Chapter 2: Test Equipment (DC Mode)

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.

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.

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).

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.

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?

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.

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.

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.

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.

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.

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

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.

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.

A problem that is created by the use of a rectifier and d’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 d’Arsonval meter movement.

The first method of damping comes from the d’Arsonval meter movement itself. In the d’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).

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.

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.

The d’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.

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.

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.

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.

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 D’Arsonval meter to produce a full scale deflection, thereby reducing the meters sensitivity.

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.

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 d’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.

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.

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.

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.

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.

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.)

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.

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-

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 I_R1 and I_R2 remains the same. I_R2 is nine times greater than I_R1 and I_R1 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.

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 d’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.

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 ammeter’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.

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.

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.

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(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 (E_R2) 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 (R_n) 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 (R_n) 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 (I_R4) through the meter (R₄) in Figure 24(C) is also very small compared to the current (I_R2) 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.

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.

Voltmeter sensitivity is expressed in ohms per volt (Ω/V). It is the resistance of the voltmeter at the full-scale reading in volts. Since the voltmeter’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 voltmeter’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 × resistance).

For example, if a meter has a full-scale current of 50μA and a resistance of 960Ω, the sensitivity could be calculated as:

The full-scale voltage reading would be calculated as:

Full-scale voltage reading = full-scale current × resistance

Full-scale voltage reading = 50μA × 960Ω

Full-scale voltage reading = 48mV

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

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.

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.

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.

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.

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.

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.

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:

The ohmmeter’s pointer deflection is controlled by the amount of battery current passing through the moving coil. Before measuring the resistance of an unknown resistor or electrical circuit, the test leads of the ohmmeter are first shorted together, as shown in Figure 29. With the leads shorted, the meter is calibrated for proper operation on the selected range. While the leads are shorted, meter current is maximum and the pointer deflects a maximum amount, somewhere near the zero position on the ohms scale. Because of this current through the meter with the leads shorted, it is necessary to remove the test leads when you are finished using the ohmmeter. If the leads were left connected, they could come in contact with each other and discharge the ohmmeter battery. When the variable resistor (rheostat) is adjusted properly, with the leads shorted, the pointer of the meter will come to rest exactly on the zero position. This indicates ZERO RESISTANCE between the test leads, which, in fact, are shorted together. The zero reading of a series-type ohmmeter is on the right-hand side of the scale, where as the zero reading for an ammeter or a voltmeter is generally to the left-hand side of the scale. (There is another type of ohmmeter which is discussed a little later on in this chapter.) When the test leads of an ohmmeter are separated, the pointer of the meter will return to the left side of the scale. The interruption of current and the spring tension act on the movable coil assembly, moving the pointer to the left side (∞) of the scale.

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.

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.

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 × 1, R × 10, and R × 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 × 1, R × 10, and R × 100 are connected to three different size resistors located within the ohmmeter. This is shown in Figure 31.

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 (R_X 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 R_X 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 × 1 and R_X is too great to allow sufficient battery current to flow to deflect the pointer away from infinity (∞). (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 × 10. With this done, assume the pointer deflects to indicate 375 ohms. This would indicate that R_X has 375 ohms × 10, or 3,750 ohms resistance. The change of range caused the deflection because resistor R × 10 has about 1/10 the resistance of resistor R × 1. Thus, selecting the smaller series resistance permitted a battery current of sufficient amount to cause a useful pointer deflection. If the R_X 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 × 100 has about 1/10 the resistance of resistor R × 10.

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

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(A) is the scale of a series ohmmeter. Notice "0" is on the right and "∞" is on the left. Figure 32(B) is the scale of a shunt ohmmeter. In the shunt ohmmeter "∞" 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 ∞ reading of the meter (full-scale deflection). R₂, R₃, and R₄ are used to provide the R × 1, R × 10, and R × 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 ∞. 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 × 1, R × 10, and R × 100. This is provided to stop current flow and prevents the battery from being discharged while the meter is not being used.

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 ∞ 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.

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

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.

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 d’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 −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.

As described above, the FUNCTION SWITCH is used to select the function desired; the −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.

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.

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 manufacturer’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.

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

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) 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.

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.

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.

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.

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 46 shows the model 8000A multimeter.