FIGURE 8-114. Double-pole single-throw knife and toggle switches.
contact is attached to the push button. The push button is either an insulator itself or is insulated from the contact. This switch is springloaded and designed for momentary contact.
Microswitches A microswitch will open or close a circuit with a very small movement of the tripping device (1/16 inch or less). This is what gives the switch its name, since micro means small.
Microswitches are usually push-button switches.
They are used primarily as limit switches to provide automatic control of landing gears, actuator motors, and the like. The diagram in figure 8-117 shows a normally closed microswitch in cross section and illustrates how these switches operate. When the operating plunger is pressed in, the spring and the movable contact are pushed, opening the contacts and the circuit.
Rotary-Selector Switches A rotary-selector switch takes the place of several switches. As shown in figure 8-118, when the knob of the switch is rotated, the switch opens one circuit and closes another. Ignition switches and voltmeter selector switches are typical examples of this kind of switch.
Relays Relays, or relay switches, are used for remote control of circuits carrying heavy currents, A relay is connected in the circuit between the unit controlled and the nearest source of power (or power bus bar) so that the cables carrying heavy current will be as short as possible.
A relay switch consists of a coil, or solenoid, an iron core, and both fixed and movable contacts.
A small wire connects one of the coil terminals (which is insulated from the housing) to the source of power through a control switch usually located in the cockpit. The other coil terminal is usually grounded to the housing. When the control switch is closed, an electromagnetic field is set up around the coil.
In one type of relay switch, an iron core is fixed firmly in place inside the coil. When the control switch is closed, the core is magnetized and pulls a soft iron armature toward it, closing the main contacts. The contacts are spring-loaded to the open position as shown in figure 8-119. When the control switch is turned off, the magnetic field collapses and the spring opens the contacts.
In another type of relay switch, part of the core is movable. A spring holds the movable part a short distance away from the fixed part, as illus- FIGURE 8-115. Double-pole double-throw knife and toggle switches.
Single-pole single-throw Double-pole single-throw Single-pole double-throw Double-pole double-throw FIGURE 8-116. Schematic representation for typical switches.
trated in figure 8-120. When the coil is energized, the magnetic field tries to pull the movable part of the core into the coil. This pull overcomes the spring tension. As the core moves inward, it brings the movable contacts, which are attached to but insulated from it, down against the stationary contacts. This completes the main circuit. When the control switch is turned off, the magnetic field collapses and the spring returns the movable core to its original position, opening the main Relays vary in construction details according to their intended use. When selecting a relay to be installed in a circuit, make sure it is designed for the job it is intended to do.
Some relay switches are made to operate continuously, while others are designed to operate only intermittently. The starter-relay switch is Stationary contact Operating plunger. Movable contact made to operate intermittently and would overheat if used continuously. The battery-relay switch can be operated continuously because its coil has a fairly high resistance which prevents overheating.
In a circuit carrying a large current, the more quickly the circuit is opened the less it will are at the relay, and the less the switch contacts will be burned. Relays used in circuits with large motors have strong return springs to open the circuit quickly.
Most of the relays use in the a.c. circuitry of an aircraft are energized by d.c. current. These devices will be discussed, as necessary, in the appropriate areas covering alternating current devices.
D. C. MEASURING INSTRUMENTS Understanding the functional design and operation of electrical measuring instruments is very important, since they are used in repairing, maintaining, and troubleshooting electrical circuits. While some meters can be used for both
d.c. and a.c. circuit measurement, only those used as d.c. instruments are discussed in this section.
The meters used for a.c., or for both a.c. and d.c., are discussed in the study of a.c. theory and Effects of Current The effect of current may be classified as follows:
chemical, physiological, photoelectric, piezoelectric, thermal, or electromagnetic.
Grooved anchor Three-bladed spring Chemical When an electric current is passed through cer- FIGURE 8-117. Cross section of a microswitch. tain solutions, a chemical reaction takes place and 320
Rotor pole Contacts FIGURE 8-118. Rotary-selector switch.
a deposit forms on one electrode. The amount of this deposit is proportional to the amount of current. Industrially, this process is useful in electroplating and electrolysis. Although the chemical effect is useful in defining the standard ampere (the amount of current which causes .001118 grams of silver to be deposited in one second from a 15 percent solution of silver nitrate), it is of no practical use in meters.
Physiological The physiological effect of current refers to the reaction of the human body to an electric current, To high power circuit An electric shock, although painful at times, is too difficult to evaluate quantitatively and is, therefore, not practical for use in meters.
Photoelectric When electrons strike certain materials, a glow appears at the point of contact. The picture tube of a television set and the scope of a radar set illustrate this effect. Using the intensity of the light produced as a means of measuring the amount of current is neither accurate nor practical.
To high power circuit Movable contactor Movable contactor Stationary Stationary contact contact Fixed core - Return spring Solenoid coil Movable core - Solenoid coil Low current control circuit Low current control circuit FIGURE 8-119. Fixed-core relay. FIGURE 8-120. Movable-core relay.
Piezoelectric Certain crystals such as quartz and Rochelle salts become deformed when a voltage is applied across two of the crystal faces. This effect is not visible to the human eye and is, therefore, impractical for use in meters.
Thermal When a current flows through a resistance, heat is produced. The amount of heat produced is equal to IR. This relationship establishes that heat varies as the square of the current. Meters which employ the thermal effect in their operation are common.
Electromagnetic Whenever electrons flow through a conductor, a magnetic field proportional to the current is created. This effect is useful for measuring current and is employed in many practical meters.
The first four effects discussed are of no practical importance as electrical measuring devices.
The last two effects, thermal and magnetic, are of practical use in meters. Since most of the meters in use have D'Arsonval movements, which operate because of the magnetic effect, only this type will be discussed in detail.
D'Arsonval Meter The basic d.c. meter movement is known as the D'Arsonval meter movement because it was first employed by the French scientist, D'Arsonval, in making electrical measurement. This type of meter movement is a current-measuring device which is used in the ammeter, voltmeter, and ohmmeter. Basically, both the ammeter and the voltmeter are current-measuring instruments, the principal difference being the method in which they are connected in a circuit. While an ohmmeter is also basically a current-measuring instrument, it differs from the ammeter and voltmeter in that it provides its own source of power and contains other auxiliary circuits.
Ammeter The D'Arsonval ammeter is an instrument designed for measuring direct current flowing in an electrical circuit and consists of the following parts: a permanent magnet, a moving element Permanent magnet Upper control — spring Coil wound on , aluminum Pointer frame
• Iron core Lower control spring Pointed shaft for needle bearing FIGURE 8-121. Moving-coil element with pointer and springs.
mounting, bearings, and a case which includes terminals, a dial, and screws. Each part and its function are described in the discussion which follows.
The permanent magnet furnishes a magnetic field which will react with the magnetic field set up by the moving element.
The moving element is mounted so that it is tree to rotate when energized by the current to be measured. A pointer which moves across a calibrated scale is attached to this element. A movingcoil mechanism is shown in figure 8-121. The controlling element is a spring, or springs, whose main function is to provide a counter or restoring force. The strength of this force increases with the turning of the moving element and brings the pointer to rest at some point on the scale. Two springs are generally used; they are wound in opposite drections to compensate for the expansion and contraction of the spring material due to temperature variation. The springs are made of nonmagnetic material and conduct current to and from the moving coil in some meters.
The moving element consists of a shaft with very hard pivot points to carry the moving coil or other movable element (figure 8-121). The pivot points are so fitted into highly polished jewels or very hard glass bearings that the moving element can rotate with very little friction. Another type of mounting has been 322
FIGURE 8-123. D'Arsonval meter movement.
scale, units of measurement, and meter uses. The terminals are made of materials having very low electrical resistance. Their funetion is to conduct the required current into and away from the meter.
Pivot FIGURE 8-122. Method of mounting moving elements.
designed in which the pivot points are reversed and the bearings are inside the moving-coil assembly. A method of mounting moving elements is shown in figure 8-122.
The bearings are highly polished jewels such as sapphires, synthetic jewels, or very hard glass.
These are usually round and have a conical depression in which the pivots rotate. They are set in threaded nuts which allow adjustment. The radius of the depression in the jewel is greater than the radius of the pivot point. This limits the area of contact surfaces and provides a bearing which, when operated dry, probably has the lowest constant friction value of any known type of bearing.
The case houses the instrument movement and protects it from mechanical injury and exposure.
It also has a window for viewing the movement of the pointer across a calibrated scale. The dial has printed on it pertinent information such as the Operation of the Meter Movement The major units are mounted in their relationship to one another (figure 8-123). Note that the coil portion of the moving element is in the magnetic field of the permanent magnet.
In order to understand how the meter works, assume that the coil of the moving element is placed in a magnetic field as shown in figure 8-124.
The coil is pivoted so that it is able to rotate back and forth within the magnetic field set up by the magnet. When the coil is connected in a circuit, current flows through the coil in the direction indicated by the arrows and sets up a magnetic field within the coil. This field has the same polarity as the adjacent poles of the magnet.
The interaction of the two fields causes the coil to rotate to a position so that the two magnetie fields are aligned. This force of rotation (torque) is proportional to the interaction between the like poles of the coil and the magnet and, therefore, to FIGURE 8-124. Effect of a coil in a magnetic field.
the amount of current flow in the coil. As a result, a pointer attached to the coil will indicate the amount of current flowing in the circuit as it moves across a graduated scale.
In the arrangement just discussed, note that any torque sufficient to overcome the inertia and friction of moving parts causes the coil to rotate until the fields align. This uncontrolled movement would cause inaccruate current readings. Therefore, the turning motion of the coil is opposed by two springs. The value of the current flowing through the coil determines the turning force of the coil. When the turning force is equal to the opposition of the springs, the coil stops moving and the pointer indicates the current reading on a calibrated scale. In some meters the springs are made of conducting material and conduct current to and from the coil. The pole pieces of the magnet form a circular air gap within which the coil is pivoted.
To obtain a clockwise rotation, the north pole of the permanent magnet and that of the coil must be adjacent. The current flowing through the coil must, therefore, always be in the same direction. The D'Arsonval movement can be used only for d.c. measurements and the correct polarity must be observed. If the current is allowed to flow in the wrong direction through the coil, the coil will rotate counterclockwise and the pointer will be damaged. Since the movement of the coil is directly proportional to the current through the coil, the scale is normally a linear scale.
Damping in order that meter readings can be made quickly and accurately, it is desirable that the moving pointer overshoot its proper position only a small amount and come to rest after not more than one or two small oscillations. The term "damping" is applied to methods used to bring the pointer of an electrical meter to rest after it has been set in motion. Damping may be accomplished by electrical means, by mechanical means, or by a combination of both.
Electrical Damping A common method of damping by electrical means is to wind the moving coil on an aluminum frame. As the coil moves in the field of the permanent magnet, eddy currents are set up in the aluminum frame. The magnetic field Closed air chamber Light vane swinging in chamber
•with small clearance FIGURE 8-125. Air damping.
produced by the eddy currents opposes the motion of the coil. The pointer will therefore swing more slowly to its proper position and come to rest quickly with very little oscillation.
Mechanical Damping Air damping is a common method of damping by mechanical means. As shown in figure 8-125, a vane is attached to the shaft of the moving element and enclosed in an air chamber.
The movement of the shaft is retarded because of the resistance which the air offers to the vane.
Effective damping is achieved if the vane nearly touches the walls of the chamber.
Meter Sensitivity The sensitivity of a meter movement is usually expressed as the amount of current required to give full-scale deflection. In addition, the sensitivity may be expressed as the number of millivolts across the meter when full-scale current flows through it. This voltage drop is obtained by multiplying the full-scale current by the resistance of the meter movement. A meter movement, whose resistance is 50 ohms and which requires 1 milliampere (ma.) for full-scale reading, may be described as a 50-millivolt 0-1 milliammeter.
Extending the Range of an Ammeter A 0-1 milliammeter movement may be used to measure currents greater than 1 ma. by connecting a resistor in parallel with the movement. The parallel resistor is called a shunt because it bypasses a portion of the current around the movement, extending the range of the ammeter.
10 ma.
0-1 ma 10 ma. 1 ma. 9 ma.
FIGURE 8-126. Meter movement with shunt.
A schematic drawing of a meter movement with a shunt connected across it to extend its range is shown in figure 8-126.
Determining the Value of a Shunt The value of a shunt resistor can be computed by applying the basic rules for parallel circuits.
If a 50 millivolt 0-1 milliammeter is to be used to measure values of current up to 10 ma., the following procedure can be used: The first step involves drawing a schematic of the meter shunted by a resistor labeled R, (shunt resistor), as shown in figure 8-127.
Since the sensitivity of the meter is known, the meter resistance can be computed. The circuit is then redrawn as shown in figure 8-128, and the branch currents can be computed, since a maximum of I ma. can flow through the meter.
The voltage drop across R, is the same as that across the meter, Rm:
E = IR = 0.001 × 50 = 0.050 volt.
R, can be found by applying Ohm's law:
Rs = Irs Kirs
0.050 = 0.009 = 5.55 ohms.
The value of the shunt resistor (5.55 Q) is very small, but this value is critical. Resistors used as shunts musts must have close tolerances, usually 1 percent.
FIGURE 8-127. Circuit schematic for shunt resistor.
Universal Ammeter Shunt The schematic drawing in figure 8-129, the universal shunt, shows an arrangement whereby two or more ranges are provided by tapping the shunt resistor at the proper points. In this arrangement, a 0-5 ma. movement with a resistance of 20 ohms is shunted to provide a 0-25 ma. range and a 0-50 ma. range.
Ammeters having a number of internal shunts are called multirange ammeters. A scale for each range is provided on the meter face (figure 8-130).
Some multimeters avoid internal switching through the use of external shunts. Changing ammeter ranges involves the selection and installation on the meter case of the proper size shunt.
MULTIMETERS Ammeters are commonly incorporated in multiple-purpose instruments such as multimeter or volt-ohm-milliammeters. These instruments vary somewhat according to the design used by different manufacturers, but most incorporate the functions of an ammeter, a voltmeter, and an ohmmeter in one unit. A typical multimeter is shown in figure 8-131. This multimeter has two 10 ma.
> 50 g 1 1 ma. 9 ma.
10 ma.
FIGURE 8-128. Equivalent meter circuit.
0.5 ma.
2012
• ONE AMPERE 50 ma. 25 ma.
FIGURE 8-129. Universal ammeter shunt.
selector switches: a function switch and a range switch. Since a multimeter is actually three meters in one case, the function switch must be placed in proper position for the type of measurement to be made. In figure 8-131, the function switch is shown in the ammeter position to measure d.c. milliamperes and the range switch is set at 1000. Set in this manner, the ammeter can measure up to 1,000 milliamperes or 1 ampere.
Multimeters have several scales, and the one used should correspond properly to the position of the range switch. If current of unknown value is to be measured, always select the highest possible range to avoid damage to the meter. The test leads should always be connected to the meter in the manner prescribed by the manufacturer. Usually the red lead is positive and the black lead is negative, or common. Many multimeters employ color coded jacks as an aid 3 - - 300 FIGURE 8-130. A multirange ammeter.
DCMA Function 1000 switch Range switch FIGURE 8-131. A multimeter set to measure one ampere.
in connecting the meter into the circuit to be tested. In figure 8-132, a multimeter properly set to measure current flow is connected into a circuit.
The precautions to be observed when using an ammeter are summarized as follows:
1. Always connect an ammeter in series with the element through which the current flow is to be measured.
2. Never connect an ammeter across a source of voltage, such as a battery or generator.
Remember that the resistance of an ammeter, particularly on the higher ranges, is extremely low and that any voltage, even a volt or so, can cause very high current to flow through the meter, causing damage to it.
3. Use a range large enough to keep the deflection less than full scale. Before measuring a current, form some idea of its magnitude. Then switch to a large enough scale or start with the highest range and work down until the appropriate scale is reached. The most accurate readings are obtained at approximately half-scale deflection. Many milliammeters have been ruined by attempts to measure amperes.
Therefore, be sure to read the lettering either on the dial or on the switch positions and choose proper scale before connecting the meter in the circuit.
4. Observe proper polarity in connecting the meter in the circuit. Current must flow 326
• DCMA BLACK + RED FIGURE 8-132. A multimeter set to measure current flow.
through the coil in a definite direction in order to move the indicator needle up-scale.
Current reversal because of incorrect connection in the circuit results in a reversed meter deflection and frequently causes bending of the meter needle. Avoid improper meter connections by observing the polarity markings on the meter.
this is done, the unit containing the resistance is commonly called a multiplier.
Extending the Voltmeter Range The value of the necessary series resistance is determined by the current required for full-scale deflection of the meter and by the range of voltage
VOLTMETER The D'Arsonval meter movement can be used either as an ammeter or a voltmeter (figure 8-133).
Thus, an ammeter can be converted to a voltmeter by placing a resistance in series with the meter coil and measuring the current flowing through it.
In other words, a voltmeter is a current-measuring instrument, designed to indicate voltage by measuring the current flow through a resistance of known value. Various voltage ranges can be obtained by adding resistors in series with the meter coil. For low-range instruments, this resistance is mounted inside the case with the D'Arsonval movement and usually consists of resistance wire having a low temperature coefficient which is wound either on spools or card frames. For higher voltage ranges, the series FIGURE 8-133. Simplified diagram of a resistance may be connected externally. When voltmeter.
meter coil and the series resistance is 100 мa 100 22 R=] - = 0.0001 - = 10,000 ohms, M 1 VOLT 100 VOLT and the series resistance alone is
• -.01 V -- 1000 VOLT R, = 10,000 - 100 = 9,900 ohms.
9,900 g 25 006 666 wn 9,999,900 R ment with the required resistances connected in Multirange voltmeters utilize one meter moveseries with the meter by a convenient switching arrangement. A multirange voltmeter circuit with three ranges is shown in figure 8-134. The total circuit resistance for each of the three ranges beginning with the 1-volt range is:
1 R = 7 = 100 = 0.01 megohm FIGURE 8-134. Multirange voltmeter schematic. 100 100 = 1 megohm to be measured. Because 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 series resistance.
For example, assume that the basic meter (microammeter) is to be made into a voltmeter with a full-scale reading of 1 volt. The coil resistance of the basic meter is 100 ohms, and
0.0001 ampere (100 microamperes) causes a fullscale deflection. The total resistance, R, of the VOLTS -I5V
0-1.5V- — O-150V FIGURE 8-135. Typical multirange voltmeter.
1,000 100 = 10 megohms.
Multirange voltmeters, like multirange ammeters, are used frequently. They are physically very similar to ammeters, and their multipliers are usually located inside the meter with suitable switches or sets of terminals on the outside of the meter for selecting ranges (see figure 8-135).
Voltage-measuring instruments are connected across (in parallel with) a circuit. If the approximate value of the voltage to be measured is not known, it is best, as in using the ammeter, to start with the highest range of the voltmeter and progressively lower the range until a suitable reading is obtained.
In many cases, the voltmeter is not a centralzero indicating instrument. Thus, it is necessary to observe the proper polarity when connecting the instrument to the circuit, as is the case when connecting the d.c. ammeter. The positive terminal of the voltmeter is always connected to the positive terminal of the source, and the negative terminal to the negative terminal of the source, when the source voltage is being measured. In any case, the voltmeter is connected so that electrons will flow into the negative terminal and out of the positive terminal of the meter. In figure 8-136 a multimeter is properly connected to a circuit to measure the voltage drop across a resistor. The function switch is set at the d.c. volts position and the range switch is placed in the 50-volt position.
Red DCV Black 0-30V
VARIABLE SOURCE FiGURE 8-136. A multimeter connected to measure a circuit voltage drop.
The function of a voltmeter is to indicate the potential difference between two points in a circuit. When the voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has low resistance, it will draw an appreciable amount of current. The effective resistance of the circuit will be lowered, and the voltage reading will consequently be lowered.
When voltage measurements are made in high-resistance circuits, it is necessary to use a high-resistance voltmeter to prevent the shunting action of the meter. The effect is less noticeable in low-resistance circuits because the shunting effect by increasing the strength of the permanent magnet, by using lighter weight materials for the moving element (consistent with increased number of turns on the coil), and by using sapphire jewel bearings to support the moving coil.
Voltmeter Accuracy The accuracy of a meter is generally expressed in percent. For example, a meter with an accuracy of 1 percent will indicate a value within 1 percent of the correct value. The statement means that, if the correct value is 100 units, the meter indication may be anywhere within the range of 99 to 101 Voltmeter Sensitivity OHMMETERS The sensitivity of a voltmeter is given in ohms Two instruments are commonly used to check per volt (Q/E) and is determined by dividing the the continuity or to measure the resistance of a resistance (Rm) of the meter plus the series circuit or circuit element. These instruments are resistance (Rs) by the full-scale reading in volts. the ohmmeter and the megger, or megohmmeter.
The ohmmeter is widely used to measure resistance sensitivity = Rm + R. and to check the continuity of electrical circuits and devices. Its range usually extends to a few E inegohms. The megger is widely used for measuring This is the same as saying that the sensitivity is insulation resistance, such as the resistance equal to the reciprocal of the current (in amperes); between the windings and the frame of electric that is, machinery, and for measuring the insulation resistance of cables, insulators, and bushings. Its sensitivity = ohms 1 range may extend to more than 1,000 megohms.
volts volts amperes When measuring very high resistances of this ohms nature, it is not necessary to find the exact value of resistance, but rather to know that the insula- Thus, the sensitivity of a 100-microampere tion is either above or below a certain standard.
movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.
The sensitivity of a voltmeter can be increased When precision measurements are required, some type of bridge circuit is used. Ohmmeters may be of the series or shunt type.
OHMS E. _ 4.5V 37.5K 13.5K 9K 4.5K ISK B 0-1 ma 2.5
MILLIAMPERES FIGURE 8-137. Ohmmeter circuit.
Series-type Ohmmeters A simplified schematic of an ohmmeter is shown in figure 8-137. E is a source of EMF; R, is a variable resistor used to zero the meter; R2 is a fixed resistor used to limit the current in the meter movement; and A and B are test terminals across which the resistance to be measured is placed.
If A and B are connected together (shortcircuited), the meter, the battery, and resistors R1 and R2 form a simple series circuit. With R1 adjusted so that the total resistance in the circuit is 4,500 ohms, the current through the meter is 1 ma. and the needle deflects full scale. Since there is no resistance between A and B, this position of the needle is labeled zero (figure 8-138). If a resistance equal to 4,500 ohms is placed between terminals A and B, the total resistance is 9,000 ohms and the current is .5 ma.
This causes the needle to deflect half scale. This half-scale reading, labeled 4.5 K ohms, is equal to the internal resistance of the meter, in this instance 4,500 ohms. If a resistance of 9,000 ohms is placed between terminals A and B, the needle deflects one-third scale. Resistances of 13.5 K and
1.5 K placed between terminals A and B will cause a deflection of one-fourth and three-fourths scale, respectively.
If terminals A and B are not connected (opencircuited), no current flows and the needle does not move. The left side of the scale is, therefore, labeled infinity to indicate an infinite resistance.
A typical ohmmeter scale is shown in figure
8-138. Note that the scale is not linear and is crowded at the high resistance end. For this reason, it is good practice to use an ohmmeter range in which the readings are not too far from mid-scale. A good rule is to use a range in which the reading obtained does not exceed ten times, or is not less than one-tenth, the mid-scale reading.
FIGURE 8-138. A typical ohmmeter scale.
The useful range of the scale shown is, by this rule, from 450 ohms to 45,000 ohms.
Most ohmmeters have more than one scale.
Additional scales are made possible by using various values of limiting resistors and battery voltages. Some ohmmeters have a special scale called a low-ohm scale for reading low resistances.
A shunt-type ohmmeter circuit is used for this scale.
Shunt-Type Ohmmeter Shunt-type ohmmeters are used to measure small values of resistance. In the circuit shown in figure 8-139, E (voltage) is applied across a limiting resistor R and a meter movement in series.
Resistance and battery values are chosen so that the meter movement deflects full scale when terminals A and B are open. When the terminals are short-circuited, the meter reads zero; the short circuit conducts all the current around the meter.
The unknown resistance Rz is placed between terminals A and B in parallel with the meter movement. The smaller the resistance value being measured, the less current flows through the meter The value of the limiting resistor R is usually made large compared to the resistance of the meter movement. This keeps the current drawn from the battery practically constant. Thus, the value of R.
determines how much of this constant current flows through the meter and how much through Rza Note that in a shunt-type ohmmeter, current is always flowing from the battery through the meter 330