1. Circuit Measurement

This chapter will acquaint you with the basics of circuit measurement and some of the devices used to measure voltage, current, resistance, power, and frequency. There are other quantities involved in electrical circuits, such as capacitance, inductance, impedance, true power, and effective power. It is possible to measure any circuit quantity once you are able to select and use the proper circuit measuring device. You will NOT know all there is to know about circuit measuring devices (test equipment) when you finish this chapter. That is beyond the scope of this chapter and even beyond the scope of this training series. However, more information on test equipment is provided in another portion of this training series.

A question which you might ask before starting this chapter is "Why do I need to know about circuit measurement?"

If you intend to accomplish anything in the field of electricity and electronics, you must be aware of the forces acting inside the circuits with which you work. Modules 1 and 2 of this training series introduced you to the physics involved in the study of electricity and to the fundamental concepts of direct and alternating current. The terms voltage (volts), current (amperes), and resistance (ohms) were explained, as well as the various circuit elements; e.g., resistors, capacitors, inductors, transformers, and batteries.

In explaining these terms and elements to you, schematic symbols and schematic diagrams were used. In many of these schematic diagrams, a meter was represented in the circuit, as shown in Figure 1.

As you recall, the current in a dc circuit with 6 volts across a 6-ohm resistor is 1 ampere. The "A" in Figure 1 is the symbol for an ammeter. An ammeter is a device that measures current. The name "ammeter" comes from the fact that it is a meter used to measure current (in amperes), and thus is called an AMpere METER, or AMMETER. The ammeter in Figure 1 is measuring a current of 1 ampere with the voltage and resistance values given.

Figure 1. A simple representative circuit.

In the discussion and explanation of electrical and electronic circuits, the quantities in the circuit (voltage, current, and resistance) are important. If you can measure the electrical quantities in a circuit, it is easier to understand what is happening in that circuit. This is especially true when you are troubleshooting defective circuits. By measuring the voltage, current, capacitance, inductance, impedance, and resistance in a circuit, you can determine why the circuit is not doing what it is supposed to do. For instance, you can determine why a radio is not receiving or transmitting, why your automobile will not start, or why an electric oven is not working. Measurement will also assist you in determining why an electrical component (resistor, capacitor, inductor) is not doing its job. The measurement of the electrical parameters quantities in a circuit is an essential part of working on electrical and electronic equipment.

2. Analog Meters in DC Mode

Circuit measurement is used to monitor the operation of an electrical or electronic device, or to determine the reason a device is not operating properly. Since electricity is invisible, you must use some sort of device to determine what is happening in an electrical circuit. Various devices called test equipment are used to measure electrical quantities. The most common types of test equipment use some kind of metering device.

2.1. In-Circuit Meters

Some electrical and electronic devices have meters built into them. These meters are known as in- circuit meters. An in-circuit meter is used to monitor the operation of the device in which it is installed. Some examples of in-circuit meters are the generator or alternator meter on some automobiles; the voltage, current, and frequency meters on control panels at electrical power plants; and the electrical power meter that records the amount of electricity used in a building.

It is not practical to install an in-circuit meter in every circuit. However, it is possible to install an in- circuit meter in each critical or representative circuit to monitor the operation of a piece of electrical equipment. A mere glance at or scan of the in-circuit meters on a control board is often sufficient to tell if the equipment is working properly.

While an in-circuit meter will indicate that an electrical device is not functioning properly, the cause of the malfunction is determined by troubleshooting. Troubleshooting is the process of locating and repairing faults in equipment after they have occurred. Since troubleshooting is covered elsewhere in this training series, it will be mentioned here only as it applies to circuit measurement.

2.2. Out-Of-Circuit Meters

In troubleshooting, it is usually necessary to use a meter that can be connected to the electrical or electronic equipment at various testing points and may be moved from one piece of equipment to another. These meters are generally portable and self-contained, and are known as out-of-circuit meters.

Out-of-circuit meters are more versatile than in-circuit meters in that the out-of-circuit meter can be used wherever you wish to connect it. Therefore, the out-of-circuit meter is more valuable in locating the cause of a malfunction in a device.

3. Basic Meter Movements

The meter movement is, as the name implies, the part of a meter that moves. A meter movement converts electrical energy into mechanical energy. There are many different types of meter movements. The first one you will learn about is based upon a principle with which you are already familiar. That principle is the interaction of magnetic fields.

3.1. Compass and Conducting Wire

You know that an electrical conductor in which current flows has a magnetic field generated around it. If a compass is placed close to the conductor, the compass will react to that magnetic field (Figure 2).

Figure 2. Compass and conductor with direct current.

If the battery is disconnected, the north end of the compass needle will point to magnetic north, as illustrated in figure Figure 2(A) by the broken-line compass needle pointing to the right. When the battery is connected, current flows through the circuit and the compass needle aligns itself with the magnetic field of the conductor, as indicated by the solid compass needle. The strength of the magnetic field created around the conductor is dependent upon the amount of current.

In figure Figure 2(A), the resistance in the circuit is 6 ohms. With the 6-volt battery shown, current in the circuit is 1 ampere. In figure Figure 2(B), the resistance has been changed to 12 ohms. With the 6-volt battery shown, current in the circuit is 1/2 or .5 ampere. The magnetic field around the conductor in figure Figure 2(B) is weaker than the magnetic field around the conductor in Figure 2(A). The compass needle in figure Figure 2(B) does not move as far from magnetic north.

If the direction of the current is reversed, the compass needle will move in the opposite direction because the polarity of the magnetic field has reversed.

In figure Figure 2(C), the battery connections are reversed and the compass needle now moves in the opposite direction.

You can construct a crude meter to measure current by using a compass and a piece of paper. By using resistors of known values, and marking the paper to indicate a numerical value, as in Figure 3, you have a device that measures current.

Figure 3. A simple meter from a compass.

This is, in fact, the way the first GALVANOMETERS were developed. A galvanometer is an instrument that measures small amounts of current and is based on the electromagnetic principle. A galvanometer can also use the principles of electrodynamics, which will be covered later in this topic.

The meter in Figure 3 is not very practical for electrical measurement. The amount the compass needle swings depends upon the closeness of the compass to the conductor carrying the current, the direction of the conductor in relation to magnetic north, and the influence of other magnetic fields. In addition, very small amounts of current will not overcome the magnetic field of the Earth and the needle will not move.

conductor?

3.2. Permanent-Magnet Moving-Coil Movement

The compass and conducting wire meter can be considered a fixed-conductor moving-magnet device since the compass is, in reality, a magnet that is allowed to move. The basic principle of this device is the interaction of magnetic fields-the field of the compass (a permanent magnet) and the field around the conductor (a simple electromagnet).

A permanent-magnet moving-coil movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as in Figure 4. When the switch is closed, causing current through the coil, the coil will have a magnetic field which will react to the magnetic field of the permanent magnet. The bottom portion of the coil in Figure 4 will be the north pole of this electromagnet. Since opposite poles attract, the coil will move to the position shown in Figure 5.

Figure 4. A movable coil in a magnetic field (no current).
Figure 5. A movable coil in a magnetic field (current).

The coil of wire is wound on an aluminum frame, or bobbin, and the bobbin is supported by jeweled bearings which allow it to move freely. This is shown in Figure 6.

Figure 6. A basic coil arrangement.

To use this permanent-magnet moving-coil device as a meter, two problems must be solved. First, a way must be found to return the coil to its original position when there is no current through the coil. Second, a method is needed to indicate the amount of coil movement.

The first problem is solved by the use of hairsprings attached to each end of the coil as shown in Figure 7. These hairsprings can also be used to make the electrical connections to the coil. With the use of hairsprings, the coil will return to its initial position when there is no current. The springs will also tend to resist the movement of the coil when there is current through the coil. When the attraction between the magnetic fields (from the permanent magnet and the coil) is exactly equal to the force of the hairsprings, the coil will stop moving toward the magnet.

Figure 7. Coil and hairsprings.

As the current through the coil increases, the magnetic field generated around the coil increases. The stronger the magnetic field around the coil, the farther the coil will move. This is a good basis for a meter.

But, how will you know how far the coil moves? If a pointer is attached to the coil and extended out to a scale, the pointer will move as the coil moves, and the scale can be marked to indicate the amount of current through the coil. This is shown in Figure 8.

Figure 8. A complete coil.

Two other features are used to increase the accuracy and efficiency of this meter movement. First, an iron core is placed inside the coil to concentrate the magnetic fields. Second, curved pole pieces are attached to the magnet to ensure that the turning force on the coil increases steadily as the current increases.

The meter movement as it appears when fully assembled is shown in Figure 9.

Figure 9. Assembled meter movement.

This permanent-magnet moving-coil meter movement is the basic movement in most measuring instruments. It is commonly called the dâ€™Arsonval movement because it was first employed by the Frenchman dâ€™Arsonval in making electrical measurements. Figure 10 is a view of the dâ€™Arsonval meter movement used in a meter.

Figure 10. A meter using d’Arsonval movement.

3.3. Compass and Alternating Current

Up to this point, only direct current examples have been used. What happens with the use of alternating current? Figure 11 shows a magnet close to a conductor carrying alternating current at a frequency of 1 hertz.

Figure 11. Compass and conductor with ac.

The compass needle will swing toward the east part of the compass (down) as the current goes positive, as represented in Figure 11(A). (The sine wave of the current is shown in the lower portion of the figure to help you visualize the current in the conductor.)

In Figure 11(B), the current returns to zero, and the compass needle returns to magnetic north (right). As the current goes negative, as in Figure 11(C), the compass needle swings toward the west portion of the compass (up). The compass needle returns to magnetic north as the current returns to zero as shown in Figure 11(D).

This cycle of the current going positive and negative and the compass swinging back and forth will continue as long as there is alternating current in the conductor.

If the frequency of the alternating current is increased, the compass needle will swing back and forth at a higher rate (faster). At a high enough frequency, the compass needle will not swing back and forth, but simply vibrate around the magnetic north position. This happens because the needle cannot react fast enough to the very rapid current alternations. The compass (a simple meter) will indicate the average value of the alternating current (remember the average value of a sine wave is zero) by vibrating around the zero point on the meter (magnetic north). This is not of much use if you wish to know the value of the alternating current. Some device, such as a rectifier, is needed to allow the compass to react to the alternating current in a way that can be useful in measuring the current.

3.4. Damping

A problem that is created by the use of a rectifier and dâ€™Arsonval meter movement is that the pointer will vibrate (oscillate) around the average value indication. This oscillation will make the meter difficult to read.

The process of "smoothing out" the oscillation of the pointer is known as DAMPING. There are two basic techniques used to damp the pointer of a dâ€™Arsonval meter movement.

The first method of damping comes from the dâ€™Arsonval meter movement itself. In the dâ€™Arsonval meter movement, current through the coil causes the coil to move in the magnetic field of the permanent magnet. This movement of the coil (conductor) through a magnetic field causes a current to be induced in the coil opposite to the current that caused the movement of the coil. This induced current will act to damp oscillations. In addition to this method of damping, which comes from the movement itself, most meters use a second method of damping.

The second method of damping used in most meter movements is an airtight chamber containing a vane (like a windmill vane) attached to the coil (Figure 12).

Figure 12. A typical meter damping system.

As the coil moves, the vane moves within the airtight chamber. The action of the vane against the air in the chamber opposes the coil movement and damps the oscillations.

An additional advantage of damping a meter movement is that the damping systems will act to slow down the coil and help keep the pointer from overshooting its rest position when the current through the meter is removed.

3.5. Indicating Alternating Current

Another problem encountered in measuring ac is that the meter movement reacts to the average value of the ac. The value used when working with ac is the effective value (rms value). Therefore, a different scale is used on an ac meter. The scale is marked with the effective value, even though it is the average value to which the meter is reacting. That is why an ac meter will give an incorrect reading if used to measure dc.

3.6. Other Meter Movements

The dâ€™Arsonval meter movement (permanent-magnet moving-coil) is only one type of meter movement. Other types of meter movements can be used for either ac or dc measurement without the use of a rectifier.

When galvanometers were mentioned earlier in this topic, it was stated that they could be either electromagnetic or electrodynamic. Electrodynamic meter movements will be discussed at this point.

3.7. Electrodynamic Meter Movement

An electrodynamic movement uses the same basic operating principle as the basic moving-coil meter movement, except that the permanent magnet is replaced by fixed coils (Figure 13). A moving coil, to which the meter pointer is attached, is suspended between two field coils and connected in series with these coils. The three coils (two field coils and the moving coil) are connected in series across the meter terminals so that the same current flows through each.

Figure 13. Electrodynamic meter movement.

Current flow in either direction through the three coils causes a magnetic field to exist between the field coils. The current in the moving coil causes it to act as a magnet and exert a turning force against a spring. If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same time, and the turning force continues in the original direction. Since reversing the current direction does not reverse the turning force, this type of meter can be used to measure both ac and dc if the scale is changed. While some voltmeters and ammeters use the electrodynamic principle of operation, the most important application is in the wattmeter. The wattmeter, along with the voltmeter and the ammeter, will be discussed later in this topic.

3.8. Moving-Vane Meter Movements

The moving-vane meter movement (sometimes called the moving-iron movement) is the most commonly used movement for ac meters. The moving-vane meter operates on the principle of magnetic repulsion between like poles (Figure 14). The current to be measured flows through a coil, producing a magnetic field which is proportional to the strength of the current. Suspended in this field are two iron vanes. One is in a fixed position, the other, attached to the meter pointer, is movable. The magnetic field magnetizes these iron vanes with the same polarity regardless of the direction of current flow in the coil. Since like poles repel, the movable vane pulls away from the fixed vane, moving the meter pointer. This motion exerts a turning force against the spring. The distance the vane will move against the force of the spring depends on the strength of the magnetic field, which in turn depends on the coil current.

Figure 14. Moving-vane meter movement.

These meters are generally used at 60-hertz ac, but may be used at other ac frequencies. By changing the meter scale to indicate dc values rather than ac rms values, moving-vane meters will measure dc current and dc voltage. This is not recommended due to the residual magnetism left in the vanes, which will result in an error in the instrument.

One of the major disadvantages of this type of meter movement occurs due to the high reluctance of the magnetic circuit. This causes the meter to require much more power than the Dâ€™Arsonval meter to produce a full scale deflection, thereby reducing the meters sensitivity.

3.9. Hot-Wire and Thermocouple Meter Movements

Hot-wire and thermocouple meter movements both use the heating effect of current flowing through a resistance to cause meter deflection. Each uses this effect in a different manner. Since their operation depends only on the heating effect of current flow, they may be used to measure both direct current and alternating current of any frequency on a single scale.

The hot-wire meter movement deflection depends on the expansion of a high-resistance wire caused by the heating effect of the wire itself as current flows through it. (See Figure 15.) A resistance wire is stretched taut between the two meter terminals, with a thread attached at a right angle to the center of the wire. A spring connected to the opposite end of the thread exerts a constant tension on the resistance wire. Current flow heats the wire, causing it to expand. This motion is transferred to the meter pointer through the thread and a pivot.

Figure 15. Hot-wire meter movement.

The thermocouple meter consists of a resistance wire across the meter terminals, which heats in proportion to the amount of current. (See Figure 16.) Attached to this wire is a small thermocouple junction of two unlike metal wires, which connect across a very sensitive dc meter movement (usually a dâ€™Arsonval meter movement). As the current being measured heats the heating resistor, a small current (through the thermocouple wires and the meter movement) is generated by the thermocouple junction. The current being measured flows through only the resistance wire, not through the meter movement itself. The pointer turns in proportion to the amount of heat generated by the resistance wire.

Figure 16. A thermocouple meter.

3.10. Ammeters

An ammeter is a device that measures current. Since all meter movements have resistance, a resistor will be used to represent a meter in the following explanations. Direct current circuits will be used for simplicity of explanation.

3.11. Ammeter Connected in Series

In Figure 17(A), R1 and R2 are in series. The total circuit resistance is R2 + R2 and total circuit current flows through both resistors. In Figure 17(B), R1 and R2 are in parallel. The total circuit resistance is

and total circuit current does not flow through either resistor.

Figure 17. A series and a parallel circuit.

If R1 represents an ammeter, the only way in which total circuit current will flow through the meter (and thus be measured) is to have the meter (R1) in series with the circuit load (R2), as shown in Figure 17(A).

In complex electrical circuits, you are not always concerned with total circuit current. You may be interested in the current through a particular component or group of components. In any case, an ammeter is always connected in series with the circuit you wish to test. Figure 18 shows various circuit arrangements with the ammeter(s) properly connected for measuring current in various portions of the circuit.

Figure 18. Proper ammeter connections.

Connecting an ammeter in parallel would give you not only an incorrect measurement, it would also damage the ammeter, because too much current would pass through the meter.

3.12. Effect on Circuit Being Measured

The meter affects the circuit resistance and the circuit current. If R1 is removed from the circuit in figure Figure 17(A), the total circuit resistance is R2. Circuit current

with the meter (R1) in the circuit, circuit resistance is R1 + R2 and circuit current

The smaller the resistance of the meter (R1), the less it will affect the circuit being measured. (R1 represents the total resistance of the meter; not just the resistance of the meter movement.)

3.13. Ammeter Sensitivity

Ammeter sensitivity is the amount of current necessary to cause full scale deflection (maximum reading) of the ammeter. The smaller the amount of current, the more "sensitive" the ammeter. For example, an ammeter with a maximum current reading of 1 milliampere would have a sensitivity of 1 milliampere, and be more sensitive than an ammeter with a maximum reading of 1 ampere and a sensitivity of 1 ampere. Sensitivity can be given for a meter movement, but the term "ammeter sensitivity" usually refers to the entire ammeter and not just the meter movement. An ammeter consists of more than just the meter movement.

3.14. Ammeter Ranges

If you have a meter movement with a sensitivity of 1 milliampere, you can connect it in series with a circuit and measure currents up to 1 milliampere. But what do you do to measure currents over 1 milliampere?

To answer this question, look at Figure 19. In Figure 19(A), 10 volts are applied to two resistors in parallel. R1 is a 10-ohm resistor and R2 is a 1.11-ohm resistor. Since voltage in parallel branches is equal-

Figure 19. Current in a parallel circuit.

In Figure 19(B), the voltage is increased to 100 volts. Now,

In Figure 19(C), the voltage is reduced from 100 volts to 50 volts. In this case,

Notice that the relationship (ratio) of IR1 and IR2 remains the same. IR2 is nine times greater than IR1 and IR1 has one-tenth of the total current.

If R1 is replaced by a meter movement that has 10 ohms of resistance and a sensitivity of 10 amperes, the reading of the meter will represent one-tenth of the current in the circuit and R2 will carry nine-tenths of the current. R2 is a SHUNT resistor because it diverts, or shunts, a portion of the current from the meter movement (R1). By this method, a 10-ampere meter movement will measure current up to 100 amperes. By adding a second scale to the face of the meter, the current can be read directly.

By adding several shunt resistors in the meter case, with a switch to select the desired resistor, the ammeter will be capable of measuring several different maximum current readings or ranges.

Most meter movements in use today have sensitivities of from 5 microamperes to 1 milliampere. Figure 20 shows the circuit of meter switched to higher ranges, the shunt an ammeter that uses a meter movement with a sensitivity of 100 microamperes and shunt resistors. This ammeter has five ranges (100 microamperes; 1, 10, and 100 milliamperes; 1 ampere) selected by a switch.

Figure 20. An ammeter with internal shunt resistors.

By adding several shunt resistors in the meter case, with a switch to select the desired resistor, the ammeter will be capable of measuring several different maximum current readings or ranges.

Most meter movements in use today have sensitivities of from 5 microamperes to 1 milliampere. Figure 20 shows the circuit of meter switched to higher ranges, the shunt an ammeter that uses a meter movement with a sensitivity of 100 microamperes and shunt resistors. This ammeter has five ranges (100 microamperes; 1, 10, and 100 milliamperes; 1 ampere) selected by a switch.

With the switch in the 100 microampere position, all the current being measured will go through the meter movement. None of the current will go through any of the shunt resistors. If the ammeter is switched to the 1 milliampere position, the current being measured will have parallel paths of the meter movement and all the shunt resistors (R1, R2, R3, and R4). Now, only a portion of the current will go through the meter movement and the rest of the current will go through the shunt resistors. When the meter is switched to the 10-milliampere position (as shown in Figure 20), only resistors R1, R2, and R3 shunt the meter. Since the resistance of the shunting resistance is less than with R4 in the circuit (as was the case in the 1-milliampere position), more current will go through the shunt resistors and less current will go through the meter movement. As the resistance decreases and more current goes through the shunt resistors. As long as the current to be measured does not exceed the range selected, the meter movement will never have more than 100 microamperes of current through it.

Shunt resistors are made with close tolerances. That means if a shunt resistor is selected with a resistance of .01 ohms (as R1 in Figure 20), the actual resistance of that shunt resistor will not vary from that value by more than 1 percent. Since a shunt resistor is used to protect a meter movement and to allow accurate measurement, it is important that the resistance of the shunt resistor is known very accurately.

Figure 20 represents an ammeter with internal shunts. The shunt resistors are inside the meter case and selected by a switch. For limited current ranges (below 50 amperes), internal shunts are most often employed.

For higher current ranges (above 50 amperes) ammeters that use external shunts are used. The external shunt resistor serves the same purpose as the internal shunt resistor. The external shunt is connected in series with the circuit to be measured and in parallel with the ammeter. This shunts (bypasses) the ammeter so only a portion of the current goes through the meter. Each external shunt will be marked with the maximum current value that the ammeter will measure when that shunt is used. Figure 21 shows an ammeter that is designed to use external shunts and a dâ€™Arsonval meter movement. Figure 21(A) shows the internal construction of the meter and the way in which the external shunt is connected to the meter and to the circuit being measured. Figure 21(C) shows some typical external shunts.

Figure 21. An ammeter employing the d’Arsonval principle and external shunts.

A shunt resistor is nothing more than a resistor in parallel with the meter movement. To measure high currents, very small resistance shunts are used so the majority of the current will go through the shunt. Since the total resistance of a parallel circuit (the meter movement and shunt resistor) is always less than the resistance of the smallest resistor, as an ammeterâ€™s range is increased, its resistance decreases. This is important because the load resistance of high-current circuits is smaller than the load resistance of low-current circuits. To obtain accurate measurements, it is necessary that the ammeter resistance be much less than the load resistance, since the ammeter is connected in series with the load.

3.14.1. Range Selection

Part of the correct use of an ammeter is the proper use of the range selection switch. If the current to be measured is larger than the scale of the meter selected, the meter movement will have excessive current and will be damaged. Therefore, it is important to always start with the highest range when you use an ammeter. If the current can be measured on several ranges, use the range that results in a reading near the middle of the scale. Figure 22 illustrates these points.

Figure 22. Reading an ammeter at various ranges.

Figure 22(A) shows the initial reading of a circuit. The highest range (250 milliamperes) has been selected and the meter indication is very small. It would be difficult to properly interpret this reading with any degree of accuracy. Figure 22(B) shows the second reading, with the next largest range (50 milliamperes). The meter deflection is a little greater. It is possible to interpret this reading as 5 milliamperes. Since this approximation of the current is less than the next range, the meter is switched as shown in Figure 22(C). The range of the meter is now 10 milliamperes and it is possible to read the meter indication of 5 milliamperes with the greatest degree of accuracy. Since the current indicated is equal to (or greater than) the next range of the ammeter (5 milliamperes), the meter should NOT be switched to the next range.

3.15. Ammeter Safety Precautions

When you use an ammeter, certain precautions must be observed to prevent injury to yourself or others and to prevent damage to the ammeter or the equipment on which you are working. The following list contains the MINIMUM precautions to observe when using an ammeter.

• Ammeters must always be connected in series with the circuit under test.

• Deenergize and discharge the circuit completely before you connect or disconnect the ammeter.

• In dc ammeters, observe the proper circuit polarity to prevent the meter from being damaged.

• Never use a dc ammeter to measure ac.

• Observe the general safety precautions of electrical and electronic devices.

3.16. Voltmeters

All the meter movements discussed so far react to current, and you have been shown how ammeters are constructed from those meter movements. It is often necessary to measure circuit properties other than current. Voltage measurement, for example, is accomplished with a VOLTMETER.

3.17. Voltmeters Connected in Parallel

While ammeters are always connected in series, voltmeters are always connected in parallel. Figure 23 (and the following figures) use resistors to represent the voltmeter movement. Since a meter movement can be considered as a resistor, the concepts illustrated are true for voltmeters as well as resistors. For simplicity, dc circuits are shown, but the principles apply to both ac and dc voltmeters.

Figure 23. Current and voltage in series and parallel circuits.

Figure 23(A) shows two resistors connected in parallel. Notice that the voltage across both resistors is equal. In Figure 23(B) the same resistors are connected in series. In this case, the voltage across the resistors is not equal. If R1 represents a voltmeter, the only way in which it can be connected to measure the voltage of R2 is in parallel with R2, as in Figure 23(A).

A voltmeter has an effect on the circuit being measured. This is called LOADING the circuit. Figure 24 illustrates the loading effect and the way in which the loading effect is kept to a minimum.

In Figure 24(A), a series circuit is shown with R1 equaling 15 ohms and R2 equaling 10 ohms. The voltage across R2 (ER2) equals 10 volts. If a meter (represented by R3) with a resistance of 10 ohms is connected in parallel with R2, as in Figure 24(B), the combined resistance of R2 and R3 (Rn) is equal to 5 ohms. The voltage across R2 and R3 is now 6.25 volts, and that is what the meter will indicate. Notice that the voltage across R1 and the circuit current have both increased. The addition of the meter (R3) has loaded the circuit.

In Figure 24(C), the low-resistance meter (R3) is replaced by a higher resistance meter (R4) with a resistance of 10 kilohms. The combined resistance of R2 and R4 (Rn) is equal to 9.99 ohms. The voltage across R2 and R4 is now 9.99 volts, the value that will be indicated on the meter. This is much closer to the voltage across R2, with no meter (R3 or R4) in the circuit. Notice that the voltage across R, and the circuit current in Figure 24(C) are much closer to the values in Figure 24(A). The current (IR4) through the meter (R4) in Figure 24(C) is also very small compared to the current (IR2) through R2. In Figure 24(C) the meter (R4) has much less effect on the circuit and does not load the circuit as much. Therefore, a voltmeter should have a high resistance compared to the circuit being measured, to minimize the loading effect.

3.19. Making a Voltmeter From a Current Sensitive Meter Movement

The meter movements discussed earlier in this chapter have all reacted to current. Various ways have been shown in which these movements can be used in ammeters. If the current and resistance are known, the voltage can be calculated by the formula E = IR. A meter movement has a known resistance, so as the movement reacts to the current, the voltage can be indicated on the scale of the meter.

In Figure 25(A), a voltmeter (represented by R2) connected across a 10-ohm resistor with 10 volts applied. The current through the voltmeter (R2) is .1 milliamperes. In Figure 25(B), the voltage is increased to 100 volts. Now, the current through the voltmeter (R2) is 1 milliampere. The voltage has increased by a factor of 10 and so has the current. This illustrates that the current through the meter is proportional to the voltage being measured.

Figure 25. Current and voltage in parallel circuit.

3.20. Sensitivity of Voltmeters

Voltmeter sensitivity is expressed in ohms per volt (â„¦/V). It is the resistance of the voltmeter at the full-scale reading in volts. Since the voltmeterâ€™s resistance does not change with the position of the pointer, the total resistance of the meter is the sensitivity multiplied by the full-scale voltage reading. The higher the sensitivity of a voltmeter, the higher the voltmeterâ€™s resistance. Since high resistance voltmeters have less loading effect on circuits, a high-sensitivity meter will provide a more accurate voltage measurement.

To determine the sensitivity of a meter movement, you need only to divide 1 by the amount of current needed to cause full-scale deflection of the meter movement. The manufacturer usually marks meter movements with the amount of current needed for full-scale deflection and the resistance of the meter. With these figures, you can calculate the sensitivity

and the full-scale voltage reading full-scale current (full-scale current Ã— resistance).

For example, if a meter has a full-scale current of 50Î¼A and a resistance of 960â„¦, the sensitivity could be calculated as:

The full-scale voltage reading would be calculated as:

Full-scale voltage reading = full-scale current Ã— resistance

Full-scale voltage reading = 50Î¼A Ã— 960â„¦

3.21. Ranges

Table 1 shows the figures for most meter movements in use today.

 CURRENT TO DEFLECT FULL SCALE RESISTANCE SENSITIVITY VOLTAGE FULL SCALE 1 mA 100 Î© 1 kÎ©/VOLT .1 V 50 Î¼A 960 Î© 20 kÎ©/VOLT .048 V 5 Î¼A 5750 Î© 200 kÎ©/VOLT .029 V

Notice that the meter movements shown in Table 1 will indicate .029 volts to .1 volt at full scale, and the sensitivity ranges from 1000 ohms per volt to 200,000 ohms per volt. The higher sensitivity meters indicate smaller amounts of voltage. Since most voltage measurements involve voltage larger than .1 volt, a method must be used to extend the voltage reading.

Figure 26 illustrates the method of increasing the voltage range of a voltmeter.

Figure 26. A voltmeter and a range resistor.

In Figure 26(A), a voltmeter with a range of 10 volts and a resistance of 1 kilohm (R2) is connected in parallel to resistor R1. The meter has .01 ampere of current (full-scale deflection) and indicates 10 volts. In Figure 26(B), the voltage has been increased to 100 volts. This is more than the meter can measure. A 9 kilohm resistor (R3) is connected in series with the meter (R2). The meter (R2) now has .01 ampere of current (full-scale deflection). But since R3 has increased the voltage capability of the meter, the meter indicates 100 volts. R3 has changed the range of the meter.

Voltmeters can be constructed with several ranges by the use of a switch and internal resistors. Figure 27 shows a voltmeter with a meter movement of 100 ohms and 1 milliampere full-scale deflection with 5 ranges of voltage through the use of a switch. In this way a voltmeter can be used to measure several different ranges of voltage.

Figure 27. A voltmeter with internal range resistors.

The current through the meter movement is determined by the voltage being measured. If the voltage measured is higher than the range of the voltmeter, excess current will flow through the meter movement and the meter will be damaged. Therefore, you should always start with the highest range of a voltmeter and switch the ranges until a reading is obtained near the center of the scale. Figure 28 illustrates these points.

Figure 28. Reading a voltmeter at various ranges.

In Figure 28(A) the meter is in the 1000-volt range. The pointer is barely above the 0 position. It is not possible to accurately read this voltage. In Figure 28(B) the meter is switched to the 250 volt range. From the pointer position it is possible to approximate the voltage as 20 volts. Since this is well below the next range, the meter is switched, as in Figure 28(C). With the meter in the 50-volt range, it is possible to read the voltage as 22 volts. Since this is more than the next range of the meter (10 volts), the meter would not be switched to the next (lower) scale.

3.22. Electrostatic Meter Movement

The final meter movement covered in this chapter is the ELECTROSTATIC METER MOVEMENT. The other meter movements you have studied all react to current, the electrostatic meter movement reacts to voltage.

The mechanism is based on the repulsion of like charges on the plates of a capacitor. The electrostatic meter movement is actually a large variable capacitor in which one set of plates is allowed to move. The movement of the plates is opposed by a spring attached to the plates. A pointer that indicates the value of the voltage is attached to these movable plates. As the voltage increases, the plates develop more torque. To develop sufficient torque, the plates must be large and closely spaced. A very high voltage is necessary to provide movement, therefore, electrostatic voltmeters are used only for HIGH VOLTAGE measurement.

3.23. Voltmeter Safety Precautions

Just as with ammeters, voltmeters require safety precautions to prevent injury to personnel and damage to the voltmeter or equipment. The following is a list of the MINIMUM safety precautions for using a voltmeter.

• Always connect voltmeters in parallel.

• Deenergize and discharge the circuit completely before connecting or disconnecting the voltmeter.

• In dc voltmeters, observe the proper circuit polarity to prevent damage to the meter.

• Never use a dc voltmeter to measure ac voltage.

• Observe the general safety precautions of electrical and electronic devices.

3.24. Ohmmeters

The two instruments most commonly used to check the continuity (a complete circuit), or to measure the resistance of a circuit or circuit element, are the OHMMETER and the MEGGER (megohm meter). The ohmmeter is widely used to measure resistance and 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 between a wire and the outer surface of the insulation, and insulation resistance of cables and insulators. The range of a megger may extend to more than 1,000 megohms.

The ohmmeter consists of a dc ammeter, with a few added features. The added features are:

1. A dc source of potential (usually a 3-volt battery) 2. One or more resistors (one of which is variable) 3. A simple ohmmeter circuit is shown in Figure 29.

Figure 29. A simple ohmmeter circuit.

3.25. Using the Ohmmeter

After the ohmmeter is adjusted for zero reading, it is ready to be connected in a circuit to measure resistance. A typical circuit and ohmmeter arrangement is shown in Figure 30.

Figure 30. Measuring circuit resistance with an ohmmeter.

The power switch of the circuit to be measured should always be in the OFF position. This prevents the source voltage of the circuit from being applied across the meter, which could cause damage to the meter movement.

The test leads of the ohmmeter are connected in series with the circuit to be measured (Figure 30). This causes the current produced by the 3-volt battery of the meter to flow through the circuit being tested. Assume that the meter test leads are connected at points a and b of Figure 30. The amount of current that flows through the meter coil will depend on the total resistance of resistors R1 and R2, and the resistance of the meter. Since the meter has been preadjusted (zeroed), the amount of coil movement now depends solely on the resistance of R1 and R2. The inclusion of R1 and R2 raises the total series resistance, decreasing the current, and thus decreasing the pointer deflection. The pointer will now come to rest at a scale figure indicating the combined resistance of R1 and R2. If R1 or R2, or both, were replaced with a resistor(s) having a larger value, the current flow in the moving coil of the meter would be decreased further. The deflection would also be further decreased, and the scale indication would read a still higher circuit resistance. Movement of the moving coil is proportional to the amount of current flow.

3.26. Ohmmeter Ranges

The amount of circuit resistance to be measured may vary over a wide range. In some cases it may be only a few ohms, and in others it may be as great as 1,000,000 ohms (1 megohm). To enable the meter to indicate any value being measured, with the least error, scale multiplication features are used in most ohmmeters. For example, a typical meter will have four test lead jacks-COMMON, R Ã— 1, R Ã— 10, and R Ã— 100. The jack marked COMMON is connected internally through the battery to one side of the moving coil of the ohmmeter. The jacks marked R Ã— 1, R Ã— 10, and R Ã— 100 are connected to three different size resistors located within the ohmmeter. This is shown in Figure 31.

Figure 31. An ohmmeter with multiplication jacks.

Some ohmmeters are equipped with a selector switch for selecting the multiplication scale desired, so only two test lead jacks are necessary. Other meters have a separate jack for each range, as shown in Figure 31. The range to be used in measuring any particular unknown resistance (RX in Figure 31) depends on the approximate value of the unknown resistance. For instance, assume the ohmmeter in Figure 31 is calibrated in divisions from 0 to 1,000. If RX is greater than 1,000 ohms, and the R x 1 range is being used, the ohmmeter cannot measure it. This occurs because the combined series resistance of resistor R Ã— 1 and RX is too great to allow sufficient battery current to flow to deflect the pointer away from infinity (âˆž). (Infinity is a quantity larger than the largest quantity you can measure.) The test lead would have to be plugged into the next range, R Ã— 10. With this done, assume the pointer deflects to indicate 375 ohms. This would indicate that RX has 375 ohms Ã— 10, or 3,750 ohms resistance. The change of range caused the deflection because resistor R Ã— 10 has about 1/10 the resistance of resistor R Ã— 1. Thus, selecting the smaller series resistance permitted a battery current of sufficient amount to cause a useful pointer deflection. If the RX 100 range were used to measure the same 3,750-ohm resistor, the pointer would deflect still further, to the 37.5-ohm position. This increased deflection would occur because resistor R Ã— 100 has about 1/10 the resistance of resistor R Ã— 10.

The foregoing circuit arrangement allows the same amount of current to flow through the meterâ€™s moving coil whether the meter measures 10,000 ohms on the R Ã— 10 scale, or 100,000 ohms on the R Ã— 100 scale.

It always takes the same amount of current 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, it is necessary to ALWAYS "zero" adjust the meter for each multiplication fact or selected.

You should select the multiplication factor (range) that will result in the pointer coming to rest as near as possible to the midpoint of the scale. This enables you to read the resistance more accurately, because the scale readings are more easily interpreted at or near midpoint.

3.27. Shunt Ohmmeter

The ohmmeter described to this point is known as a series ohmmeter, because the resistance to be measured is in series with the internal resistors and the meter movement of the ohmmeter. Another type of ohmmeter is the SHUNT OHMMETER. In the shunt ohmmeter, the resistance to be measured shunts (is in parallel with) the meter movement of the ohmmeter. The most obvious way to tell the difference between the series and shunt ohmmeters is by the scale of the meter. Figure 32 shows the scale of a series ohmmeter and the scale of a shunt ohmmeter.

Figure 32. Series and shunt ohmmeter scales.

Figure 32(A) is the scale of a series ohmmeter. Notice "0" is on the right and "âˆž" is on the left. Figure 32(B) is the scale of a shunt ohmmeter. In the shunt ohmmeter "âˆž" is on the right and "0" is on the left. A shunt ohmmeter circuit is shown in Figure 33.

In Figure 33, R1 is a rheostat used to adjust the âˆž reading of the meter (full-scale deflection). R2, R3, and R4 are used to provide the R Ã— 1, R Ã— 10, and R Ã— 100 ranges. Points A and B represent the meter leads. With no resistance connected between points A and B the meter has full-scale current and indicates âˆž. If a resistance is connected between points A and B, it shunts some of the current from the meter movement and the meter movement reacts to this lower current. Since the scale of the meter is marked in ohms, the resistance of the shunting resistor (between points A and B) is indicated. Notice that the switch has an OFF position, as well as positions for R Ã— 1, R Ã— 10, and R Ã— 100. This is provided to stop current flow and prevents the battery from being discharged while the meter is not being used.

Figure 33. A shunt ohmmeter with internal range resistors.

The shunt ohmmeter is connected to the circuit to be measured in the same way the series ohmmeter is connected. The only difference is that on the shunt ohmmeter the âˆž reading is adjusted, while on the series ohmmeter the 0 reading is adjusted. Shunt ohmmeters are not commonly used because they are limited generally to measuring resistances from 5 ohms to 400 ohms. If you use a shunt ohmmeter, be certain to switch it to the OFF position when you are finished using it.

difference between the two types of ohmmeters? Q48. List the four safety precautions observed when using ohmmeters.

3.28. Ohmmeter Safety Precautions

The following safety precautions and operating procedures for ohmmeters are the MINIMUM necessary to prevent injury and damage.

• Be certain the circuit is deenergized and discharged before connecting an ohmmeter.

• Do not apply power to a circuit while measuring resistance.

• When you are finished using an ohmmeter, switch it to the OFF position if one is provided and remove the leads from the meter.

• Always adjust the ohmmeter for 0 (or âˆž in shunt ohmmeter) after you change ranges before making the resistance measurement.

3.29. Multimeter

A MULTIMETER is the most common measuring device used in the Navy. The name multimeter comes from MULTIple METER, and that is exactly what a multimeter is. It is a dc ammeter, a dc voltmeter, an ac voltmeter, and an ohmmeter, all in one package. Figure 34 is a picture of a typical multimeter.

Figure 34. A typical multimeter.

The multimeter shown in Figure 34 may look complicated, but it is very easy to use. You have already learned about ammeters, voltmeters, and ohmmeters; the multimeter is simply a combination of these meters.

Most multimeters use a dâ€™Arsonval meter movement and have a built-in rectifier for ac measurement. The lower portion of the meter shown in Figure 34 contains the function switches and jacks (for the meter leads).

The use of the jacks will be discussed first. The COMMON or -jack is used in all functions is plugged into the COMMON jack. The +jack is used for the second meter lead for any of the functions printed in large letters beside the FUNCTION SWITCH (the large switch in the center). The other jacks have specific functions printed above or below them and are self-explanatory (the output jack is used with the dB scale, which will not be explained in this chapter). To use one of the special function jacks, except +10 amps, one lead is plugged into the COMMON jack, and the FUNCTION SWITCH is positioned to point to the special function (small letters). For example, to measure a very small current (20 microamperes), one meter lead would be plugged into the COMMON jack, the other meter lead would be plugged into the 50A AMPS jack, and the FUNCTION SWITCH would be placed in the 50V/IA AMPS position. To measure currents above 500 milliamperes, the +10A and âˆ’10A jacks would be used on the meter with one exception. One meter lead and the FUNCTION SWITCH would be placed in the 10MA/AMPS position.

3.30. Multimeter Controls

As described above, the FUNCTION SWITCH is used to select the function desired; the âˆ’DC, +DC, AC switch selects dc or ac (the rectifier), and changes the polarity of the dc functions. To measure resistance, this switch should be in the +DC position.

The ZERO OHMS control is a potentiometer for adjusting the 0 reading on ohmmeter functions. Notice that this is a series ohmmeter. The RESET is a circuit breaker used to protect the meter movement (circuit breakers will be discussed in chapter 2 of this module). Not all multimeters have this protection but most have some sort of protection, such as a fuse. When the multimeter is not in use, it should have the leads disconnected and be switched to the highest voltage scale and AC. These switch positions are the ones most likely to prevent damage if the next person using the meter plugs in the meter leads and connects the meter leads to a circuit without checking the function switch and the dc/ac selector.

3.31. Multimeter Scales

The numbers above the uppermost scale in Figure 35 are used for resistance measurement. If the multimeter was set to the R x 1 function, the meter reading would be approximately 12.7 ohms.

Figure 35. A multimeter scale and reading.

The numbers below the uppermost scale are used with the uppermost scale for dc voltage and direct current, and the same numbers are used with the scale just below the numbers for ac voltage and alternating current. Notice the difference in the dc and ac scales. This is because the ac scale must indicate effective ac voltage and current. The third scale from the top and the numbers just below the scale are used for the 2.5-volt ac function only. The lowest scale (labeled DB) will not be discussed. The manufacturerâ€™s technical manual will explain the use of this scale.

The table in Figure 35 shows how the given needle position should be interpreted with various functions selected.

As you can see, a multimeter is a very versatile measuring device and is much easier to use than several separate meters.

Table 2 illustrates an interesting point about multimeters. It was mentioned earlier in this chapter that both voltmeters and ammeters have an effect upon the circuits they measure.

To keep this effect to a minimum, it is necessary that the voltmeter have a high resistance (sensitivity expressed in ohms per volt) and the ammeter have a low resistance compared to the circuit being measured.

Table 2 shows the figures associated with three meter movements available for use in multimeters. The last two columns indicate the value of shunt resistance and the overall resistance of the shunt and meter movement necessary to compensate all three movements to an ammeter sensitivity (full-scale current) of 1 milliampere. Notice that as the voltmeter sensitivity increases, the resistance of the ammeter decreases. This shows how a meter movement used in a voltmeter will have a high effective resistance and the same meter movement used in an ammeter will have a low effective resistance because of the shunt resistors.

3.32. Parallax Error

Most multimeters (and some other meters) have a mirror built into the scale. Figure 36 shows the arrangement of the scale and mirror.

Figure 36. A multimeter scale with mirror.

The purpose of the mirror on the scale of a meter is to aid in reducing PARALLAX ERROR. Figure 37 will help you understand the idea of parallax.

Figure 37(A) shows a section of barbed wire fence as you would see it from one side of the fence. Figure 37(B) shows the fence as it would appear if you were to look down the fine of fence posts and were directly in line with the posts. You see only one post because the other posts, being in line, are hidden behind the post you can see. Figure 37(C) shows the way the fence would appear if you moved to the right of the line of posts. Now the fence posts appear to the right of the post closest to you. Figure 37(D) shows the line of fence posts as you would see them if you moved to the left of the front post. This apparent change in position of the fence posts is called PARALLAX.

Figure 37. Parallax illustration (barbed-wire fence).

Parallax can be a problem when you are reading a meter. Since the pointer is slightly above the scale (to allow the pointer to move freely), you must look straight at the pointer to have a correct meter reading. In other words, you must be in line with the pointer and the scale. Figure 38 shows the effect of parallax error.

Figure 38. A parallax error in a meter reading.

Figure 38(A) shows a meter viewed correctly. The meter reading is 5 units. Figure 38(B) shows the same meter as it would appear if you were to look at it from the right. The correct reading (5) appears to the right of the pointer because of parallax.

The mirror on the scale of a meter, shown in Figure 36, helps get rid of parallax error. If there is any parallax, you will be able to see the image of the pointer in the mirror. If you are looking at the meter correctly (no parallax error) you will not be able to see the image of the pointer in the mirror because the image will be directly behind the pointer. Figure 39 shows how a mirror added to the meter in Figure 38 shows parallax error. Figure 39(A) is a meter with an indication of 5 units. There is no parallax error in this reading and no image of the pointer is seen in the mirror. Figure 39(B) shows the same meter as viewed from the right. The parallax error is shown and the image of the pointer is shown in the mirror.

Figure 39. A parallax error on a meter with a mirrored scale.

3.33. Multimeter Safety Precautions

As with other meters, the incorrect use of a multimeter could cause injury or damage. The following safety precautions are the MINIMUM for using a multimeter.

• Deenergize and discharge the circuit completely before connecting or disconnecting a multimeter.

• Never apply power to the circuit while measuring resistance with a multimeter.

• Connect the multimeter in series with the circuit 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.

• When you are finished with a multimeter, switch it to the OFF position, if available. If there is no OFF position, switch the multimeter to the highest ac voltage position.

• 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.

• Be certain to read ac measurements on the ac scale of a multimeter.

• Observe the general safety precautions for electrical and electronic devices.

4. Analog Multimeters in DC Mode

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.

 Caution 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.

4.1. Measuring Resistance, Voltage, and Current With a VOM

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 40. The Simpson 260 is a typical VOM used in the Navy today.

Figure 40. Simpson 260 Series 6XLP Volt-Ohm-Milliammeter (VOM).

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, âˆ’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 (âˆ’) or with the word COMMON.

4.1.1. 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:

 Caution 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.

• 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 41 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.

Figure 41. Functional block diagram of an ohmmeter circuit.

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 42 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 Ã— 1, R Ã— 100, or R Ã— 10,000).

Figure 42. Ohmmeter scale.

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, letâ€™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 Ã— 1 position, you connect the meter across the resistor. The meter point then deflects between 200 and the point labeled with the infinity symbol (âˆž) on the extreme left side of the scale. Because the R Ã— 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 Ã— 100) to obtain a more easily read value.

In the R Ã— 100 position, you again zero the meter. This time, the pointer moves to the 24 mark on the scale. Because the R Ã— 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 Ã— 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 Ã— 10, R Ã— 100, or R Ã— 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.

4.1.2. Measuring DC Voltages

You set the multimeter to operate as a dc voltmeter by placing the function switch in either of two positions: +DC or âˆ’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 43 is a functional block diagram of dc voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc voltages.

Figure 43. Functional block diagram of dc voltage circuits.

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 âˆ’DC, letâ€™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.

For explanation purposes, letâ€™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.

 Caution 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 letâ€™s discuss how you take a voltage measurement on a component within a circuit. As an example, letâ€™s measure the voltage drop across the resistor shown in Figure 44.

Figure 44. Measuring the voltage drop of a resistor.

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.

4.1.3. Measuring Output 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.

 Caution 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.

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.

4.1.4. Measuring Current

The multimeter can function as an ammeter to measure current flow.

 Caution 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).

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