appreciable changes occur in the electrolyte during charge or discharge. As a result, the battery charge cannot be determined by a specific gravity check of the electrolyte. The electrolyte level should be maintained just above the tops of the Operation of Nickel-Cadmium Cells When a charging current is applied to a nickelcadmium battery, the negative plates lose oxygen and begin forming metallic cadmium. The active material of the positive plates, nickel-hydroxide, becomes more highly oxidized. This process continues while the charging current is applied or until all the oxygen is removed from the negative plates and only cadmium remains.
Toward the end of the charging cycle the cells emit gas. This will also occur if the cells are overcharged. This gas is caused by decomposition of the water in the electrolyte into hydrogen at the negative plates and oxygen at the positive plates.
The voltage used during charging, as well as the temperature, determines when gassing will occur.
To completely charge a nickel-cadmium battery, some gassing, however slight, must take place;
thus, some water will be used.
The chemical action is reversed during discharge. The positive plates slowly give up oxygen, which is regained by the negative plates. This process results in the conversion of the chemical energy into electrical energy. During discharge the plates absorb a quantity of the electrolyte. On recharge, the level of the electrolyte rises and, at full charge, the electrolyte will be at its highest level. Therefore, water should be added only when the battery is fully charged.
The nickel-cadmium battery is usually interchangeable with the lead-acid type. When replacing a lead-acid battery with a nickel-cadmium battery, the battery compartment must be clean, dry, and free of all traces of acid from the old battery. The compartment must be washed out and neutralized with ammonia or boric acid solution, allowed to dry thoroughly, and then painted with an alkali-resisting varnish.
The pad in the battery sump jar should be saturated with a three percent (by weight) solution of boric acid and water before connecting the battery vent system.
Servicing Nickel-Cadmium Batteries There are significant differences in the servicing methods required for the nickel-cadmium batteries and those of the lead-acid batteries. The most important points to be observed are as follows:
A separate storage and maintenance area should be provided for nickel-cadmium batteries. The electrolyte is chemically opposite to the sulphuric acid used in a lead-acid battery. Fumes from a lead-acid battery can contaminate the electrolyte in a nickel-cadmium battery. This precaution should include equipment, such as hand tools and syringes, used with leadacid batteries. Indeed, every possible precaution must be taken to keep anything containing acid away from the nickelcadmium battery shop.
The potassium hydroxide electrolyte used in nickel-cadmium batteries is extremely Protective goggles, rubber gloves, and rubber aprons should be used to handle and service batteries. Suitable washing facilities should be provided in case electrolyte is spilled on clothing or the skin. Such exposures should be rinsed immediately with water or vinegar, lemon juice, or a boric acid solution. When potassium hydroxide and distilled water are mixed to make electrolyte, the potassium hydroxide should be added slowly to the water, not vice versa.
Severe arcing may result if a wire brush is used to clean a battery. The vent plugs should be closed during the cleaning process and the battery should never be cleaned with acids, solvents, or any chemical solution. Spilled electrolyte can react with carbon dioxide to form crystals of potassium carbonate. These, which are nontoxic and noncorrosive, can be loosened with a fiber brush and wiped off with a damp cloth. When potassium carbonate forms on a properly serviced battery, it may indicate the battery is overcharging because the voltage regulator is out of Additional water should never be added to the battery earlier than three or four hours after it has been fully charged.
Should it be necessary to add water, use only distilled or demineralized water.
(5) Since the electrolyte does not react chemically with the cell plates, the specific gravity of the electrolyte does not change appreci- 314
ably. Thus, it is not possible to determine the state of charge of the battery with a hydrometer; nor can the charge be determined by a voltage test because the voltage of a nickel-cadmium battery remains constant during 90 percent of the discharge At rection, constant-voltage or the constant-current method. For the constant potential charging, maintain the charging voltage constant until the charging current decays to 3 amperes or less assuring that the battery cell temperature does not exceed 100° F.
constant current charging start the charge and continue until the voltage reaches the desired potential, then reduce the current level to 4 amperes continuing the charging until its desired voltage or until the battery temperature exceeds 100° F. and the voltage begins to decline.
The troubleshooting chart outlined in figure 8-111 can be used as a guide in troubleshooting battery malfunctions.
CIRCUIT PROTECTIVE AND CONTROL DEVICES Electricity, when properly controlled, is of vital importance to the operation of aircraft.
When it is not properly controlled, it can become dangerous and destructive. It can destroy aircraft components or complete aircraft; it can injure personnel and even cause their death.
It is of the greatest importance, then, that all necessary precautions be taken to protect the electrical circuits and units in the aircraft and to keep this force under proper control at all times.
Protective Devices When an aircraft is built, the greatest care is taken to ensure that each electrical circuit is fully insulated from all others so that the current in a circuit will follow its intended individual path. Once the aircraft is put into service, however, there are many things that can happen to alter the original circuitry. Some of these changes can cause serious trouble if not detected and corrected in time.
Perhaps the most serious trouble in a circuit is a direct short. The term, "direct short," describes a situation in which some point in the circuit, where full system voltage is present, comes in direct contact with the ground or return side of the circuit. This establishes a path for current flow that contains no resistance other than that present in the wires carrying the current, and these wires have very little resistance.
According to Ohm's law, if the resistance in a circuit is small, the current will be great. When a direct short occurs, there will be an extremely heavy current flowing through the wires. Suppose, for instance, that the two leads from a battery to a motor came in contact with each other. Not only would the motor stop running because of the current going through the short, but the battery would become discharged quickly (perhaps destroyed); there would also be danger of fire.
The battery cables in this example would be very large wires, capable of carrying very heavy currents. Most wires used in aircraft electrical circuits are considerably smaller and their currentcarrying capacity is quite limited. The size of the wires used in any given circuit is determined by the amount of current the wires are expected to carry under normal operating condidions. Any current flow greatly in excess of normal, such as the case of a direct short, would cause a rapid generation of heat.
If the excessive current flow caused by the short is left unchecked, the heat in the wire will continue to increase until something gives way. Perhaps a portion of the wire will melt and open the circuit so that nothing is damaged other than the wires involved. The probability exists, however, that much greater damage would result. The heat in the wires could char and burn their insulation and that of other wires bundled with them, which could cause more shorts. If a fuel or oil leak is 315
OBSERVATION PROBABLE CAUSE CORRECTIVE ACTION High trickle charge-When charg- Defective cells.
ing at constant voltage of 28.5
(10.1) volts, current does not drop below 1 amp after a 30-minute charge.
While still charging, check individual cells. Those below .5 volts are defective and should be replaced. Those between .5 and 1.5 volts may be defective or may be unbalanced, those above 1.5 volts are all right.
High trickle charge after replacing Cell imbalance.
defective cells, or battery fails to meet amp-hour capacity check.
Discharge battery and short out individual cells for eight hours. Charge battery using constant current method. Check capacity and if O.K., recharge using constant current method.
Battery fails to deliver rated ca- Cell imbalance or faulty Repeat capacity check, discharge and pacity. cells. constant current charge a maximum of three times. If capacity does not develop, replace faulty cells.
No potential available. Complete battery failure. Check terminals and all electrical connections. Check for dry cell. Check for high trickle charge.
Excessive white crystal deposits on Excessive spewage.
cells. (There will always be some potassium carbonate present due to normal gassing.)
Battery subject to high charge current, high temperature or high liquid level. Clean battery, constant current charge and check liquid level. Check charger operation.
Distortion of cell case. Overcharge or high heat. Replace cell.
Foreign material in cells-black or Impure water, high heat, Adjust specific gravity and electrolyte gray particles. high concentration of level. Check battery for cell imbalance KOH or improper water or replace defective cell.
level.
Excessive corrosion of hardware. Defective or damaged Replace parts.
plating.
Heat or blue marks on hardware. Loose connections caus- Clean hardware and properly torque ing overheating of inter- connectors.
cell connector or hardware.
Excessive water consumption. Cell imbalance. Proceed as above for cell imbalance.
Dry cell.
FIGURE 8-111. Nickel-cadmium troubleshooting chart.
near any of the hot wires, a disastrous fire might be started.
To protect aircraft electrical systems from damage and failure caused by excessive current, several kinds of protective devices are installed in the systems. Fuses, circuit breakers, and thermal protectors are used for this purpose.
Circuit protective devices, as the name implies, all have a common purpose-to protect the units and the wires in the circuit. Some are designed primarily to protect the wiring and to open the circuit in such a way as to stop the current flow when the current becomes greater than the wires can safely carry. Other devices are designed to protect a unit in the circuit by stopping the current flow to it when the unit becomes excessively warm.
Fuses A fuse is a strip of metal that will melt when current in excess of its carefully determined capacity flows through it. The fuse is installed in the circuit so that all the current in the circuit passes through it. In most fuses, the strip of metal is made of an alloy of tin and bismuth. Other fuses are made of copper and are called current limiters;
these are used primarily to sectionalize an aircraft circuit.
A fuse melts and breaks the circuit when the current exceeds the rated capacity of the fuse, but a current limiter will stand a considerable overload for a short period of time. Since the fuse is intended to protect the circuit, it is quite important that its capacity match the needs of the circuit in which it is used. When a fuse is replaced, the applicable manufacturer's instructions should be consulted to be sure a fuse of the correct type and capacity is installed. Fuses are installed in two type fuse holders in aircraft. "Plug-in holders" are used for small type and low capacity fuses.
"Clip" type holders are used for heavy high capacity fuses and current limiters.
Circuit Breakers A circuit breaker is designed to break the circuit and stop the current flow when the current exceeds a predetermined value. It is commonly used in place of a fuse and may sometimes eliminate the need for a switch. A circuit breaker differs from a fuse in that it "trips" to break the circuit and it may be reset, while a fuse melts and must be replaced.
There are several types of circuit breakers in general use in aircraft systems. One is a magnetic type. When excessive current flows in the circuit, it makes an electromagnet strong enough to move a small armature which trips the breaker. Another type is the thermal overload switch or breaker.
This consists of a bimetallic strip which, when it becomes overheated from excessive current, bends away from a catch on the switch lever and permits the switch to trip open.
Most circuit breakers must be reset by hand.
When the circuit breaker is reset, if the overload condition still exists, the circuit breaker will trip again to prevent damage to the circuit.
Thermal Protectors A thermal protector, or switch, is used to protect a motor. It is designed to open the circuit automatically whenever the temperature of the motor becomes excessively high. It has two positions, open and closed. The most common use for a thermal switch is to keep a motor from overheating. If a malfunction in the motor causes it to overheat, the thermal switch will break the circuit intermittently.
The thermal switch contains a bimetallic disk, or strip, which bends and breaks the circuit when it is heated. This occurs because one of the metals expands more than the other when they are subjected to the same temperature. When the strip or disk cools, the metals contract and the strip returns to its original position and closes the circuit.
Control Devices The units in the electrical circuits in an aircraft are not all intended to operate continuously or automatically. Most of them are meant to operate at certain times, under certain conditions, to perform very definite functions. There must be some means of controlling their operation. Either a switch or a relay, or both, may be included in the circuit for this purpose.
SWITCHES Switches control the current flow in most aircraft electrical circuits. A switch is used to start, to stop, or to change the direction of the current flow in the circuit. The switch in each circuit must be able to carry the normal current of the circuit and must be insulated heavily enough for the voltage of the circuit.
Knife switches are seldom used on aircraft. They
• 317
FIGURE 8-112. Single-pole single-throw knife and toggle switches.
are included here to simplify the operation of the toggle switch. Toggle switches operate much the same as knife switches, but their moving parts are enclosed. They are used in aircraft circuits more than any other kind of switch.
Toggle switches, as well as some other type of switches, are designated by the number of poles, throws, and positions they have. A pole of a switch is its movable blade or contactor. The number of poles is equal to the number of circuits, or paths for current flow, that can be completed through the switch at any one time. The throw of a switch indicates the number of circuits, or paths for current, that it is possible to complete through the switch with each pole or contactor. The number of positions a switch has is the number of places at which the operating device (toggle, plunger, etc.) will come to rest and at the same time open or close one or more circuits.
As shown in figure 8-112, when it is possible to complete only one circuit through a switch, the switch is a single-pole single-throw (spst) switch.
A single-pole switch through which two circuits can be completed (not at the same time) is a single-pole double-throw (spdt) switch. (See figure 8-113.)
A switch with two contactors, or poles, each of which completes only one circuit, is a double-pole single-throw (dpst) switch. Double-pole singlethrow knife and toggle switches are illustrated in figure 8-114.
A double-pole switch that can complete two circuits, one circuit at a time through each pole, is a double-pole double-throw (dpdt) switch. Both a knife and a toggle switch illustrating this type of switch are shown in figure 8-115.
The schematic representations for the most commonly used switches are shown in figure 8-116.
A toggle switch that is spring-loaded to the OFF position and must be held in the ON position to complete the circuit is a momentary contact twoposition switch. One that will come to rest at either of two positions, opening the circuit in one position and closing it in another, is a two-position switch. A toggle switch that will come to rest at any one of three positions is a three-position switch.
A switch that stays open, except when it is held in the closed position, is a normally open switch (usually identified as NO). One that stays closed, except when it is held in the open position, is a normally closed switch (NC). Both kinds are spring-loaded to their normal position and will return to that position as soon as they are released.
Push-Button Switches Push-button switches have one stationary contact and one movable contact. The movable FIGURE 8-113. Single-pole double-throw knife and toggle switches.
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.