Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-24 A B Single wire Conductor Typical Wire No. SX001A22 12PB501-*APJ356 NM200A22 RF300A22 Shielded single wire Shielded double wire Shielded triple wire Shielding installation agency to another. For further understanding of a wire numbering system, consult the appropriate wiring guide published by the agency that drew the prints. Regardless of the specifics of the wire numbering system, organizations that exercise professional wiring practices have the installed wire marked in some manner. This is an aid to the technician who has to troubleshoot or modify the system at a later date.
Types of Resistors
Fixed Resistor Fixed resistors have built into the design a means of opposing current. [Figure 12-48] The general use of a resistor in a circuit is to limit the amount of current flow. There are a number of methods used in construction and sizing of a resistor that control properties, such as resistance value, the precision of the resistance value, and the ability to dissipate heat. While in some applications the purpose of the resistive element is used to generate heat, such as in propeller anti-ice boots, heat typically is the unwanted loss of energy. Carbon Composition The carbon composed resistor is constructed from a mixture of finely grouped carbon/graphite, an insulation material for filler, and a substance for binding the material together.
The amount of graphite in relation to the insulation material determines the ohmic or resistive value of the resistor. This mixture is compressed into a rod, which is fitted with axial leads or “pigtails.” The finished product is sealed in an insulating coating for isolation and physical protection. There are other types of fixed resistors in common use. Included in this group are: • Carbon film • Metal-oxide • Metal film • Metal glaze The construction of a film resistor is accomplished by depositing a resistive material evenly on a ceramic rod. This resistive material can be graphite for the carbon film resistor, nickel chromium for the metal film resistor, metal and glass for the metal glaze resistor, and metal and an insulating oxide for the metal-oxide resistor.
Resistor Ratings Color Code It is very difficult to manufacture a resistor to an exact standard of ohmic values. Fortunately, most circuit requirements are not extremely critical. For many uses, the actual resistance in ohms can be 20 percent higher or lower than the value marked on the resistor without causing difficulty. The percentage variation between the marked value and the actual value of a resistor is known as the “tolerance” of a resistor. A resistor coded for a 5 percent tolerance is not more than 5 percent higher or lower than the value indicated by the color code. The resistor color code is made up of a group of colors, numbers, and tolerance values. Each color is represented by a number, and in most cases, by a tolerance value. [Figure 12-49] When the color code is used with the end-to-center band marking system, the resistor is normally marked with bands of color at one end of the resistor. The body or base color of the resistor has nothing to do with the color code, and in no way indicates a resistance value. To prevent confusion, this body is never the same color as any of the bands indicating resistance value.
12-25 Resistor color code Color Number Tolerance Black 0 — Brown 1 1% Red 2 2% Orange 3 3% Yellow 4 4% Green 5 5% Blue 6 6% Violet 7 7% Gray 8 8% White 9 9% Gold — 5% Silver — 10% No color — 20% Color Band Decoding When the end-to-center band marking system is used, either three or four bands mark the resistor. 1. The first color band (nearest the end of the resistor) indicates the first digit in the numerical resistance value. This band is never gold or silver in color. 2. The second color band always indicates the second digit of ohmic value. It is never gold or silver in color. [Figure 12-50] 3. The third color band indicates the number of zeros to be added to the two digits derived from the first and second bands, except in the following two cases: (a) If the third band is gold in color, the first two digits must be multiplied by 10 percent.
(b) If the third band is silver in color, the first two digits must be multiplied by 1 percent. 4. If there is a fourth color band, it is used as a multiplier for percentage of tolerance, as indicated in the color code chart in Figure 12-49. If there is no fourth band, the tolerance is understood to be 20 percent. value of a resistor marked with the end-to-center band system. This resistor is marked with three bands of color, which must be read from the end toward the center. There is no fourth color band; therefore, the tolerance is understood to be 20 percent. 20 percent of 250,000 Ω, equals 50,000 Ω.
Since the 20 percent tolerance is plus or minus, Maximum resistance = 250,000 Ω + 50,000 Ω = 300,000 Ω Minimum resistance = 250,000 Ω − 50,000 Ω = 200,000 Ω The following paragraphs provide a few extra examples of resistor color band decoding. Figure 12-51 contains a resistor with another set of colors. This resistor code should be read as follows: The resistance of this resistor is 86,000 ±10 percent ohms. The maximum resistance is 94,600 ohms, and the minimum resistance is 77,400 ohms. Another example is the resistance of the resistor in Figure 12-52 is 960 ±5 percent ohms. The maximum resistance is 1,008 ohms, and the minimum resistance is 912 ohms.
Sometimes circuit considerations dictate that the tolerance must be smaller than 20 percent. Figure 12-53 shows an example of a resistor with a 2 percent tolerance. The resistance value of this resistor is 2,500 ±2 percent ohms. The maximum resistance is 2,550 ohms, and the minimum resistance is 2,450 ohms. third color band. The color code value of black is zero, and the third band indicates the number of zeros to be added to the first two digits. In this case, a zero number of zeros must be added to the first two digits; therefore, no zeros are added. Thus, the resistance value is 10 ±1 percent ohms. The maximum resistance is 10.1 ohms, and the minimum resistance is 9.9 ohms. There are two exceptions to the rule stating the third color band indicates the number of zeros. The first of these exceptions is illustrated in Figure 12-55. When the third band is gold in color, it indicates that the first two digits must be multiplied by 10 percent. The value of this resistor in this case is: 10 × 0.10 ±2% = 1 = 0.02 ohms When the third band is silver, as is the case in Figure 12-56, the first two digits must be multiplied by 1 percent. The value 12-26 Gray Blue Orange Silver 8 6 000 10% White Blue Brown Gold 9 6 0 5% Red Green Red Red 2 5 00 2% Brown Black Black Brown 1 0 1% 1st Band 2nd Band 3rd Band Red Green Yellow Color Numerical Value Significance 1st band — red 2 1st digit 2nd band — green 5 2nd digit 3rd band — yellow 4 No. of zeroes to add Yellow Green Silver (Multiplier) Silver 4 5 1% 10% Brown Black Gold (Multiplier) Red 1 0 10% 2% of the resistor is 0.45 ±10 percent ohms.
Wire-Wound Wire-wound resistors typically control large amounts of current and have high power ratings. Resistors of this type are constructed by winding a resistance wire around an insulating rod usually made of porcelain. The windings are coated with an insulation material for physical protection and heat conduction. Both ends of the windings are connected to terminals, which are used to connect the resistor to a circuit. [Figure 12-57] A wire-wound resistor with tap is a special type of fixed resistor that can be adjusted. These adjustments can be made by moving a slide bar tap or by moving the tap to a preset incremental position. While the tap may be adjustable, the adjustments are usually set at the time of installation to a specific value and then operated in service as a fixed resistor. Another type of wire-wound resistor is constructed of Manganin wire, used when high precision is needed.
Variable Resistors Variable resistors are constructed so that the resistive value can be changed easily. This adjustment can be manual or automatic, and the adjustments can be made while the system that it is connected to is in operation. There are two basic types of manual adjustors: the rheostat and the potentiometer. Rheostat A rheostat is a variable resistor used to vary the amount of current flowing in a circuit. [Figure 12-58] Figure 12-59 shows a rheostat connected in series with an ordinary resistance in a series circuit. As the slider arm moves from point A to B, the amount of rheostat resistance (AB) is increased. Since the rheostat resistance and the fixed resistance are in series, the total resistance in the circuit also increases, and the current in the circuit decreases. On the other hand, if the slider arm is moved toward point A, the total resistance decreases and the current in the circuit increases.
12-27 Potentiometer The potentiometer is considered a three-terminal device. [Figure 12-60] As illustrated, terminals 1 and 2 have the entire value of the potentiometer resistance between them. Terminal 3 is the wiper or moving contact. Through this wiper, the resistance between terminals 1 and 3 or terminals 2 and 3 can be varied. While the rheostat is used to vary the current in a circuit, the potentiometer is used to vary the voltage in a circuit. A typical use for this component can be found in the volume controls on an audio panel and input devices for flight data recorders, among many other applications.
In Figure 12-61A, a potentiometer is used to obtain a variable voltage from a fixed voltage source to apply to an electrical load. The voltage applied to the load is the voltage between points 2 and 3. When the slider arm is moved to point 1, the entire voltage is applied to the electrical device (load); when the arm is moved to point 3, the voltage applied to the load is zero. The potentiometer makes possible the application of any voltage between zero and full voltage to the load. The current flowing through the circuit of Figure 12-61 leaves the negative terminal electron flow of the battery and divides one part flowing through the lower portion of the potentiometer (points 3 to 2) and the other part through the load. Both parts combine at point 2 and flow through the upper portion of the potentiometer (points 2 to 1) back to the positive terminal of the battery. In View B of Figure 12-61, a potentiometer and its schematic symbol are shown.
In choosing a potentiometer resistance, the amount of current drawn by the load should be considered, as well as the current flow through the potentiometer at all settings of the slider arm. The energy of the current through the potentiometer is dissipated in the form of heat. It is important to keep this wasted current as small as possible by making the resistance of the potentiometer as large as practicable. In most cases, the resistance of the potentiometer can be several times the resistance of the load. function as a rheostat. Linear Potentiometers In a linear potentiometer, the resistance between both terminal and the wiper varies linearly with the position of the wiper.
To illustrate, one quarter of a turn on the potentiometer results in one quarter of the total resistance. The same relationship exists when one-half or three-quarters of potentiometer movement. [Figure 12-63] Tapered Potentiometers Resistance varies in a nonlinear manner in the case of the tapered potentiometer. [Figure 12-64] Keep in mind that one-half of full potentiometer travel does not necessarily correspond to one-half the total resistance of the potentiometer. Thermistors The thermistor is a type of a variable resistor that is temperature sensitive. [Figure 12-65] This component has a negative temperature coefficient, which means that as the sensed temperature increases, the resistance of the thermistor decreases.
Photoconductive Cells The photoconductive cell is similar to the thermistor. Like the thermistor, it has a negative temperature coefficient. Unlike the thermistor, the resistance is controlled by light intensity. This kind of component can be found in radio control heads where the intensity of the ambient light is sensed through the photoconductive cell resulting in the backlighting of the control heads to adjust to the flight deck lighting conditions. [Figure 12-66]
Circuit Protection Devices
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. Most wires used in aircraft electrical circuits are small gauge, and their current carrying capacity is quite limited. The size of the wires used in any given circuit is determined 12-28 A B 1 2 3 Load Shaft Terminals Resistive strip Wiper Rotary potentiometer construction 1 2 3 A 25% 70 Ω 30 Ω 100 Ω B 50% 30 Ω 70 Ω 100 Ω C 75% 10 Ω 90 Ω 100 Ω AB 1 2 3 by the amount of current the wires are expected to carry under normal operating conditions. Any current flow 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 could cause a portion of the wire to melt and at the very least, open the circuit.
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, thermal protectors, and arc fault circuit breakers 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.
Fuse Fuses are used to protect the circuit from over current conditions. [Figure 12-67A] 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, which melts and opens the circuit when the current exceeds 12-29 T A 25% 70 Ω 30 Ω 100 Ω B 50% 30 Ω 70 Ω 100 Ω C 75% 10 Ω 90 Ω 100 Ω the rated capacity of the fuse. For example, if a 5-amp fuse is placed into a circuit, the fuse allows currents up to 5 amps to pass. Because 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 replacing a fuse, consult the applicable manufacturer’s instructions to be sure a fuse of the correct type and capacity is installed. Fuses are installed in two types of fuse holders in aircraft. “Plug-in holders” or in-line holders are used for small and low capacity fuses. “Clip” type holders are used for heavy high capacity fuses and current limiters. Current Limiter The current limiter is very much like the fuse. However, the current limiter link is usually made of copper and will stand a considerable overload for a short period of time. Like the fuse, it opens up in an over current condition in heavy current circuits such as 30 amp or greater. These are used primarily to sectionalize an aircraft circuit or bus. Once the limiter is opened, it must be replaced. The schematic symbol for the current limiter shows two triangles pointing to each other with a line on both sides of the triangles. [Figure 12-67B].
Circuit Breaker The circuit breaker is commonly used in place of a fuse and is designed to break the circuit and stop the current flow when the current exceeds a predetermined value. Unlike the fuse, the circuit breaker can be reset; whereas the fuse or current limiter must be replaced. [Figure 12-68] 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. If the overload condition still exists, the circuit breaker trips again to prevent damage to the circuit. At this point, it is usually not advisable to continue resetting the circuit breaker, but to initiate troubleshooting to determine the cause. Repeated resetting of a circuit breaker can lead to circuit or component damage or worse, the possibility of a fire or explosion. Automatic reset type circuit breakers are not allowed in aircraft. Circuit breakers are commonly grouped on a circuit breaker panel that is accessible to the flight crew. Figure 12-69 shows a circuit breaker and a circuit breaker panel.
Arc Fault Circuit Breaker In recent years, the arc fault circuit breaker has begun to provide an additional layer of protection beyond that of the thermal protection already provided by conventional circuit breakers. The arc fault circuit breaker monitors the circuit for an electrical arcing signature, which can indicate possible wiring faults and unsafe conditions. These conditions can lead to fires or loss of power to critical systems. The arc fault circuit breaker is only beginning to make an appearance in the aircraft industry and is not widely used like the thermal type of circuit breaker.
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 12-30 A B malfunction in the motor causes it to overheat, the thermal switch breaks the circuit intermittently. The thermal switch contains a bimetallic disk, or strip, that 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
Components in the electrical circuits are typically 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. Figure 12-70 shows various switches used in aircraft electrical systems. An understanding of some basic definitions of the switch is necessary before any of the switch types are discussed. The number of poles, throws, and positions they have designates toggle switches, as well as some other type of switches.
Pole—the switch’s 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. Throw—indicates the number of circuits, or paths for current, that it is possible to complete through the switch with each pole or contactor. Positions—indicates the number of places at which the operating device (toggle, plunger, and so forth) comes to rest and at the same time open or close one or more circuits. Toggle Switch Single-Pole, Single-Throw (SPST) The single-pole, single-throw (SPST) switch allows a connection between two contacts. One of two conditions exist. Either the circuit is open in one position or closed in the other position. [Figure 12-71] Single-Pole, Double-Throw (SPDT) The single-pole, double-throw (SPDT) switch is shown in can be made between one contact and the other.
Double-Pole, Single-Throw (DPST) The double-pole, single-throw (DPST) switch connection can be made between one set of contacts and either of two other sets of contacts. [Figure 12-73] Double-Pole, Double-Throw (DPDT) The schematic symbol for the double-pole, double-throw (DPDT) switch is shown in Figure 12-74. This type of switch makes a connection from one set of contacts to either of two other sets of contacts. A toggle switch that is spring-loaded to the OFF position 12-31 system. and must be held in the ON position to complete the circuit is a momentary contact two-position switch. One that comes 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 comes 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 return to that position as soon as they are released. Locking toggles require the operator to pull out on the switch toggle before moving it in to another position. Once in the new position, the switch toggle is release back into a lock, which then prevents the switch from inadvertently being moved. Microswitches A microswitch opens or closes 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 pushbutton switches. They are used primarily as limit switches to provide automatic control of landing gears, actuator motors, and the like. Figure 12-75 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. Figure 12-76 shows a pushbutton microswitch. Rotary Selector Switches A rotary selector switch takes the place of several switches. 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.
[Figure 12-77] Pushbutton Switches Pushbutton switches have one stationary contact and one 12-32 movable contact. The movable contact is attached to the pushbutton. The pushbutton is either an insulator itself or is insulated from the contact. This switch is spring loaded and designed for momentary contact. Lighted Pushbutton Switches Another more common switch found in today’s aircraft is the lighted pushbutton switch. This type of switch takes the form of a 5⁄8-inch to 1-inch cube with incandescent or LED lights to indicate the function of the switch. Switch designs come in a number of configurations; the two most common are the alternate action and momentary action and usually have a two-pole or four-pole switch body. Other less common switch actions are the alternate and momentary holding coil configurations. The less known holding or latching coil switch bodies are designed to have a magnetic coil inside the switch body that is energized through two contacts in the base of the switch. When the coil is energized and the switch is pressed, the switch contacts remain latched until power is removed from the coil. This type of design allows for some degree of remote control over the switch body. [Figure 12-78] The display optics of the lighted pushbutton switch provide the crew with a clear message that is visible under a wide range of lighting conditions with very high luminance and wide viewing angles. While some displays are simply a transparent screen that is backlit by an incandescent light, the higher quality and more reliable switches are available in sunlight readable displays and night vision (NVIS) versions. Due to the sunlight environment of the flight deck, displays utilizing standard lighting techniques “washout” when viewed in direct sunlight.
Sunlight readable displays are designed to minimize this effect. Lighted pushbutton switches can also be used in applications where a switch is not required and the optics are only for indications. This type of an indicator is commonly called an annunciator. 12-33 Stationary contact Moveable contact Operating plunger Grooved anchor Three-bladed spring Dual In-Line Parallel (DIP) Switches The acronym “DIP” switch is defined as Dual In-Line Parallel switch in reference to the physical layout. DIP switches are commonly found in card cages, and line replaceable units (LRU) and are used in most cases to adjust gains, control configurations, and so forth. Each one of the switches is generally an SPST slide or rocker switch. The technician may find this switch in packages ranging in size from DIP2 through DIP32. Some of the more common sizes are DIP4 and DIP8.
Switch Guards Switch guards are covers that protect a switch from unintended operation. Prior to the operation of the switch, the guard is usually lifted. Switch guards are commonly found on systems such as fire suppression and override logics for various systems. Figure 12-79A shows a traditional switch with a guard while Figure 12-79B shows a pushbutton switch with a guard. The guard needs to be moved before the switch can be pushed.
Relays
A relay is simply an electromechanical switch where a small amount of current can control a large amount of current. [Figure 12-80] When a voltage is applied to the coil of the relay, the electromagnet is energized due to the current. When energized, an electromagnetic field pulls the common (C) or arm of the relay down. When the arm or common is pulled down, the circuit between the arm and the normally closed (NC) contacts is opened and the circuit between the arm and the normally open (NO) contacts are closed. When the energizing voltage is removed, the spring returns the arm contacts back to the normally closed (NC) contacts. The relay usually has two connections for the coil. The (+) side is designated as X1 and the ground-side of the coil is designated as X2.
Series DC Circuits
The series circuit is the most basic electrical circuit and provides a good introduction to basic circuit analysis. The series circuit represents the first building block for all of the circuits to be studied and analyzed. Figure 12-81 shows this simple circuit with nothing more than a voltage source or battery, a conductor, and a resistor. This is classified as a series circuit because the components are connected end-to- end, so that the same current flows through each component equally. There is only one path for the current to take and the battery and resistor are in series with each other. Next is to make a few additions to the simple circuit in Figure 12-81.