DIRECTION
OF CURRENT FIGURE 8-99. Magnetic field around a conductor with two loops.
generator charges the battery over a long period of time and restores the chemical energy. Leadacid and nickel-cadmium batteries are the types of storage batteries in general use.
LEAD-ACID BATTERIES Lead-acid batteries used in aircraft are similar to automobile batteries. The cells of a battery are connected in series. Each cell contains positive plates of lead peroxide, negative plates of spongy lead, and electrolyte (sulphuric acid and water).
In discharging, the chemical energy stored in the battery is changed to electrical energy; in charging, the electrical energy supplied to the battery is changed to chemical energy and stored.
It is possible to charge a storage battery many times before it deteriorates permanently.
Lead-acid Cell Construction The components of a typical lead-acid cell are shown in figure 8-105. Each plate consists of a framework called a grid, made of lead and antimony, to which the active material (spongy lead or lead peroxide) is attached. The positive and negative plates, (1) of figure 8-105, are so assembled that each positive plate is between two negative plates. Thus, the end plate in each cell is a negative plate. Between the plates are FIGURE 8-100. Magnetic field of a coil.
FIGURE 8-101. Electromagnet.
porous separators (7) which keep the positive and negative plates from touching each other and and shorting out the cell. The separators have vertical ribs on the side facing the positive plate.
This construction permits the electrolyte to circulate freely around the plates. In addition, it provides a path for sediment to settle to the bottom of the cell.
Each cell is sealed in a hard rubber casing through the top of which are terminal posts and a hole into which is screwed a nonspill vent cap
(4). The hole provides access for testing the strength of the electrolyte and adding water. The vent plug permits gases to escape from the cell with a minimum of leakage of electrolyte, regardless of the position the airplane might assume.
In figure 8-106, the construction of the vent plug is shown. In level flight, the lead weight permits venting of gases through a small hole. In inverted flight, this hole is covered by the lead weight.
The individual cells of the battery are connected in series by means of cell straps, as illustrated in figure 8-107. The complete assembly is enclosed in an acid-resisting metal container (battery box), which serves as electrical shielding and mechanical protection. The battery box has a removable top.
It also has a vent-tube nipple at each end. When the battery is installed in an airplane, a vent tube is attached to each nipple. One tube is the intake tube and is exposed to the slipstream. The other is the exhaust vent tube and is attached to the battery drain sump, which is a glass jar containing a felt pad moistened with a concentrated solution of sodium bicarbonate (baking soda). With this arrangement, the airstream is directed through the battery case where battery gases are picked up, neutralized in the sump, and then expelled overboard without damage to the airplane.
To facilitate installation and removal of the battery in some aircraft, a quick-disconnect assembly is used to connect the power leads to the FIcURE 8-102. Left-hand rule applied to a coil. FIGURE 8-103. Solenoid with iron core.
Relay Bus Bar Battery M Cockpit switch Motor FIGURE 8-104. Use of a solenoid in a circuit.
battery. This assembly, which is shown in figure 8-108, attaches the battery leads in the aircraft to a receptacle mounted on the side of the battery.
The receptacle covers the battery terminal posts and prevents accidental shorting during the installation and removal of the battery. The plug consists of a socket and a handwheel with a course-pitch thread. It can be readily connected to the receptacle by the handwheel. Another advantage of this assembly is that the plug can be installed in only one position, eliminating the possibility of reversing the battery leads.
Upright position Inverted position Gases escape Electrolyte Vent Vent Lead weight Lead weight seated not sealed seated FIGURE 8-106. Nonspill battery vent plug.
Operation of Lead-acid Cells A lead-acid cell contains positive plates coated with lead peroxide (P602); negative plates made of lead (Pb); and a liquid electrolyte, consisting of sulphuric acid (H2S04) and water (H20). During discharge, lead sulfate (PISO,) is formed on both the positive and negative plates, the acid content of the electrolyte is decreased, and its water content is increased. As discharge continues, the amount of lead sulfate on the plates increases until the sulfate coatings become so thick that the weakened electrolyte cannot effectively reach the active materials (lead and lead peroxide). When this happens, chemical reaction is retarded and the output of the cell is reduced. In practice, the cell is not permitted to be discharged to this extent because thick coatings of lead sulfate are difficult to remove in charging. Additionally, a cell ap-
• Vent plug Cells
1. Plates 3. Supporting ribs 6. Cell cover
2. Cell container 4. Vent cap 7. Separators
5. Terminal post - Cell strap FIGURE 8-105. Lead-acid cell construction. FIGURE 8-107. Connection of storage battery.
To give a high discharge current and a high terminal voltage under load, a battery must have low internal resistance. This characteristic can be achieved through extensive plate area. Therefore, each cell contains several sets of plates. All the positive plates of a cell are connected by one connecting bar, and all the negative plates by another. Thus, the plates are connected in parallel, further decreasing the internal resistance of the cell. The open circuit cell voltage is not affected;
it remains the same as that of a single pair of plates.
FIGURE 8-108. A battery quick-disconnect assembly.
proaching a state of total discharge is of little use because the high internal resistance (IR) caused by the sulfate coatings on its plates reduces the current to a value too low for practical use.
When a cell is being charged, lead sulfate is removed from both the positive and negative plates, and sulphuric acid is again formed. In the process, the water content of the electrolyte is of the deal re is decreased and the density of the electrolyte is increased.
The open-circuit voltage of a lead-acid cell, that is, its voltage when there is no load drawing current, is approximately 2.2 volts. This voltage is the same for every lead-acid cell regardless of its plate size and remains at this value until the cell is practically dead, regardless of its state of discharge. When the cell approaches total discharge, its voltage begins to drop rapidly.
The closed-circuit voltage of a cell, that is, its voltage under load, decreases gradually as the cell is discharged. This gradual decrease in terminal voltage is due to a gradual increase in the internal resistance of the cell caused by sulphation of the plates. At the end of normal discharge, the internal resistance of a lead-acid cell is more than twice as high as it is when fully charged. The difference between the open-circuit and closedcircuit terminal voltages is due to the voltage drop inside the cell. This is equal to the current the load draws multiplied by the internal resistance in the cell. Therefore, the discharging voltage that a lead-acid cell can supply under closedcircuit conditions is equal to the open-circuit voltage of the cell minus the IR drop in the cell.
Lead-acid Battery Ratings The voltage of a battery is determined by the number of cells connected in series to form the battery. Although the voltage of one lead-acid cell just removed from a charger is approximately
2.2 volts, a lead-acid cell is normally rated at only 2 volts, because it soon drops to that value.
A battery rated at 12 volts consists of 6 lead-acid cells connected in series, and a battery rated at 24 volts is composed of 12 cells.
The capacity of a storage battery is rated in ampere-hours (amperes furnished by the battery times the amount of time current can be drawn).
This rating indicates how long the battery may be used at a given rate before it becomes completely discharged.
Theoretically, a 100 ampere-hour battery will furnish 100 amperes for 1 hour, 50 amperes for 2 hours, or 20 amperes for 5 hours. Actually, the ampere-hour output of a particular battery depends on the rate at which it is discharged.
Heavy discharge current heats the battery and decreases its efficiency and total ampere-hour output. For airplane batteries, a period of 5 hours has been established as the discharge time in rating battery capacity, However, this time of 5 hours is only a basis for rating and does not necessarily mean the length of time during which the battery is expected to furnish current. Under actual service conditions, the battery can be completely discharged within a few minutes, or it may never be discharged if the generator provides sufficient charge.
The ampere-hour capacity of a battery depends upon its total effective plate area. Connecting batteries in parallel increases ampere-hour capacity. Connecting batteries in series increases the total voltage but not the ampere-hour capacity. In multiengine airplanes, where more 310
than one battery is used, the batteries are usually connected in parallel. The voltage is equal to that of one battery, but the ampere-hour capacity is increased. The total capacity is the sum of the ampere-hour ratings for the individual batteries.
Factors Affecting Lead-acid Battery Life Various factors cause deterioration of a battery and shorten its service life. These include overdischarging, which causes excess sulphation and too rapid charging or discharging, resulting in overheating of the plates and shedding of active material. The accumulation of shedded material, in turn, causes shorting of the plates and results in internal discharge. A battery that remains in a low or discharged condition for a long period of time may be permanently damaged. In addition to causing deterioration of the battery, these factors also decrease battery capacity.
Lead-acid Battery Testing Methods The state of charge of a storage battery depends upon the condition of its active materials, primarily the plates. However, the state of charge of a battery is indicated by the density of the electrolyte and is checked by a hydrometer, an instrument which measures the specific gravity (weight as compared with water) of liquids.
The hydrometer commonly used consists of a small sealed glass tube weighted at its lower end so it will float upright, as shown in figure 8-109.
Within the narrow stem of the tube is a paper scale with a range of 1.100 to 1.300. When a hydrometer is used, a quantity of electrolyte sufficient to float the hydrometer is drawn up into the syringe. The depth to which the hydrometer sinks into the electrolyte is determined by the density of the electrolyte, and the scale value indicated at the level of the electrolyte is its specific gravity. The more dense the electrolyte, the higher the hydrometer will float; therefore, the highest number on the scale (1.300) is at the lower end of the hydrometer scale.
In a new, fully charged aircraft storage battery, the electrolyte is approximately 30 percent acid and 70 percent water (by volume) and is 1.300 times as heavy as pure water. During discharge, the solution (electrolyte) become less dense and its specific gravity drops below 1.300. A specific gravity reading between 1.300 and 1.275 indicates a high state of charge; between 1.275 and 1.240, a medium state of charge; and between 1.240 and
1.200, a low state of charge. Aircraft batteries are generally of small capacity but are subject to heavy loads. The values specified for state of charge are therefore rather high. Hydrometer tests are made periodically on all storage batteries installed in aircraft. An aircraft battery in a low state of charge may have perhaps 50 percent charge remaining, but is nevertheless considered low in the face of heavy demands which would soon exhaust it. A battery in such a state of charge is considered in need of immediate recharging.
When a battery is tested using a hydrometer, the temperature of the electrolyte must be taken into consideration. The specific gravity readings on the hydrometer will vary from the actual specific gravity as the temperature changes. No correction is necessary when the temperature is between 70° F. and 90° F., since the variation is not great enough to be considered. When temperatures are greater than 90° F. or less than 70° F., it is necessary to apply a correction factor. Some hydrometers are equipped with a correction scale inside the tube. With other hydrometers it is necessary to refer to a chart provided by the manufacturer.
In both cases, the corrections should be added to, or subtracted from, the reading shown on the hydrometer.
The specific gravity of a cell is reliable only if nothing has been added to the electrolyte except occasional small amounts of distilled water to replace that lost as a result of normal evaporation.
Hydrometer readings should always be taken before adding distilled water, never after. This is necessary to allow time for the water to mix thoroughly with the electrolyte and to avoid drawing up into the hydrometer syringe a sample which does not represent the true strength of the solution.
Extreme care should be exercised when making the hydrometer test of a lead-acid cell. The electrolyte should be handled carefully, for sulphuric acid will burn clothing and skin. If the acid does contact the skin the area should be washed thoroughly with water and then bicarbonate of soda applied.
Lead-acia Battery Charging Methods A storage battery may be charged by passing direct current through the battery in a direction opposite to that of the discharge current. Because of the internal resistance (IR) in the battery, the 311
High charge Medium charge Low charge FICURE 8-109. Hydrometer (specific gravity readings).
voltage of the external charging source must be greater than the open-circuit voltage. For example, the open-circuit voltage of a fully charged 12-cell, lead-acid battery is approximately 26.4 volts (12 X 2.2 volts), but approximately 28 volts are required to charge it. This larger voltage is needed for charging because of the voltage drop in the battery caused by the internal resistance.
Hence, the charging voltage of a lead-acid battery must equal the open-circuit voltage plus the IR drop within the battery (product of the charging current and the internal resistance).
When a storage battery is being charged, it generates a certain amount of hydrogen and Constant oxygen. Since this is an explosive mixture, it is voltage charging important that steps be taken to prevent ignition circut of the gas mixture. The vent caps should be Motor generator loosened and left in place. No open flames, sparks, Rheostat or other source of ignition should be permitted in the vicinity. Before disconnecting or connecting a battery to the charge, always turn off the power by means of a remote switch.
(A) Constant current charging circuit (B) FIGURE 8-110. Battery charging methods.
Batteries are charged by either the constantvoltage or constant-current method. In the constant-voltage method (A of figure 8-110), a motorgenerator set with a constant, regulated voltage forces the current through the battery. In this method, the current at the start of the process is high but automatically tapers off; reaching a value of approximately 1 ampere when the battery is fully charged. The constant-voltage method requires less time and supervision than does the constant-current method.
In the constant-current method (B of figure 8-110), the current remains almost constant during the entire charging process.
This method requires a longer time to charge a battery fully and, toward the end of the process, presents the danger of overcharging, if care is not In the aircraft, the storage battery is charged by direct current from the aircraft generator system.
This method of charging is the constant-voltage since the generator voltage is held constant by use of a voltage regulator.
NICKEL-CADMIUM BATTERIES Nickel-cadmium batteries have been available for some time, but they did not come into extensive use in aviation until the increase in the number of commercial and executive jet aircraft made them economically practicable. The many advantages of the nickel-cadmium battery were well known, but its initial cost was several times that of the lead-acid battery. The increasing use of the nickel-cadmium battery (often referred to as "ni-cad") stems largely from the low maintenance cost derived from the long service life of the battery. Additionally, the nickel-cadmium battery has a short recharge time, excellent reliability, and good starting capability.
Nickel-Cadmium Cell Construction As in the lead-acid type, the cell is the basic unit of the nickel-cadmium battery. It consists of positive and negative plates, separators, electrolyte, cell vent, and cell container. The positive plates are made from a porous plaque on which nickel-hydroxide has been deposited. The negative plates are made from similar plaques on which cadmium-hydroxide is deposited. In both cases the porous plaque is obtained by sintering nickel powder to a fine-mesh wire screen. Sintering is a process which fuses together extremely small granules of powder at a high temperature.
After the active positive and negative materials are deposited on the plaque, it is formed and cut into the proper plate size. A nickel tab is then welded to a corner of each plate and the plates are assembled with the tabs welded to the proper terminals. The plates are separated from each other by a continuous strip of porous plastic.
The electrolyte used in the nickel-cadmium battery is a 30 percent solution (by weight) of potassium hydroxide (KOH) in distilled water.
The specific gravity of the electrolyte remains between 1.240 and 1.300 at room temperature. No 313
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.