Chapter 3 Basic Meters
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-1 UNCLASSIFIED 3 BASIC METERS LEARNING OBJECTIVES
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.
3.1 INTRODUCTION 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.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-2 UNCLASSIFIED 3.2 METERS 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.
3.2.1 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? 3.2.2 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.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-3 UNCLASSIFIED 3.2.2.1 Galvanometer 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.
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. Figure 3-1 Simplified galvanometer
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-4 UNCLASSIFIED 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.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-5 UNCLASSIFIED 3.2.2.2 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.
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? Figure 3-2 D'Arsonval meter movement
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-6 UNCLASSIFIED 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.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-7 UNCLASSIFIED
Figure 3-3 Detailed view of the basic D'Arsonval meter movement
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-8 UNCLASSIFIED 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? 3.2.3 DC Ammeter 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.
3.2.3.1 Shunts 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?
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-9 UNCLASSIFIED 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
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-10 UNCLASSIFIED 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.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-11 UNCLASSIFIED 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 (E coil) by using Ohm's law, as follows:
Ecoil = I × R
= 0.0001 amperes × 100 ohms
= 0.01 volt
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 (R shunt), as follows:
Rshunt = E I
0.01 volt 0.999 ampere
= 0.01 ohm
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-12 UNCLASSIFIED 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:
Rshunt = E I
0.01 volt 49.999 amperes
= 0.0002 ohm
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 (E coil) across the meter coil (and the shunt) at full-scale deflection, as follows:
E coil = I × R
= 0.001 amperes × 50 ohms
= 50 millivolts
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-13 UNCLASSIFIED The current that flows through the shunt (Ishunt) is the difference between the line current and the meter current, as figured below:
Ishunt = Itotal − Imeter
= 10mA − 1mA
= 9 mA or 0.009 ampere
The shunt resistance (Rshunt) may then be figured, as follows:
Rshunt = E I
0.05 volt 0.009 amperes
= 5.55 ohms
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 = I m /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:
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-14 UNCLASSIFIED
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.
Figure 3-5 Ways of connecting internal meter shunts
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-15 UNCLASSIFIED 3.2.3.2 Ammeter Connections 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 portion of it) directly to the meter terminals. The resulting excessive current burns up the meter coil and renders the meter useless until repaired.
Q-11. In what manner are current-measuring instruments connected to a circuit? If you do not know the approximate value of current in the circuit, you should take a reading at the highest range of the ammeter; then you should switch progressively to lower ranges until a suitable reading is obtained. Most ammeter scales indicate the current being measured in increasing values from left to right. If you connect the meter without observing proper polarity, the pointer may be deflected backwards (from right to left). This action often damages the meter movement. You should ensure that the ammeter is always connected so that the current will flow into the negative terminal and out the positive terminal. Figure 3-6 shows various circuit arrangements and the proper ammeter connection methods to measure current in various portions of the circuit.
Q-13. (True or False) The larger the current required to produce full-scale deflection of the meter coil, the better the sensitivity of the meter.
Figure 3-6 Proper ammeter connection
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-16 UNCLASSIFIED Good sensitivity is especially important in ammeters to be used in circuits in which small currents flow. As the meter is connected in series with the load, the current flows through the meter. If the internal resistance of the meter is a large portion of the load resistance, an effect known as METER-LOADING will occur. Meter-loading is the condition that exists when the insertion of a meter into a circuit changes the operation of that circuit. This condition is not desirable. The purpose of inserting a meter into a circuit is to allow the measurement of circuit current in the normal operating condition. If the meter changes the circuit operation and changes the amount of current flow, the reading you obtain will be in error. An example of this is shown in figure 3-7.
Q-14. What condition exists when the insertion of a meter into a circuit changes the operation of the circuit? In view A of figure 3-7, the circuit to be tested has an applied voltage of 100 millivolts and a resistance of 100 ohms. The current normally flowing in this circuit is 1 milliampere. In view B, an ammeter that requires 1 milliampere for full-scale deflection and that has an internal resistance of 100 ohms has been inserted. Since 1 milliampere of current flow is shown in view A, you might naturally assume that with the meter inserted into the circuit, a full-scale deflection will occur. You might also assume that the 1 milliampere of circuit current will be measured. However, neither of these assumptions is correct. With the ammeter inserted into the circuit, as shown in view B, the total resistance of the circuit is 200 ohms. With an applied voltage of 100 millivolts, applying Ohm’s law shows the actual current (I circuit) to be 0.5 milliampere.
Figure 3-7 Ammeter loading effect
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-17 UNCLASSIFIED I = E R
Icircuit = 100 × 10−3 volts 200 ohms
= 0.5 × 10−3 ampere
= 0.0005 ampere or 0.5 milliampere
Since the meter reads 0.5 milliampere instead of the normal value of current, the meter reveals that a definite loading effect has taken place. In cases such as this, the use of ammeters, which have a lower internal resistance and a better current sensitivity, is desirable.
3.2.4 DC Voltmeter Up to this point, we have been discussing the 100-microampere D’Arsonval movement and its use as an ammeter. However, it can also be used to measure voltage if a MULTIPLIER (high resistance) is placed in series with the moving coil of the meter. For low-voltage instruments, this resistance is physically mounted inside the meter case with the D’Arsonval movement. The series resistance is constructed of a wire-wound resistance that has a low temperature coefficient wound on either a spool or card frame. For high-voltage ranges, the series resistance can be connected externally. A simplified diagram of a voltmeter is shown in figure 3-8.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-18 UNCLASSIFIED
Q-15. What modification is made to the D’Arsonval meter movement to enable the meter to measure voltage? Keep in mind that the D’Arsonval meter movement uses current flow to produce a magnetic field that is proportional to the current. The meter movement is, therefore, an indicator of current flow rather than voltage. The addition of the series resistance is what allows the meter to be calibrated in terms of voltage; that is, the meter movement of a voltmeter operates because of the current flow through the meter, but the scale is marked in volts. For example, the meter movement shown in figure 3-9 has an internal resistance of 100 ohms, requires 100 microamperes for full-scale deflection, and has a voltage drop of 10 millivolts when full-scale deflection is reached. If you were to place this meter directly across a 10-volt source, an excessive current (in milliamperes) would flow. The meter would be destroyed because of the excessive current flowing through the meter movement. This can be seen in the following Ohm’s law application:
Figure 3-8 Internal construction and circuit of a simplified voltmeter
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-19 UNCLASSIFIED
Using this equation, you can see that a current through the meter of 100 milliamperes is excessive and will cause damage.
I = E R
= 10 volts 100 ohms
= 100 milliamperes
Since the normal voltage drop for the meter is 10 millivolts at full-scale deflection, some means must be supplied to drop the extra 9.99 volts without applying it directly to the meter. This is done by the addition of a multiplier resistor, as shown in figure 3-9.
Figure 3-9 Use of multiplier resistors with D'Arsonval meter movement
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-20 UNCLASSIFIED 3.2.4.1 Extending Voltmeter Ranges The value of series resistance is determined by the current required for full-scale deflection and by the range of the voltages to be measured. Since the current through the meter circuit is directly proportional to the applied voltage, the meter scale can be calibrated directly in volts for a fixed value of series resistance. For example, let’s assume that the basic meter is to be made into a voltmeter with a full-scale deflection of 1 volt. The coil resistance of the basic meter is 100 ohms, and 100 microamperes of current causes full-scale deflection. The resistance (R meter) required to limit the total current in the circuit to 100 microamperes can be found as follows:
Rmeter = E I
1 volt 100 microamperes
= 10 kilohms
Because the meter coil already measures 100 ohms, the series resistance required is equal to 10 kilohms minus 100 ohms, or 9.9 kilohms.
Q-16. What factors determine the value of the multiplier resistor? Multi-range voltmeters use one meter movement. The required resistances are connected in series with the meter by a switching arrangement. A schematic diagram of a multi- range voltmeter with three ranges is shown in figure 3-10. The total meter resistance (R meter) for each of the three ranges, beginning with the 1-volt range, is figured by the application of Ohm’s law, as follows:
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-21 UNCLASSIFIED
1-volt range
Rmeter = 1 . 0001 = 10 kilohms
100-volt range
Rmeter = 100 . 0001 = 10 megohm
1,000-volt range
Rmeter = 1000 . 0001 = 10 megohms
The actual value of the multiplying series resistor ( Rseries) for each of these circuits is 100 ohms less than the total resistance. This allows for the resistance of the meter coil (Rcoil).
Figure 3-10 Multi-range voltmeter
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-22 UNCLASSIFIED 3.2.4.2 Voltmeter Circuit Connections When voltmeters are used, a primary rule of safety is that such voltage-measuring instruments must always be connected in parallel with (across) a circuit. If you are unsure of the level of the voltage to be measured, take a reading at the highest range of the voltmeter and progressively (step by step) lower the range until a suitable reading is obtained. In many cases, the voltmeter you will be using will not be a center-zero- (0 reading is in the center) indicating instrument. Observing the correct polarity is important when connecting the instrument to the circuit. Voltmeter polarity is the same as for the dc ammeter; that is, current flows from negative to positive.
Q-17. In what manner are voltage-measuring instruments connected to the circuit to be measured? 3.2.4.3 Influence of a Voltmeter in a Circuit The purpose of a voltmeter is to indicate the potential difference between two points in a circuit. When a voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has a low resistance, it will draw a substantial amount of current. This action lowers the effective resistance of the circuit and changes the voltage reading. When you are making voltage measurements in high-resistance circuits, use a HIGH-RESISTANCE VOLTMETER to prevent the shunting action of the voltmeter. The effect is less noticeable in low-resistance circuits because the shunting effect is less. The problem of voltmeter shunting (sometimes called circuit loading) is illustrated in figure 3-11.
Figure 3-11 Shunting action caused by a voltmeter
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-23 UNCLASSIFIED Q-18. When making voltage measurements in a high-resistance circuit, you should always use a voltmeter with what relative value of resistance? In view A of figure 3-11, a source of 150 volts is applied to a series circuit consisting of two 10-kilohm resistors. View A shows the voltage drop across each resistor to be 75 volts. In the 150-volt range, the voltmeter to be used has a total internal resistance of 10 kilohms. View B shows the voltmeter connected across the circuit. The parallel combination of R2 and the meter now present a total resistance of 5 kilohms. Because of the addition of the voltmeter, the voltage drops change to 100 volts across R1 and 50 volts across R2. Notice that this is not the normal voltage drop across R2. Actual circuit conditions have been altered because of the voltmeter.
3.2.4.4 Voltmeter Sensitivity The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (R meter), plus the series resistance (Rseries), by the full- scale reading in volts. In equation form, sensitivity is expressed as follows:
sensitivity = Rm + Rs E
This is the same as saying the sensitivity is equal to the reciprocal of the full-scale deflection current. In equation form, this is expressed as follows:
sensitivity = ohms volt
= 1 volt ohms
= 1 ampere
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-24 UNCLASSIFIED Therefore, the sensitivity of a 100-microampere movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.
sensitivity = 1 ampere
= 1 . 0001
10,000 ohms per volt
Q-19. What term is used to express the sensitivity of a voltmeter? 3.2.5 Meters Used For Measuring Resistance The two instruments you will use most often to check continuity, or to measure the resistance of a circuit or circuit component, are the OHMMETER and the MEGGER (MEGOHMMETER). The ohmmeter is widely used to measure resistance and to check the continuity of electrical circuits and devices. Its range usually extends to only a few megohms. The megger is widely used for measuring insulation resistance, such as that between a wire and the outer surface of its insulation, and the insulation resistance of cables and insulators. The range of a megger can be extended to more than 1,000 megohms.
Q-20. What instrument is used for measuring the insulation resistance of cables?
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-25 UNCLASSIFIED 3.2.5.1 The Ohmmeter A simple ohmmeter circuit is shown in figure 3-12. The ohmmeter consists of the dc milliammeter, discussed earlier in this chapter, and the added features shown below:
• A source of dc potential; and • One or more resistors (one of which is variable).
Q-21. What added features enable a dc milliammeter to function as an ohmmeter?
Figure 3-12 Simple ohmmeter circuit
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-26 UNCLASSIFIED The deflection of the pointer of an ohmmeter is controlled by the amount of battery current passing through the moving coil. Before you can measure the resistance of an unknown resistor or electrical circuit, you must calibrate the ohmmeter to be used. If the value of resistance to be measured can be estimated within reasonable limits, select a range on the ohmmeter that will give approximately half-scale deflection when the resistance is inserted between the probes. If you cannot estimate the resistance to be measured, then set the range switch on the highest scale. Whatever range you select, the meter must be calibrated to read zero before the unknown resistance is measured.
To calibrate the meter, you first short the test leads together, as shown in figure 3-12. With the test leads shorted, a complete series circuit exists. The complete series circuit consists of the 3-volt source, the resistance of the meter coil (R meter), the resistance of the zero-adjust rheostat, and the series multiplying resistor (Rseries). The shorted test leads cause current to flow and the meter pointer to deflect.
Notice that the zero point on the ohmmeter scale (as opposed to the zero points for voltage and current) is located at the extreme right side of the scale. With the test leads shorted, the zero-adjust potentiometer is set so that the pointer rests on the zero mark. Therefore, a full-scale deflection indicates zero resistance between the leads.
Q-22. A full-scale deflection on an ohmmeter scale indicates what resistance between the leads? If you change the range on the meter, you must "zero" (calibrate) the meter again to obtain an accurate reading. When you separate the test leads, the pointer of the meter will return to the left side of the scale. This action, as explained earlier, is caused by the restoring force of the spring tension acting on the movable coil assembly. The reading at the left side of the scale indicates an infinite resistance.
After you have adjusted the ohmmeter for zero reading, it is ready to be connected to a circuit to measure resistance. A typical circuit and ohmmeter arrangement is shown in figure 3-13. You must ensure that the power switch of the circuit to be measured is in the de-energized (OFF) position. This prevents the source voltage of the circuit from being applied to the meter, a condition that could cause severe damage to the meter movement.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-27 UNCLASSIFIED
Remember that the ohmmeter is an open circuit when the test leads are separated. To take a resistance reading with a meter, you must provide a path for current flow produced by the meter’s battery. In view A of figure 3-13, the meter is connected at points A and B to produce this path. Connecting these test leads places resistors R1 and R2 in series with the resistance of the meter coil, the zero-adjust potentiometer, and the series multiplying resistor. Since you previously calibrated the meter, the amount of coil movement now depends only on the resistances of R1 and R2.
The addition of R1 and R2 into the meter circuit raises the total series resistance and decreases the current. This decreases the amount of pointer deflection. The pointer comes to rest at a scale reading that indicates the combined resistance of R1 and R2. If you were to replace either R1 or R2, or both, with a resistor having a larger ohmic value, the current flow in the moving coil of the meter would be decreased even more. This would further decrease the pointer deflection, and the scale indication would read a still higher circuit resistance. View B is a simplified version of the circuitry in view A.
From our ohmmeter discussion, two facts should be apparent: (1) Movement of the moving coil is proportional to the amount of current flow, and (2) the scale reading of the ohmmeter is inversely proportional to current flow in the moving coil.
Figure 3-13 Measuring circuit resistance with an ohmmeter.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-28 UNCLASSIFIED The amount of circuit resistance to be measured may vary over a wide range. In some cases, it may only be a few ohms; in other cases, it may be as great as 1 megohm. Scale multiplication features are built into most ohmmeters so that they will indicate any ohmic value being measured and offer the least amount of error. Most ohmmeters are equipped with a selector switch for selecting the multiplication scale desired. For example, view A of figure 3-14 shows a typical meter that has a six-position switch. The positions are marked on the meter in multiples of 10, from R × 1 through R × 100K.
Figure 3-14 Ohmmeter with multiplication switch
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-29 UNCLASSIFIED The range used to measure any particular unknown resistance (Rx in view A of figure 3- 14) depends on the approximate ohmic value of the unknown resistance. For instance, the ohmmeter scale of the figure is calibrated in divisions from 0 to infinity. Note that the divisions are easier to read on the right-hand portion of the scale than on the left. For this reason, if Rx is greater than 1,000 ohms and if you are using the R × 1 range, you will be unable to accurately read the indicated resistance. This happens because the combined series resistance of resistors Rx is too large for range R × 1 to allow enough battery current to flow to deflect the pointer away from infinity. You need to turn the range switch to the R × 10 position to obtain the 1,000-ohm reading.
Let’s assume that you have changed the range switch to the R × 10 position and the pointer now deflects to a reading of 375 ohms, as shown in view B of figure 3-14. This would indicate to you that unknown resistance R x has 3,750 (375 times 10) ohms of resistance. The change of range caused the deflection because resistor R × 10 has only 1/10 the resistance of resistor R × 1. Therefore, selecting the smaller series resistance allowed a battery current of sufficient value to cause a readable pointer deflection. If the R × 100 range were used to measure the same 3,750 ohm resistor, the pointer would deflect still further to the 37.5-ohm position, as shown in view C. This increased deflection would occur because resistor R × 100 has only 1/10 the resistance of resistor R × 10.
Q-23. The R × 100 resistance selection on an ohmmeter has what amount of resistance compared to the R × 10 selection? The circuit arrangement in view A of figure 3-14 allows the same amount of current to flow through the moving meter coil. The same amount is allowed to flow whether the meter measures 10,000 ohms on the R × 1 scale, 100,000 ohms on the R × 10 scale, or 1,000,000 ohms on the R × 100 scale.
The same amount of current must always be used to deflect the pointer to a certain position on the scale (midscale position, for example), regardless of the multiplication factor being used. Since the multiplier resistors are of different values, you must always "zero" the meter for each multiplication scale selected. When selecting a range on the ohmmeter, select the one that will result in the pointer coming to rest as close to the midpoint of the scale as possible. This will enable you to read the resistance more accurately because scale readings are more easily interpreted at or near midpoint.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-30 UNCLASSIFIED 3.2.5.2 The Megohmmeter An ordinary ohmmeter cannot be used for measuring multimillion ohm values of resistances, such as those in conductor insulation. To test for such insulation breakdown, you need to use a much higher potential than that supplied by the battery of an ohmmeter. This potential is placed between the conductor and the outside of the insulation. A megger (megohmmeter) is used for these tests. The megger, shown in figure 3-15, is a portable instrument consisting of two main elements: (1) a hand-driven dc generator, which supplies the necessary voltage for making the measurement, and (2) the instrument portion, which indicates the value of the resistance you are measuring. The instrument portion is of the opposed-coil type, as shown in view A. Coils a and b are mounted on movable member c. A fixed angular relationship exists between coils, and they are free to turn as a unit in a magnetic field. Coil b tends to move the pointer counterclockwise, and coil a tends to move it clockwise.
Figure 3-15 Megger internal circuit and external view
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-31 UNCLASSIFIED Coil a is connected in series with R3 and unknown resistance Rx. The combination of coil a, R3, and Rx forms a direct series path between the + and − brushes of the dc generator. Coil b is connected in series with R2, and this combination is also connected across the generator. Notice that the movable member (pointer) of the instrument portion of the megger has no restoring springs. Therefore, when the generator is not being operated, the pointer will float freely and may come to rest at any position on the scale.
The guard ring, shown in view A of figure 3-15, shunts any leakage currents to the negative side of the generator. This prevents such current from flowing through coil a and affecting the meter reading.
Q-24. What is the purpose of the guard ring in a megohmmeter? If the test leads are open, no current will flow in coil a. However, current will flow internally through coil b and deflect the pointer to infinity. This reading indicates a resistance too large to measure. When a resistance, such as R x, is connected between the test leads, current also flows in coil a; the pointer tends to move clockwise. At the same time, coil b still tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position in which the two forces are balanced. This position depends upon the value of Rx, which controls the amount of the current in coil a. Because changes in voltage affect both coils in the same proportion, the position of the moving element is independent of the voltage. If you short the test leads together, the pointer will come to rest at zero because the current in coil a is relatively large. Since R3 limits the current, the instrument will not be damaged under these circumstances. The external appearance of one type of megger is shown in view B of figure 3-15.
Most meggers you will use are rated at 500 volts; however, there are other types. Meggers are usually equipped with friction clutches, which are designed to slip if the generator is cranked faster than its rated speed. This prevents the generator speed and output voltage from exceeding rated values. A 1,000-volt generator is available for extended ranges. When an extremely high resistance, such as 10,000 megohms or more, is to be measured, a high voltage is needed to cause enough current flow to actuate the meter movement.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-32 UNCLASSIFIED CAUTION
When using a megger, you can easily be injured or damage equipment if you do not observe the following MINIMUM safety precautions:
• Use meggers on high-resistance measurements only (such as insulation measurements or to check two separate conductors on a cable). • Never touch the test leads while the handle is being cranked. • De-energize and discharge the circuit completely before connecting a megger. • Whenever possible, disconnect the component being checked from other circuitry before using a megger
Q-25. Most meggers you will use are rated at what voltage? Q-26. The development of excessive test voltages is avoided by the use of meggers equipped with what device? 3.3 ELECTRODYNAMOMETER-TYPE METERS The electrodynamometer-type meter differs from the galvanometer types we have just studied in that two fixed coils are used to produce the magnetic field instead of a permanent magnet. Two movable coils are also used in the electrodynamometer meter. The electrodynamometer meter is most commonly found in various types of power meters.
Q-27. What components in an electrodynamometer-type meter movement produce the magnetic field? As shown in figure 3-16, the fixed coils are connected in series and positioned coaxially (in line) with a space between them. The two movable coils are also positioned coaxially and are connected in series. The two pairs of coils (fixed pair and movable pair) are also connected in series with each other. The movable coil is pivot-mounted between the fixed coils. The main shaft on which the movable coils are mounted is restrained by spiral springs that restore the pointer to zero when no current is flowing through the coil. These springs also act as conductors for delivering current to the movable coils. Since these conducting springs are very small, the meter cannot carry a high value of current.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-33 UNCLASSIFIED
Q-28. What is the limiting factor as to the amount of current an electrodynamometer meter movement can handle? 3.3.1 Meter Accuracy The meter is mechanically damped by means of aluminum vanes that move in enclosed air chambers. Although very accurate, electrodynamometer-type meters do not have the sensitivity of the D’Arsonval-type meter movement. For this reason, you will not find them used outside of the laboratory environment to a large extent.
Figure 3-16 Internal construction of an electrodynamometer
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-34 UNCLASSIFIED 3.3.2 Meter Movement The primary advantage of the electrodynamometer-type meter movement is that it can be used to measure alternating as well as direct current. If you apply alternating current to the standard galvanometer-type meter, it will not produce a usable reading. Instead, the meter will vibrate at or near the zero reading. On one-half cycle of the ac, the meter is deflected to the left and on the other half cycle to the right. Since the frequencies you will be measuring are 60 hertz or greater, the meter is incapable of mechanically responding at this speed. The result is simply a vibration near the zero point; in addition, no useful reading of voltage or current is obtained. This problem does not exist with the electrodynamometer-type movement. Current flow through the stationary (fixed) coils sets up a magnetic field. Current flow through the moving coils sets up an opposing magnetic field. With two magnetic fields opposing, the pointer deflects to the right. If the current reverses direction, the magnetic fields of both sets of coils will be reversed. With both fields reversed, the coils still oppose each other, and the pointer still deflects to the right. Therefore, no rectifying devices are required to enable the electrodynamometer meter movement to read both ac and dc. Rectifying devices are required for the D’Arsonval-type meter movement to enable it to be used for measuring ac voltages and currents.
Q-29. What is the primary advantage of the electrodynamometer-type meter over the D’Arsonval-type meter?
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-35 UNCLASSIFIED 3.3.3 Voltmeter When an electrodynamometer is used as a voltmeter, no problems in construction are encountered because the current required is not more than 0.1 ampere. This amount of current can be handled easily by the spiral springs. When the electrodynamometer is used as a voltmeter, its internal connections and construction are as shown in view A of figure 3-17. Fixed coils a and b are wound of fine wire since the current flow through them will not exceed 0.1 ampere. They are connected directly in series with movable coil c and the series current-limiting resistor.
Figure 3-17 Circuit arrangement of electrodynamometer for use as a voltmeter and an ammeter
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-36 UNCLASSIFIED 3.3.4 Ammeter When the electrodynamometer is used as an ammeter, a special type of construction must be used. This is because the large currents that flow through the meter cannot be carried through the moving coils. In the ammeter in view B of figure 3-17, stationary coils a and b are wound of heavier wire to carry up to 5.0 amperes. An inductive shunt (XL) is wired in parallel with the moving coils and permits only a small part of the total current to flow through the moving coil. The current flowing through the moving coil is directly proportional to the total current flowing through the instrument. The shunt has the same ratio of reactance to resistance as the moving coil does. Therefore, the instrument will be reasonably correct at frequencies at which it is used if ac currents are to be measured.
3.3.5 Wattmeter Electric power is measured by means of a wattmeter. This instrument is of the electrodynamometer type. As shown in figure 3-18, it consists of a pair of fixed coils, known as current coils, and a moving coil, called the voltage (potential) coil. The fixed current coils are wound with a few turns of a relatively large conductor. The voltage coil is wound with many turns of fine wire. It is mounted on a shaft that is supported in jeweled bearings so that it can turn inside the stationary coils. The movable coil carries a needle (pointer) that moves over a suitably graduated scale. Coil springs hold the needle at the zero position in the absence of a signal.
Figure 3-18 Simplified electrodynamometer wattmeter circuit
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-37 UNCLASSIFIED 3.3.5.1 Wattmeter Connection The current coil of the wattmeter is connected in series with the circuit (load), and the voltage coil is connected across the line. When line current flows through the current coil of a wattmeter, a field is set up around the coil. The strength of this field is in phase with and proportional to the line current. The voltage coil of the wattmeter generally has a high-resistance resistor connected in series with it. The purpose for this connection is to make the voltage-coil circuit of the meter as purely resistive as possible. As a result, current in the voltage circuit is practically in phase with line voltage. Therefore, when voltage is impressed on the voltage circuit, current is proportional to and in phase with the line voltage. Figure 3-19 shows the proper way to connect a wattmeter into a circuit.
3.3.5.2 Wattmeter Errors Electrodynamic wattmeters are subject to errors arising from such factors as temperature and frequency. For example, heat through the coils eventually causes the small springs attached to the pointer to lengthen and lose tension, which produces deflection errors. Large currents through the wattmeter also produce a noticeable deflection error. These errors are caused by the heat (I 2R) loss through coils from the application of high currents. Because of this, the maximum current range of electrodynamic wattmeters is normally restricted to approximately 20 amperes. The voltage range of wattmeters is usually limited to several hundred volts because of heat dissipation within the voltage circuit. However, the voltage range can be extended by the use of voltage multipliers.
Figure 3-19 Wattmeter connection
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-38 UNCLASSIFIED Good-quality, portable wattmeters usually have an accuracy of 0.2 to 0.25 percent. You must remember, though, that electrodynamic wattmeter errors increase with frequency. For the higher frequency and power ranges, special types of wattmeters are made specifically for those ranges. We will discuss two such wattmeters in chapter 5 of this module
3.3.5.3 Wattmeter Overloads The wattmeter consists of two circuits, either of which will be damaged if too much current passes through them. You should be especially aware of this fact because the reading on the instrument will not tell you whether or not the coils are being overheated. If an ammeter or voltmeter is overloaded, the pointer will indicate beyond the upper limit of its scale. In the wattmeter, both the current and potential circuit may carry such an overload that their insulations burn; yet the pointer may be only part of the way up the scale. This is because the position of the pointer depends upon the power factor of the circuit as well as upon the voltage and current. Therefore, a low power-factor circuit will provide a very low reading on the wattmeter. The reading will be low, even when the current and voltage circuits are loaded to the maximum safe limit. The safe rating for each wattmeter is always distinctly rated, not in watts, but in volts and amperes.
3.4 TECHNIQUES FOR METER USE We have considered the more common meters; now let’s consider some of the techniques employed in their use. The techniques suggested here are not all-inclusive. You will find, as you develop your technical skills, other variations and techniques in use. Consider the techniques for measuring current in a circuit. You can accomplish this by placing an ammeter in series with the circuit or by measuring the voltage across a resistor of known value and using Ohm’s law to figure current. This last technique has the advantage of eliminating the necessity of opening the circuit to connect the ammeter.
3.4.1 Continuity Tests Open circuits are those in which the flow of current is interrupted by a broken wire, defective switch, or any means by which the current cannot flow. The test used to detect open circuits (or to see if the circuit is complete or continuous) is continuity testing.
An ohmmeter (which contains its own batteries) is excellent for use in a continuity test. Normally, continuity tests are performed in circuits where the resistance is very low, such as the resistance of a copper conductor. An open is indicated in these circuits by a very high or infinite resistance between two continuously connected points.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-39 UNCLASSIFIED Figure 3-20 shows a continuity test of a cable that connects two electronic units. Notice that both plugs are disconnected and the ohmmeter is in series with conductor D under test. The power should be off. When checking conductors A, B, and C (connection of ohmmeter to conductors not shown), the current from the ohmmeter flows through plug 2 (female) through conductor A, B, or C to plug 1 (female). From plug 1, current passes through the jumper to the chassis, which is "grounded" to the ship’s structure. The metal structure serves as the return path to the chassis of unit 2 and completes the circuit through the series-connected ohmmeter. The ohmmeter indicates a low resistance because no break exists in conductors A, B, or C. However, checking conductor D reveals an open. The ohmmeter is shown indicating maximum resistance because current cannot flow in an open circuit. With an open circuit, the ohmmeter needle is all the way to the left since it is a series-type ohmmeter (reads right to left).
Where conditions are such that the ship’s structure cannot be used as the return path, one of the other conductors (known to be good) may be used. For example, to check D, you can connect a jumper from pin D to pin A of plug 1 (female) and the ohmmeter leads to pins D and A of plug 2 (female). This technique will also reveal the open in the circuit.
Figure 3-20 Continuity test
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-40 UNCLASSIFIED 3.4.2 Testing For Grounds Grounded circuits are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship. Grounds can have many causes. The two most common are the fraying of insulation from a wire and moisture-soaked insulation. The fraying of insulation from a wire allows bare wire to come into contact with the metal ground. Moisture-soaked insulation causes reduced insulation resistance (also classified as a ground).
Grounds are usually indicated by blown fuses or tripped circuit breakers. Blown fuses or tripped circuit breakers, however, can also result from a short circuit other than a ground. A high-resistance ground can also occur when current is increased significantly but not enough to rupture the fuse or trip the circuit breaker.
CAUTION
Before testing any circuit, ensure the circuit under test has been de-energized and checked with a safety shorting probe.
In testing for grounds, you may use a megger or an ohmmeter. Measuring the resistance to ground from points in a circuit determines if the point is grounded. Referring again to figure 3-20, you can see one possible means of testing a cable for grounds. If the jumper is removed from pin D of plug 1 (female), a test for ground can be made for each conductor in the cable. You can do this by connecting one meter lead to ground and the other to each of the pins of either of the plugs. A low resistance indicates that some part of that conductor or one of the plug assemblies is grounded. Both plugs must be removed from their units; if only one plug is removed, a false indication is possible because a conductor may be grounded through the unit.
3.4.3 Testing For Shorts A short circuit, other than a grounded one, is one where two conductors touch each other directly or through another conducting element. Two conductors with frayed insulation may touch and cause a short. Too much solder on the pin of a connector may short to the adjacent pin. In a short circuit, enough current may or may not flow to blow a fuse or open a circuit breaker. A short may occur between two cables carrying signals but might not be indicated by a blown fuse.
Shorts occur in many components, such as transformers, motor windings, and capacitors. The major test method used to detect shorts in such components is to measure resistance. The indicated resistance is then compared with the resistance given on schematics or in the equipment technical manuals to determine whether the measured value is within specifications.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-41 UNCLASSIFIED An ohmmeter is the device used to check for shorts. You can use the ohmmeter to detect a short between two conductors by measuring the resistance between them (be sure electrical power has been disconnected). A low resistance reading indicates a short. You can test the circuit in figure 3-20 for a short by first removing the jumper and disconnecting both plugs; you then measure the resistance between the two suspended conductors.
WARNING
The following section discusses voltage measurements on live circuits. BE SURE YOU ALWAYS FOLLOW PRESCRIBED SAFETY RULES WHEN MEASURING VOLTAGES.
3.4.4 Voltage Tests Voltage tests must be made with the power applied; therefore, the prescribed safety precautions must be followed to prevent injury to personnel and damage to the equipment. You will find in your maintenance work that the voltage test is of utmost importance. It is used not only in isolating casualties to major components but also in the maintenance of subassemblies, units, and circuits. Before checking a circuit voltage, you should check the voltage of the power source to be sure that the normal voltage is being applied to the circuit.
The voltmeter is used for voltage tests. In using the voltmeter, make certain that the meter used is designed for the type of current (ac or dc) to be tested and has a scale with a suitable range. Since defective parts in a circuit can cause higher than normal voltages to be present at the point of test, the highest voltmeter range available should be used first. Once you have obtained a reading, determine if a lower scale can be used that will cause no damage to the meter movement. If so, use the lower scale. This provides a more accurate reading.
Another consideration in the circuit voltage test is the resistance and current in the circuit. A low resistance in a high-current circuit could result in considerable voltage drop, whereas the same resistance in a low-current circuit may be minimal. Abnormal resistance in part of a circuit can be checked with either an ohmmeter or a voltmeter. Where practical, an ohmmeter should be used because the test is then carried out with a "dead" circuit.
The majority of the electronic circuits you will encounter in equipment will be low- current circuits, and most voltage readings will be direct current. Also, many of the schematics will indicate the voltages at various test points. Therefore, if you suspect that a certain stage is defective, you can check the voltage by connecting a voltmeter from the test point to ground. If the suspected stage is not defective, the voltmeter readings should match the voltages given on the schematic.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-42 UNCLASSIFIED Some technical manuals also contain voltage charts on which all the voltage measurements are tabulated. These charts usually indicate the sensitivity of the meter (for example, 20,000 ohms/volt) used to obtain the voltage readings for the chart. To obtain comparable results, you must use a voltmeter of the same sensitivity (or greater) as that specified. Make certain that the voltmeter is not "loading" the circuit while taking a measurement. If the meter resistance is not considerably higher than the circuit resistance, the reading will be markedly lower than the true circuit voltage because of the voltmeter’s loading effect. (To calculate meter resistance, multiply the rated ohms-per- volt sensitivity value of the meter by the scale in use. For example, a 1,000-ohms-per-volt meter set to the 300-volt scale will have a resistance of 300,000 ohms.)
3.4.5 Resistance Tests Before checking the resistance of a circuit or of a part, make certain that the power has been turned off. Also make sure capacitors in the associated circuit are fully discharged. To check continuity, always use the lowest ohmmeter range. If the highest range is used, the meter may indicate zero, even though appreciable resistance is present in the circuit. Conversely, to check a high resistance, use the highest scale since the lower range scale may indicate infinity, even though the resistance is less than a megohm. In making resistance tests, you must remember that even though the external ohmmeter leads are connected in parallel with the circuit to be measured, the internal meter circuitry is electrically connected in series.
In making resistance tests, take into account that other circuits containing resistances and capacitances may be in parallel with the circuit to be measured. Erroneous conclusions may be drawn from readings obtained in such cases. Remember, a capacitor blocks the dc flow from the ohmmeter. To obtain an accurate reading when other parts are connected across the suspected circuit, disconnect one end of the circuit to be measured from the equipment. For example, many of the resistors in major components and subassemblies are connected across transformer windings. To obtain a valid resistance measurement, you must isolate the resistors to be measured from the shunt resistances of the coils of the transformers.
Resistance tests are also used to check a component for grounds. In these tests, the component to be tested should be disconnected from the rest of the circuit so that no normal circuit ground will exist. Dismounting the component to be checked is not necessary. The ohmmeter is set for a high-resistance range. Then the ohmmeter is connected between ground and each electrically separate circuit of the component being tested. Any resistance reading less than infinity indicates at least a partial ground. You can also check capacitors suspected of being short-circuited by measuring the resistance. To check a capacitor suspected of being open, temporarily shunt a known good capacitor then recheck the performance of the circuit.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-43 UNCLASSIFIED CAUTION
To avoid possible damage to equipment during resistance tests, observe the following precautions:
• Always connect an ammeter in series—never in parallel. • Connect a voltmeter in parallel. • Never connect an ohmmeter to a live circuit. • Observe polarity when using a dc ammeter or a dc voltmeter. • View meters directly from the front. When viewed from an angle off to the side, an incorrect reading will result because of OPTICAL PARALLAX. (Parallax was covered in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.) • Always choose an instrument suitable for the measurement desired. • Select the highest range first and then switch to the proper range. • In using a meter, choose a scale that will result in an indication as near midscale as possible. • Do not mount or use instruments in the presence of a strong magnetic field. • Remember, a low internal resistance voltmeter (low sensitivity) may shunt the circuit being measured and result in incorrect readings.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-44 UNCLASSIFIED 3.5 SUMMARY The important points of this chapter are summarized in the following paragraphs. You should be familiar with these points before continuing with your study of test equipment.
A permanent-magnet, moving-coil meter movement (D’ARSONVAL movement) uses the interaction of magnetic fields to produce movement.
DAMPING is used to smooth out the vibration and to help prevent overshooting of the meter pointer.
ELECTRODYNAMOMETER movements are usually used in wattmeters. They operate much like the D’Arsonval meter movement, except field coils are used instead of a permanent magnet. Electrodynamometer movements measure either ac or dc without the use of a rectifier.
A SHUNT is a physically large, low-resistance conductor connected in parallel with the meter terminals. It carries the majority of the load current so that only a small portion of the total current will flow through the meter coil.
An AMMETER measures current and is always connected in series with the circuit being measured. An ammeter should have a low resistance so that the effect of the ammeter on the circuit will be kept to a minimum.
VOLTMETERS are used to measure voltage and are always connected in parallel with the circuit being measured. A voltmeter should have a high resistance compared to the circuit being measured to minimize the loading effect. Voltmeter sensitivity is expressed in ohms per volt.
OHMMETERS are used to measure resistance and to check continuity. An ohmmeter is electrically connected in series with the resistance being measured. The ohmmeter range, which allows a midscale deflection, should be used.
A MEGOHMMETER (MEGGER) is used to measure very high resistance, such as the insulation of wiring.
A WATTMETER is usually an electrodynamometer and is used to measure power.
A CONTINUITY TEST is accomplished with an ohmmeter. This test is used to check for opens (or to see if the circuit is complete or continuous).
GROUNDED CIRCUITS are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship or chassis. In testing for grounds, you may use either an ohmmeter or a megger.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-45 UNCLASSIFIED A SHORT CIRCUIT, other than a grounded one, is where two conductors touch each other directly or through another conducting element. An ohmmeter is used to test for shorts.
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-46 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q29.
A-1. Self-excited.
A-2. Phosphor bronze ribbons.
A-3. The pointer arrangement and the light and mirror arrangement.
A-4. Coil balance.
A-5. Hairspring.
A-6. Hairspring.
A-7. Makes it possible to have a more linear scale than if the poles were flat.
A-8. Shunt.
A-9. Zero-temperature coefficient.
A-10. Midscale.
A-11. In series.
A-12. Negative, positive.
A-13. False.
A-14. Meter-loading.
A-15. A multimeter (high resistance) is placed in series with the coil of the meter.
A-16. The current required for full-scale deflection, and the range of the voltage to be measured.
A-17. In parallel.
A-18. High.
A-19. Ohms per volt.
A-20. Megohmmeter (megger).
NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED
3-47 UNCLASSIFIED A-21. 1. A source of dc potential. 2. One or more resistors (one of which is variable).
A-22. Zero.
A-23. 1/10.
A-24. Shunts leakage current, which prevents false readings.
A-25. 500.
A-26. Friction clutches.
A-27. Fixed coils.
A-28. Size of spiral conducting.
A-29. The electrodynamometer-type meter can be used to measure both ac and dc currents.