures for troubleshooting with a logic probe.
9. Describe the proper procedures for troubleshooting using the Huntron Tracker 2000.
In chapters 3 and 4, you studied the more common pieces of test equipment. As a technician, you
will routinely use this test equipment to troubleshoot and perform maintenance on electronic equipment.
However, the equipments you will study in this chapter may or may not be found in your shop. This is
because these equipments have specific or specialized uses. Unless your rating is involved with the
equipment with which they are used, you may not have reason to use them. They are presented here so
that you will be familiar with their overall function should the need arise. The equipments you will study
devices.
As a technician, you will use a POWER METER to measure power. There are various types of
power meters, some of which are called WATTMETERS. Figure 5-1 shows the AN/URM-120 wattmeter,
which is one type of power meter commonly used in the Navy. This particular power meter measures
most likely an antenna.
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but not by being placed directly between the output of the transmitter and the load. Let's discuss the
direct-measuring power meter first; then we'll talk about an indirect-measuring power meter.
rf power from 50 to 1,000 watts at 2 MHz to 30 MHz and 10 to 500 watts at 30 MHz to 1,000 MHz.
portion of the frequency and power ranges, are provided with the wattmeter. These devices couple the rf
signal into the wattmeter and detect the signal. The coupler-detector knob projects through the top of the
wattmeter case, as shown on the AN/URM-120 wattmeter in figure 5-1.
A nameplate on the top of the POWER RANGE knob indicates the power range. The POWER
RANGE knob can be rotated 360º to the desired power range. The coupler-detector rotates 180º inside the
metal case for either forward or reverse power measurements. Also located inside the metal case are the
indicating meter and cable for interconnecting the meter to the coupler-detector. The LOCKING knob
locks the coupler-detector and POWER RANGE knobs in place.
to connect the instrument between the power source and the load. The upper and lower parts of the
wattmeter.
wattmeter in the transmission line between the load and the rf power source. To measure incident power with the wattmeter, rotate the arrow on the COUPLER-DETECTOR knob toward the load, and position
the POWER RANGE knob for peak meter reading. To measure reflected power, position the arrow toward the rf power source.
particular direction, either to (incident) or from (reflected) the load. It will be unaffected by a wave traveling in the opposite direction. A diode rectifier in the coupler rectifies the energy detected by the
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coupler. This detected rf energy is measured across a known impedance to obtain either incident or
reflected power.
Always de-energize and tag the rf power source before measuring incident power. Insert the proper coupler-detector for the rf power being measured into the wattmeter case. Remove the wire shunt (not
either at the load or the rf source. Ensure that all connections are tight.
Position the POWER RANGE knob to a value higher than the rated power of the rf source.
If the rated power to be measured is not known, place the POWER RANGE knob in the highest power position before turning on the power source.
Rotate the coupler-detector so that the arrow indicating power flow points toward the load. Turn on
the rf power source. Rotate the POWER RANGE knob to the proper range for measuring and observe the
point at which the indicating meter peaks.
Q-1. To measure incident power, you must rotate the coupler-detector of the wattmeter so that the arrow indicating power flow points toward which end of the transmission line?
coupler-detector is rotated so that the arrow points toward the rf source.
transmission line, and place the wire shunt on the meter terminals.
ratio (VSWR) of the load and the power absorbed by the load. (VSWR is covered in NEETS, Module 10,
Introduction to Wave Propagation, Transmission Lines, and Antennas.) The VSWR can be determined from a chart provided in the wattmeter technical manual, or it can be calculated (as shown in the
reflected power as measured by the wattmeter):
Where:
Pr = 0.5 watts
5-4
The example above results in a standing wave ratio expressed as 1.3 to 1. In a perfectly matched
transmission line where there is no reflected power (Pr = 0), the standing wave ratio would be 1 to 1. A
standing wave ratio of 1.5 to 1 indicates a 5-percent reflection of energy (loss) and is considered to be the
maximum allowable loss. So, our example is within allowable limits.
If the standing wave ratio is greater than 1.5 to 1, then the transmission line efficiency has decreased
and troubleshooting is necessary. An excellent discussion of the reasons for standing wave ratio increases
is presented in EIMB, Test Methods and Practices, NAVSEA 0967-LP-000-0130.
reading from the incident power reading made by the wattmeter (30 watts - 0.5 watts = 29.5 watts).
are taken while the power meter is connected in series with the transmission line. Another type of power
meter used by the Navy measures power indirectly. An example of an indirect-measuring power meter is
described in the next section.
An example of an indirect-measuring power meter is the HP-431C, shown in figure 5-2. The
controls, connectors, and indicators for the power meter are illustrated in figure 5-3. This power meter can
be operated from either an ac or dc primary power source. The ac source can be either 115 or 230 volts at
50 to 400 hertz. The dc source is a 24-volt rechargeable battery. Overall circuit operation of the power
meter is shown in the block diagram in figure 5-4.
5-7
The HP-431C power meter indirectly measures microwave frequency power by using two bridge circuitsthe detection bridge and the compensation and metering bridge. The detection bridge
heating of the thermistors in that bridge caused by microwave power. (Thermistors were covered in
chapter 2 of this module.) The compensation and metering bridge contains thermistors that are affected by
the same microwave power heating as those of the detection bridge.
An unbalance in the metering bridge produces a 10-kHz error signal. This error signal, plus the 10-
SYNCHRONOUS DETECTOR. The synchronous detector produces a dc current (Idc) that is proportional
to the 10-kHz error signal. The Idc error signal is fed back to the compensation and metering bridge, where
balance. The dc output of the synchronous detector also operates the meter circuit for a visual indication
of power.
Q-2. What condition produces the 10-kHz error signal generated by the metering bridge in the HP- 431C power meter?
dBm) full scale in the 10 MHz to 18 GHz for a 50-ohm coaxial system and 2.6 GHz to 40 GHz for a
waveguide system.
5-8
the maintenance of electronic equipment. These sources, called SIGNAL GENERATORS, are used to test
and align all types of transmitters and receivers. They are also used to troubleshoot various electronic
devices and to measure frequency.
and amplitudes with the necessary modulation for testing or measuring circuits. (Modulation was
discussed in NEETS, Module 12, Modulation Principles.) It is important that the amplitude of the signal generated by the signal generator be correct. In many signal generators, output meters are included in the
equipment to adjust and maintain the output at standard levels over wide ranges of frequencies.
When using the signal generator, you connect the output test signal into the circuit being tested. You
oscilloscopes. In many signal generators, calibrated networks of resistors, called ATTENUATORS, are
provided. You use attenuators in signal generators to regulate the voltage of the output signal. Only
regulated to avoid overloading the circuit receiving the signal.
Q-3. In signal generators, what device is used to regulate the voltage of the output signal?
There are many types of signal generators. They are classified by use and the frequency range
special types, which combine frequency ranges.
AUDIO SIGNAL GENERATORS produce stable af signals used for testing audio equipment.
range. These signal generators are used to test video amplifiers and other wideband circuits. In both audio
OSCILLATOR (or oscillators), one or more AMPLIFIERS, and an OUTPUT CONTROL.
5-9
mixing the signals of two separate rf oscillators. One is fixed in frequency, and the other is variable. The
difference between the frequencies of the two oscillators is equal to the desired audio or video frequency.
directly produced. In these signal generators, a resistance-capacitance circuit is the frequency-determining
part of the oscillator. The frequency varies when either the resistance or the capacitance is changed in value.
In other signal generators, however, the capacitance alone is often chosen as the only variable element. The change in frequency that can be produced by this method is limited, and it is usually
necessary to cover the entire range of the generator in frequency steps. This is usually accomplished by
values. The circuits in the oscillator are switched one at a time to provide the desired portion of the af
range.
one or two power amplifiers, which are coupled by means of rc networks. The output of the final power
amplifier is often coupled to the output control (attenuator) by means of an output transformer.
The output control section regulates the amplitude of the signal. A commonly used af signal
generator is the model SPN audio oscillator shown in figure 5-6. The model SPN is a programmable
over a 1-Hz to 1.3-MHz frequency range. Additionally, the equipment provides a square wave output and
means for swept operation with an external signal and can be remotely controlled via an IEEE-488 bus.
that describes its use.
5-10
sections: an OSCILLATOR CIRCUIT, a MODULATOR, and an OUTPUT CONTROL CIRCUIT. The
modulator modulates the rf signal of the oscillator. In addition, most rf generators are provided with
the generated signal. Metal shielding surrounds the unit to prevent signals from the oscillator from
affecting the circuit under test.
Q-4. Name the three main sections of a typical rf signal generator.
A block diagram of a representative rf signal generator is shown in figure 5-7. The function of the
of the generator. The type of oscillator circuit used depends on the range of frequencies for which the
generator is designed. In lower frequency rf signal generators, the oscillating circuit consists of one of a
group of coils combined with a variable capacitor. One of the coils is selected by the position of a range
selector switch that connects the coil to a capacitor to provide an inductance-capacitance circuit. The
inductive-capacitance circuit then has the correct range of resonant frequencies.
superimposed on the rf signal produced by the oscillator. The modulating signal may be provided by an
audio oscillator within the generator. This is termed INTERNAL MODULATION. It may also be derived
from an external source. This is termed EXTERNAL MODULATION. In some signal generators, either
of these two methods of modulation can be employed. In addition, a means of disabling the modulator
section is available so that the pure, unmodulated signal from the oscillator can be used when desired.
Q-5. What is the function of the modulating circuit?
The type of modulation selected depends on the application of the particular signal generator. The
modulating signal may be a sine wave, a square wave, or pulses of varying duration. In some special
generators, provision is made for pulses over a wide range of repetition rates and widths.
5-11
level meter. The output level meter provides an indication and permits control of the output voltage of the
generator. The attenuator allows you to select the amount of this output. The attenuator is made up of a
group of resistors that form a voltage-dropping circuit.
It is controlled by a control calibrated in microvolts. When the control is adjusted so that the output
meter reads unity (1.0), the reading on the attenuator control gives the exact value (no multiplication
factor) of the output in microvolts. If an output voltage at a lower value is desired, the attenuator control
is varied until the meter indicates some decimal value less than 1. This decimal is multiplied by the
attenuator reading to give the actual output in microvolts.
An rf signal generator currently in wide use by the Navy is the HP 8640B (figure 5-8). The HP
extended to 1,024 MHz by using adapters.
This completes our discussion of signal generators. The following section deals with an instrument
that measures frequencythe FREQUENCY COUNTER.
The signal generators you studied in the previous section provide signals for use in testing, aligning, and troubleshooting electronic equipment. Now, we will study the FREQUENCY COUNTER, an
instrument that measures frequencies. Frequency counters are used to measure frequencies already in existence. An example of a typical frequency counter, the model 5328A, is shown in figure 5-9.
5-12
The following description is for the model 5328A counter only. Other counters use different
techniques to derive the displayed frequency. The outputs available vary from counter to counter. For
further information on other types of counters and their uses, refer to EIMB, Test Equipment, NAVSEA 0967-LP-000-0040.
The model 5328A is a portable, solid-state electronic frequency counter. It is used to precisely
time interval average, and ratio of electronic frequency signals. This frequency counter can also provide a
1-MHz and 10-MHz output signal through the back-panel BNC.
Q-6. What frequencies are provided through the back-panel BNC?
counter section, input section, power supply, and interface bus section. Additionally, two separate (front
panel) input channels provide for time interval measurements. Each channel has an attenuator, trigger
slope selector, level control, ac or dc coupling, and an oscilloscope marker output. A third input channel
a fused-protected connection.
rate, and reset display. Also, a push-button control on the front panel allows the unit to be used in the
operational or standby mode (power applied to the crystal oven to eliminate warm-up). Rear-panel
connectors provide the gated output frequencies and an input for an external frequency standard. A
detachable front cover is used to store accessory cables and connectors.
Figures 5-10 and 5-11 show all the front- and rear-panel operating controls and indicators. Refer to
10 and 5-11, respectively.
5-14
Table 5-1.—Front Panel Controls and Connectors
NO. NAME DESCRIPTION 1 LINE switch This is a two-position switch that can be selected for STBY or OPER. When in
base to maintain a constant temperature for the crystal. When in OPER, the
power supplies normal power to the unit for operation.
2 RESET This button, when pressed, resets the display and internal count to zero. When
annunciator LEDs for a LED test.
FUNCTION This is a 10-position selector used to select the mode of operation.
channel A.
channel C.
channel A.
measurement of the signal at channel A. The number of periods over which the
average is made is determined by the RESOLUTION switch selection.
T. I. AVG A!B This position, when selected, allows the counter to make a time interval
measurement of a signal applied to channel A. The number of time intervals
switch selection made.
T. I. A!B This position, when selected, allows the counter to make a time interval
measurement. The start signal would be applied to channel A and the stop
signal applied to channel B.
3
switch, and RESET.
frequency at channel C to the frequency at channel A.
frequency at channel B to the frequency at channel A.
3
Top This position is blank and has no function.
measurements and N for totalizing and averaging measurements. This also
determines how long the main gate is open for frequency measurements.
N
1
10
.01
.1
1
10
100
10
1
.1
approximately 2 milliseconds to an indefinite hold of the display.
6 500 MHz, 50 " This is the channel C input connector.
channel is triggered by 0 to 300 mV levels with less than 20 nanoseconds delay.
5-15
Table 5-1.—Front Panel Controls and Connectors—Continued
8, 13 AC/DC These control the selection of ac or dc coupling for the input signal. When the
input amplifier control switch (No. 18) is in COM A, channel B coupling is
determined by setting of channel A coupling switch.
9, 12 ATTEN switches These select the attenuation of the input signal. The signal amplitude is reduced
by 10 in ×10 and by 100 in ×100. When the input amplifier control switch (No.
attenuation switch.
10, 11 INPUT A and B These are the input BNC connectors for channels A and B.
15, 20 LEVEL A/B These controls are used in conjunction with the ATTEN (Nos. 9, 12) to select
the voltage at which triggering will occur. With X1 attenuator selected, the
level is variable at ±2.5 volts, and ×100 at ±250 volts.
The light will blink for the associated channel when triggering is occurring.
signal is above triggering level.
negative slope of the input signal.
channels A and B in SEP (separate) position. When in COM A (common A)
channel B input circuitry. Channel B coupling and attenuation are then
determined by the channel A setting.
input.
being taken.
24 LED display This is a nine-digit LED display that shows the numerical measurement taken.
NO. NAME DESCRIPTION 25 OVFL This indicates an overflow of one or more of the most significant digits
(leftmost from the decimal point) are not displayed.
26 RMT Lights when the unit is in remote operation.
27 GATE Lights when the counter's main gate is open and a measurement is in progress.
Table 5-2.—Rear-Panel Controls and Connectors
NO. NAME FUNCTION 1 VOLTAGE
Used to select 115 or 230 volt operation.
2 LINE FUSE This requires the insertion of a 2.0 amp fuse for 115 volt or 1.0 amp fuse for
230 volt operation.
3 Input ac connector Used to connect the input ac to the unit.
is selected for measurement. In the ON position, the measurement is armed by
an input other than the input being measured.
base.
connected.
Applies a high to the output when the main gate is open.
instructions.
5-16
time period (inverse of frequency), ratio, and time interval. We will start with frequency. When the
accumulating the number of cycles, n, of the input signal that occurs over the time period, t. This is
expressed by:
The basic counter elements necessary to perform this measurement are shown in figure 5-12. The
the internal circuitry of the counter. As figure 5-12 indicates, the output of the amplifier/trigger
corresponds directly to the input signal.
crystal oscillator used for the time base element from which time, t, is derived. DECADE DIVIDERS
decade steps. This frequency can be controlled by the FREQ RESOLUTION, N switch. The time, t, is
determined by the period of this pulse train.
The heart of the counter is the MAIN GATE. When the gate is opened, pulses from the
amplifier/trigger are allowed to pass through. The opening and closing of the main gate is controlled by
the decade divider output to the main gate flip-flop. The output of the MAIN GATE is then sent to the
gate is closed.
figure 5-10, FREQUENCY RESOLUTION, N selection switch).
5-17
main gate. With the FUNCTION selector switch in the PER A position, the input signal controls the
figure 5-13). The duration of the count is one cycle or period of the input signal. When the FUNCTION
a single period. This is the basis for multiple period averaging. Period and period averaging techniques
are used to increase measurement accuracy on low-frequency measurements.
of the signal frequency at channel C or B to the signal frequency at channel A. Using the same
measure the ratio of f 2/f. The signal at frequency f can be divided into decade steps in the same manner as
multiple period averaging for higher resolution.
interval. This is done by placing the FUNCTION selector in the T.I.AVG AB position. The START input
opens the main gate, and the STOP input closes it. The START input is applied to channel A, and the
STOP input is applied to channel B. The decade divider output (clock pulses) is counted, and the display
shows the elapsed time between START and STOP signals, as shown in figure 5-15.
5-18
selection switch]). The input amplifier, main gate, and DCAs (elements of the time interval counter) must
meaningless. Clock frequencies of 1, 10, and 100 MHz, and other 10n frequencies, are preferred, since the
interval. This explains why the conventional time interval counter is presently limited to 10 nanoseconds,
a clock frequency of 100 MHz. One GHz is beyond reach, and a clock frequency of 200 MHz would
directly.
5-19
The time interval averaging technique is based on the fact that if the ±1 count error is truly random, it can be reduced by averaging a number of measurements. The words "truly random" are significant. For
frequency that is asynchronous to the instrument's clock. Under these conditions, the resolution of the
measurement is:
instrument. The 5328A can obtain 10-picoseconds resolution. Most time interval averaging has one
severe limitation: The clock period limits the minimum measurable time interval. With the FUNCTION
selector switch in the T.I. AVG AB position, synchronizers are used to remove this limitation. These
synchronizers enable the 5328A to measure intervals as short as 100 picoseconds.
asynchronous to the square wave clock. When these signals are applied to the main gate, with no
synchronizers, an output similar to the third waveform results. Since the DCAs are designed to count at
accumulated in the DCAs will be in error, as shown in the fourth waveform. This problem is alleviated by
is open. They detect and reconstruct the leading edges, making the pulses applied to the DCAs the same
duration as the clock, as shown in the fifth waveform. Occasionally, when the input time interval
repetition is synchronous with the internal clock, time interval averaging cannot be performed.
This ends our discussion on electronic frequency counters. Now, we'll study an area of electronics
5-20
components. A test instrument of value for any technician who works on digital equipment is the LOGIC
PROBE, which is an integrated circuit-testing device.
of input and output combinations involved in circuits. The two-state conditions in logic circuits are often
referred to as (1) low or high, (2) on or off, or (3) one or zero (1 or 0).
Other terminology may also be used. Any particular integrated circuit (IC) can be tested by simply
comparing it to a known good one. The LOGIC PROBE is a device that can be of great value in
troubleshooting digital integrated logic circuits.
The ideal logic probe has the following characteristics:
• It will detect a steady logic level.
• It will detect a train of logic levels.
• It will detect an open circuit.
• It will detect a high-speed transient pulse.
• It will have over-voltage protection.
• It will be small, light, and easy to handle.
digital integrated logic circuitry. It can immediately show you whether a specific point in the circuit is
low, high, open, or pulsing. Some probes have a feature that detects and displays high-speed transient
pulses as small as 10 nanoseconds wide. These probes are usually connected directly to the power supply
of the device being tested, although a few have internal batteries.
Most IC failures show up in a circuit as a constant high or low level. Because of this, logic probes
provide a quick, inexpensive way of locating the fault. They can also display the single, short-duration
pulse that is hard to detect on an oscilloscope. Figure 5-17 shows a basic logic probe.
5-21
power supply. If a separate power supply is used, the ground points of the power supply and circuit under
test should be connected together.
logic state existing in the circuit under test. The LED will provide any of four indications: (1) off, (2) dim
(about one-half brilliance), (3) bright (full brilliance), and (4) flashing on and off. The LED is normally in