Module 16—Introduction to Test Equipment
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Upon completing this chapter, you should be able to:
1. Describe the basic theory of the galvanometer.
2. Describe the basic theory of the D’Arsonval meter movement.
3. State the proper procedure for connecting an ammeter to a circuit.
4. Define ammeter sensitivity.
5. State the proper procedure for connecting a voltmeter to a circuit.
6. Describe possible effects on a circuit caused by the connection of a voltmeter.
7. Define voltmeter sensitivity.
8. Describe the internal operation of an ohmmeter with the use of a block diagram.
9. Describe the operating procedure for using a megohmmeter.
10. Describe the use of the electrodynamometer-type meter as a voltmeter, ammeter, and wattmeter.
11. Describe the factors that limit wattmeter capability.
12. Describe an open circuit, a ground, a short, and the tests used to check for these conditions.
When troubleshooting, testing, or repairing electronic equipment, you will use various meters and other types of test equipment to check for proper circuit voltages, currents, resistances, and to determine if the wiring is defective. You may be able to connect these test instruments to a circuit and take readings without knowing just how the instruments operate. However, to be a competent technician, you need to be able to do more than merely read a test instrument. You need a basic knowledge of how test instruments operate. This chapter discusses the operating principles of some of the test instruments you will use in equipment troubleshooting.
The best and most expensive measuring instrument is of no use to you unless you know what you are measuring and what each reading indicates. Remember that the purpose of a meter is to measure quantities existing within a circuit. For this reason, when the meter is connected to the circuit, it must not change the condition of the circuit.
METER POWER SOURCE
Meters are either SELF-EXCITED or EXTERNALLY EXCITED. Self-excited meters operate from their own power sources. Externally excited meters get their power from the circuit to which they are connected. Most common meters (voltmeters, ammeters, and ohmmeters) that you use in your work operate on the electromagnetic principle. All measuring instruments must have some form of indicating device, usually a meter, to be of any use to you. The most basic indicating device used in instruments that measure current and voltage operates by using the interaction between the magnetic fields associated with current flow in the circuit. Before continuing, you might want to review the properties of magnetism and electromagnetism in NEETS, Module 1, Introduction to Matter, Energy, and Direct Current.
Q-1. What meters operate from their own power sources?
BASIC METER MOVEMENT
A stationary, permanent-magnet, moving-coil meter is the basic meter movement used in most measuring instruments used for servicing electrical equipment. When current flows through the coil, a resulting magnetic field reacts with the magnetic field of the permanent magnet and causes the movable coil to rotate. The greater the intensity of current flow through the coil, the stronger the magnetic field produced; the stronger the magnetic field produced, the greater the rotation of the coil. The GALVANOMETER is an example of one type of stationary, permanent-magnet, moving-coil measuring instrument.
The galvanometer is used to measure very low currents, such as those in bridge circuits. In modified form, the galvanometer has the highest sensitivity of any of the various types of meters in use today. A simplified diagram of a galvanometer is shown in figure 3-1. It is different from other instruments used for the same purpose because its movable coil is suspended by means of metal ribbons instead of a shaft and jewel-bearing arrangement often used in other instruments.
Figure 3-1.—Simplified galvanometer.
The movable coil is wrapped around the aluminum frame of the galvanometer. The coil is suspended between the poles of the magnet by means of thin, flat ribbons of phosphor bronze. These ribbons provide a conduction path for the current between the circuit being tested and the movable coil. The ribbons allow the coil to twist in response to the interaction of the applied current through the coil and the magnetic
field of the permanent magnet. They also provide the restoring force for the coil. Basically, the restoring force is that force necessary to return the movable frame to its resting position after a reading. The ribbons restrain or provide a counterforce to the magnetic force acting on the coil. When the driving force of the coil current is removed, the restoring force provided by the ribbons returns the coil to its zero position.
Q-2. What physical component of a galvanometer provides the restoring force for the coil?
To determine the amount of current flow, we must have a means to indicate the amount of coil rotation. Either of two methods may be used: (1) the POINTER arrangement or (2) the LIGHT AND MIRROR arrangement.
Q-3. In a galvanometer, what two methods are used to indicate the amount of coil rotation?
In the pointer arrangement, one end of the pointer is mechanically connected to the rotating coil; as the coil moves, the pointer also moves. The other end of the pointer moves across a graduated scale and indicates the amount of current flow. The overall simplicity of this arrangement is its main advantage. However, a disadvantage of this arrangement is that it introduces a mechanical coil balancing problem, especially if the pointer is long.
Q-4. What is the primary disadvantage of the pointer arrangement for indicating coil rotation?
In the light and mirror arrangement, the use of a mirror and a beam of light simplifies the problem of coil balance. When this arrangement is used to measure the turning of the coil, a small mirror is mounted on the supporting ribbon, as shown in figure 3-1. An internal light source is directed to the mirror and then reflected to the scale of the meter. As the movable coil turns, so does the mirror. This causes the light reflection to move across the graduated scale of the meter. The movement of the reflection is proportional to the movement of the coil; therefore, the intensity of the current being measured by the meter is accurately indicated.
If the beam of light and mirror arrangement is used, the beam of light is swept to the right or left across a translucent screen (scale). The translucent screen is divided uniformly with the zero reading located at center scale. If the pointer arrangement is used, the pointer is moved in a horizontal plane to the right or left across a scale that is divided uniformly with the zero reading at the center. The direction in which the beam of light or the pointer moves depends on the direction (polarity) of current through the coil.
D’Arsonval Meter Movement
Most dc instruments use meters based on some form of the D’Arsonval meter movement. In D’Arsonval-type meters, the length of the conductor and the strength of the field between the poles of the magnet are fixed. Therefore, any change in current causes a proportional change in the force acting on the coil. Figure 3-2 is a simplified diagram showing the principle of the D’Arsonval movement.
Figure 3-2.—D'Arsonval meter movement.
In the figure, only one turn of wire is shown; however, in an actual meter movement, many turns of fine wire would be used, each turn adding more effective length to the coil. The coil is wound on an aluminum frame (bobbin) to which the pointer is attached. Oppositely wound hairsprings (only one is shown in the figure) are also attached to the bobbin, one at either end. The circuit to the coil is completed through the hairsprings. In addition to serving as conductors, the hairsprings serve as the restoring force that returns the pointer to the zero position when no current flows.
Q-5. What component of the D’Arsonval meter movement completes the circuit for current flow to the coil?
COIL MOVEMENT.—As we discussed previously, the deflecting (moving) force on the coil is proportional to the current flowing through the coil. This deflecting force tends to cause the coil to rotate against the restraining force of the hairsprings. When the deflecting force and the restraining force are equal, the coil and the pointer stop moving. As we have just stated, the deflecting force is proportional to the current in the coil, the angle (amount) of rotation is proportional to the deflecting force; therefore, the angle of rotation is proportional to the current through the coil. When current stops flowing through the coil, the deflecting force stops, and the restoring force of the springs returns the pointer to the zero position.
Q-6. What component supplies restoring force to the coil of the D’Arsonval meter movement?
DIRECTION OF FORCE.—The current through the single turn of wire is in the direction indicated in the figure (away from you on the right-hand side and toward you on the left-hand side). If we apply the right-hand motor rule, the direction of force is upward on the left-hand side and downward on the right-hand side; therefore, the direction of motion of the coil and pointer is clockwise. If the current were reversed in the wire, the direction of motion of the coil and pointer would be reversed. For a review of the right-hand rule for motors, refer to NEETS, Module 5, Introduction to Generators and Motors.
PRINCIPLE OF OPERATION.—A more detailed view of the basic D'Arsonval movement, as it is used in ammeters and voltmeters, is shown in figure 3-3. The principle of operation is the same as that discussed in the simplified version. The iron core is rigidly supported between the pole pieces; it serves to
concentrate the flux in the narrow space between the iron core and the pole piece. Current flows into one hairspring, through the coil, and out the other hairspring. The restoring forces of the spiral springs return the pointer to the normal zero position when the current through the coil is interrupted. Conductors connect the hairsprings with the outside terminals of the meter. If the instrument is not DAMPED to absorb the energy of the moving element, the pointer will oscillate (vibrate) for a period of time before coming to a stop in its final position. Damping is an energy-absorbing system that prevents this.
Figure 3-3.—Detailed view of the basic D'Arsonval meter movement.
DAMPING.—This is accomplished in many D'Arsonval movements by means of the motion of the aluminum bobbin on which the coil is wound. As the bobbin rotates in the magnetic field, an electromotive force is induced into it as it cuts through the lines of force. Induced currents flow in the bobbin in a direction opposite to the motion; this causes the bobbin to go beyond its final position only once before stopping. The overall sensitivity of the meter can be increased by the use of a lightweight rotating assembly (bobbin, coil, and pointer) and by the use of jewel bearings, as shown in figure 3-3.
POLE CONSTRUCTION.—Note that the pole pieces in figures 3-2 and 3-3 have curved faces. You can see the advantage of this type of construction if you remember that lines of force enter and leave a magnetic field in the air gap at right angles to the coil, regardless of the angular position of the coil. Because of this type of construction, a more linear scale is possible than if the pole faces were flat.
Q-7. What advantage is gained by using pole pieces with curved faces in the D’Arsonval meter movement?
The movable coil of the D'Arsonval meter movement we have been discussing up to now uses small- size wire in its windings. This small-size wire places limits on the amount of current that can be safely passed through the coil. Therefore, the basic D'Arsonval movement discussed can be used to indicate or measure only very small currents. Certain circuit changes must be made to the basic D'Arsonval meter movement for it to be practical in everyday use. To measure large currents, you must use a SHUNT with the meter.
A shunt is a physically large, low-resistance conductor connected in parallel (shunt) with the meter terminals. It is used to carry the majority of the load current. Such a shunt is designed with the correct amount of resistance so that only a small portion of the total current flows through the meter coil. The meter current is proportional to the total load current. If the shunt is of such a value that the meter is calibrated in milliamperes, the instrument is called a MILLIAMMETER. If the shunt has such a value that the meter must be calibrated in terms of amperes, it is called an AMMETER.
Q-8. What structurally large, low-resistance conductor is connected in parallel with the meter movement to prevent damage?
SHUNT RESISTANCE.—A single, standardized meter movement is normally used in all ammeters, no matter what the range is for a particular meter. For example, meters with working ranges of 0 to 10 amperes, 0 to 5 amperes, or 0 to 1 ampere all use the same meter movement. The various ranges are achieved through the use of different values of shunt resistance with the same meter movement. The designer of the ammeter simply calculates the correct shunt resistance required to extend the range of the meter movement to measure any desired value of current. This shunt is then connected across the meter terminals. Shunts may be located inside the meter case (internal shunts) with the proper switching arrangements for changing them. They may also be located outside the meter case (external shunts) with the necessary leads to connect them to the meter.
EXTERNAL SHUNTS.—An external-shunt circuit is shown in figure 3-4, view A. Typical external shunts are shown in view B. View C shows a meter movement mounted within the case. The case provides protection against breakage, magnetic shielding in some cases, and portability.
Figure 3-4.—Dc ammeter using the D'Arsonval movement with external shunts.
SHUNT CONSTRUCTION.—The shunt strips (view B of figure 3-4) are usually made of the alloy Manganin. Manganin has a temperature coefficient of almost zero. The zero-temperature coefficient property is desirable because of the heavy currents that often flow through shunts producing heat. A zero- temperature coefficient material is not affected by this heat; therefore, it remains stable in temperature. Most other materials increase their resistance as they are heated. If shunts were made of these materials, they would carry less current. More and more current would flow through the meter movement, and the chances of damage would increase. Using shunts constructed with zero-temperature coefficient materials eliminates this problem.
Q-9. What type of temperature coefficient material does not produce increased heat in response to increased current flow?
The ends of the shunt strips are embedded in heavy copper blocks. The blocks are attached to the meter coil leads and the line terminals. To ensure accurate readings, you should not interchangeably use the meter leads for a particular ammeter with those for a meter of a different range. Slight changes in lead length and size may vary the resistance of the meter circuit. If this happens, current will also change and cause incorrect meter readings. External shunts are generally used where currents greater than 50 amperes must be measured.
SHUNT SELECTION.—When using an external-shunt ammeter, you should select a suitable shunt so that the scale deflection can be easily read. For example, if the scale has 150 divisions and the load current you want to measure is known to be between 50 and 100 amperes, a 150-ampere shunt would be the correct choice. Under these conditions, each division of the scale represents 1 ampere. In other words,
a full-scale deflection of the pointer would rest on the 150th division mark, indicating that 150 amperes of load current is flowing. At half-scale deflection, the pointer would rest on the 75th division mark, indicating that 75 amperes of load current is flowing.
A shunt having exactly the same current rating as the expected normal load current should never be selected. If you were to select such a shunt, higher than normal load currents could possibly drive the pointer off scale and damage the meter movement. A good choice of shunt values will place the indicating needle somewhere near the midscale indication when the load current you are reading is normal. For example, assume that the meter scale is divided into 100 equal divisions and you want to measure a
current of 60 amperes. The shunt to use would be a 100-ampere shunt. This would make each division of the scale equal to 1 ampere. The meter indication would fall on the 60th division showing that 60 amperes of load current is flowing. Therefore, an allowance (40 amperes) remains for unexpected surge currents.
Q-10. A good choice of shunt resistance will place the indicating pointer near what part of the meter scale with a normal load?
INTERNAL SHUNTS FOR METERS IN THE 0- TO 50-AMPERE RANGE.—When measuring current ranges below 50 amperes, you will most often use internal shunts (Rshunt). In this way, you can easily change the range of the meter by means of a switching arrangement. A switch will select the correct internal shunt with the necessary current rating and resistance. Before you can calculate the required resistance of the shunt for each range, the total resistance of the meter movement must be known. For example, suppose you desire to use a 100-microampere D'Arsonval meter with an internal coil resistance of 100 ohms to measure line currents up to 1 ampere. The meter will deflect to its full-scale position when the current through the deflection coil is 100 microamperes.
Since the coil resistance is 100 ohms, you can calculate the coil's voltage (Ecoil) by using Ohm's law, as follows:
When the pointer is deflected to full scale, 100 microamperes of current flows through the coil and 0.01 volt drops across it. Remember, 100 microamperes is the maximum safe current for this meter movement. Exceeding this value will damage the meter. The shunt must carry any additional load current.
The meter coil has a 0.01 volt drop across it, and, because the shunt and coil are in parallel, the shunt also has a voltage drop of 0.01 volt. The current that flows through the shunt is the difference between the full-scale meter current and the line current being fed into the shunt. In this case, meter current is 100 microamperes. Full-scale deflection is desired only when the total current is 1 ampere. Therefore, the shunt current must equal 1 ampere minus 100 microamperes, or 0.9999 ampere. Ohm's law is again used to provide the approximate value of required shunt resistance (Rshunt), as follows:
To increase the range of the 100-microampere meter to 1 ampere (full-scale deflection), place a 0.01- ohm shunt in parallel with the meter movement.
You can convert the 100-microampere instrument to a 10-ampere meter by using a proper shunt. The voltage drop for a full-scale deflection is still 0.01 volt across the coil and the shunt. The meter current is still 100 microamperes. The shunt current must therefore be 9.9999 amperes under full-scale deflection. Again, this is an approximate figure found by the application of Ohm’s law.
You can also convert the same instrument to a 50-ampere meter by using the proper shunt resistance, as follows:
INTERNAL SHUNTS FOR METERS IN THE MILLIAMPERE RANGE.—The above method of computing the shunt resistance is satisfactory in most cases; however, it can only be used when the line current is in the ampere range and the meter current is relatively small compared to the load current. In such cases, you can use an approximate value of resistance for the shunt, as was done above. However, when the line current is in the milliampere range and the coil current becomes an appreciable percentage of the line current, a more accurate calculation must be made. For example, suppose you desire to use a meter movement that has a full-scale deflection of 1 milliampere and a coil resistance of 50 ohms to measure currents up to 10 milliamperes. Using Ohm's law, you can figure the voltage (Ecoil) across the meter coil (and the shunt) at full-scale deflection, as follows:
The current that flows through the shunt (Ishunt) is the difference between the line current and the meter current, as figured below:
The shunt resistance (Rshunt) may then be figured, as follows:
Notice that, in this case, the exact value of shunt resistance has been used rather than an approximation.
The formula for determining the resistance of the shunt is given by Rs = Im/Is times Rm, where Rs is the shunt resistance in ohms; Im is the meter current at full-scale deflection; Is is the shunt current at full- scale deflection; and Rm is the resistance of the meter coil. If the values given in the previous example are used in this equation, it will yield 5.55 ohms, the value previously calculated.
SWITCHING SHUNT VALUES.—Various values of shunt resistance can be used, by means of a suitable switching arrangement, to increase the number of current ranges that can be covered by the meter. Two switching arrangements are shown in figure 3-5. View A is the simpler of the two arrangements when a number of shunts are used to calculate the values of the shunt resistors. However, it has two disadvantages:
Figure 3-5.—Ways of connecting internal meter shunts.
1. When the switch is moved from one shunt resistor to another, the shunt is momentarily removed from the meter. The line current then flows through the meter coil. Even a momentary surge of current could easily damage the coil.
2. The contact resistance (resistance between the blades of the switch when they are in contact) is in series with the shunt, but not with the meter coil. In shunts that must pass high currents, this contact resistance becomes an appreciable part of the total shunt resistance. Because the contact resistance is of a variable nature, the ammeter indication may not be accurate.
The generally preferred method of range switching is shown in (figure 3-5, view B). Although only two ranges are shown, as many ranges as needed can be used. In this type of circuit, the contact resistance of the range-selector switch is external to the shunt and meter in each range position. The contact resistance in this case has no effect on the accuracy of the current measurement.
When you are using ammeters, a primary rule of safety is that such current-measuring instruments must always be connected in series with a circuit, never in parallel with it. When an ammeter is connected across a constant-potential source of appreciable voltage, the low internal resistance of the meter bypasses the circuit resistance. This results in the application of the source voltage (or a good
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics