# Chapter 12: Fundamentals of Electricity and Electronics ## DC 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. The best and most expensive measuring instrument is of no use unless the technician knows what is being measured and what each reading indicates. The purpose of the meter is to measure quantities existing in a circuit. For this reason, when a meter is connected to a circuit, it must not change the characteristics of that circuit. Meters are either self-excited or externally excited. Those that are self-excited operate from a power source within the meter. Externally-excited meters get their power source from thecircuit that they are connected to. The most common analog meters in use today are the voltmeter, ammeter, and ohmmeter. All of which operate on the principles of electromagnetism. The fundamental principle behind the operation of the meter is the interaction between magnetic fields created by a current gathered from the circuit in some manner. This interaction is between the magnetic fields of a permanent magnet and the coils of a rotating magnet. The greater the current through the coils of the rotating magnet, the stronger the magnetic field produced. A stronger field produces greater rotation of the coil. While some meters can be used for both DC and AC circuit measurement, only those used as DC instruments are discussed in this section. The meters used for AC, or for both AC and DC, are discussed in the study of AC theory and circuitry. ### D’Arsonval Meter Movement This basic DC type of meter movement—first employed by the French scientist, d’Arsonval, in making electrical measurement—is a current measuring device, which is used in the ammeter, voltmeter, and ohmmeter. The pointer is deflected in proportion to the amount of current through the coil. 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 (self-excited) of power and contains other auxiliary circuits. ### Current Sensitivity and Resistance The current sensitivity of a meter movement is the amount of current required to drive the meter movement to a full-scale deflection. A simple example would be a meter movement that has 1 mA sensitivity. What this indicates is that meter movement requires 1 mA of current to move the needle to a full-scale indication. Likewise, a half-scale deflection requires only 0.5 mA of current. Additionally, what is called movement resistance is the actual DC resistance of the wire used to construct the meter coil. ![[FIGURE 12-149.png|350]] In a standard d’Arsonval meter, movement may have a current sensitivity of 1 mA and a resistance of 50 Ω. If the meter is going to be used to measure more than 1 mA, then additional circuitry is required to accomplish the task. This additional circuitry is a simple shunt resistor. The purpose of the shunt [[resistor]] is to bypass current that exceeds the 1 mA limitation of the meter movement. To illustrate this, assume that the 1 mA meter in question is needed to measure 10 mA. The shunt resistor used should carry 9 mA while the remaining 1 mA is allowed to pass through the meter. *[[FIGURE 12-149.png|[Figure 12-149] ]]* To determine the proper shunt resistance for this situation: $\begin{align} R_{SH} &= \text{ Shunt resistance} \\ R_M &= \text{ Meter resistance } = 50 Ω \end{align}$ Because the shunt resistance and the 50 Ω meter resistance are in parallel, the voltage drop across both of them is the same. $E_{SH} = E_M $ Using Ohm’s Law, this relationship can be rewritten as: $\begin{align} &E_{SH} = I_{SH} \times R_{SH} \\ &E_M = I_M \times R_M \\ &I_{SH} \times R_{SH} = I_M \times R_M \end{align}$ Simply solve for $R_{SH}$ $R_{SH}= \frac{I_M \times R_M}{I_{SH}} $ Substituting the values $R_{SH} =\frac{ImA \times 50 Ω}{9 mA} = 5.56 Ω $ ### Damping To make meter readings 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 produced by the eddy currents opposes the motion of the coil. The pointer therefore swings more slowly to its proper position and comes to rest quickly with very little oscillation. ### Mechanical Damping ![[FIGURE 12-150.png|350]] Air damping is a common method of damping by mechanical means. As shown in *[[FIGURE 12-150.png|Figure 12-150]]*, 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 that the air offers to the vane. Effective damping is achieved if the vane nearly touches the walls of the chamber. ### A Basic Multirange Ammeter Building upon the basic meter previously discussed is the more complex and useful multirange meter, which is more practical. The basic idea of a multirange ammeter is to make the meter usable over a wide range of voltages. In order to accomplish this, each range must utilize a different shunt resistance. The example give in this text is that of a two-range meter. However, once the basics of a two-range multirange ammeter are understood, the concepts can easily be transferred to the design of meters with many selectable ranges. ![[FIGURE 12-151.png|350]] *[[FIGURE 12-151.png|Figure 12-151]]* shows the schematic of an ammeter with two selectable ranges. This example builds upon the previous 10 mA range meter by adding a 100 mA range. With the switch selected to the 10 mA range, the meter indicates 10 mA when the needle is deflected to full scale and likewise indicates 100 mA at full scale when selected to 100 mA. The value of the 100 mA shunt resistor is determined the same way the 10 mA shunt resistor was determined. Recall that the meter movement can only carry 1 mA. This means that in a 100 mA range the remaining current of 99 mA must pass through the shunt resistor. $R_{SH}= \frac{I_M \times R_M}{I_{SH}} $ Substituting the values: $R_{SH}= \frac{1mA \times 50 Ω }{99 mA} = 0.51 Ω $ #### Precautions The precautions to observe when using an ammeter are summarized as follows: 1. Always connect ammeter in series with the element through which the current flow is to be measured. 2. Never connect an ammeter across a source of voltage, such as a battery or generator. Remember that the resistance of an ammeter, particularly on the higher ranges, is extremely low and that any voltage, even a volt or so, can cause very high current to flow through the meter, causing damage to it. 3. Use a range large enough to keep the deflection less than full scale. Before measuring a current, form some idea of its magnitude. Then switch to a large enough scale or start with the highest range and work down until the appropriate scale is reached. The most accurate readings are obtained at approximately half-scale deflection. Many milliammeters have been ruined by attempts to measure amperes. Therefore, be sure to read the lettering either on the dial or on the switch positions and choose proper scale before connecting the meter in the circuit. 4. Observe proper polarity in connecting the meter in the circuit. Current must flow through the coil in a definite direction in order to move the indicator needle up scale. Current reversal because of incorrect connection in the circuit results in a reversed meter deflection and frequently causes bending of the meter needle. Avoid improper meter connections by observing the polarity markings on the meter. ## The Voltmeter The voltmeter uses the same type of meter movement as the ammeter but employs a different circuit external to the meter movement. ![[FIGURE 12-152.png|350]] As shown before, the voltage drop across the meter coil is a function of current and the coil resistance. In another example, 50 μA × 1,000 Ω = 50 mV. In order for the meter to be used to measure voltages greater than 50 mV, there must be added a series resistance to drop any excess voltage greater than that which the meter movement requires for a full scale deflection. The case of the voltmeter, this resistance is called multiplier resistance and is designated as $R_M$. *[[FIGURE 12-152.png|[Figure 12-152] ]]* The voltmeter only has one multiplier resistor for use in one range. In this example, the full scale reading is 1 volt. $R_M$ is determined in the following way: The meter movement drops 50 mV at a full scale deflection of 50 µA. The multiplying resistor $R_M$ must drop the remaining voltage of 1 V - 50 mV = 950 mV. Since $R_M$ is in series with the movement, it also carries 50 µA at full scale. $RM =\frac{950 mV}{50 µA } = 19kΩ $ Therefore, for 1 volt full scale deflection, the total resistance of the voltmeter is 20k Ω. That is, the multiplier resistance and the coil resistance. ### Voltmeter Sensitivity Voltmeter sensitivity is defined in terms of resistance per volt (Ω/V). The meter used in the previous example has a sensitivity of 20k Ω and a full scale deflection of 1 volt. ### Multiple Range Voltmeters The simplified voltmeter in *[[FIGURE 12-152.png|Figure 12-152]]* has only one range (1 volt), which means that it can measure voltages from 0 volts to 1 volt. In order for the meter to be more useful, additional multiplier resistors must be used. One resistor must be used for each desired range. ![[FIGURE 12-153.png|350]] For a 50 μA movement, the total resistance required is 20kΩ for each volt of full scale reading. In other words, the sensitivity for a 50 μA movement is always 20kΩ regardless of the selected range. The full-scale meter current is 50 µA at any range selection. To find the total meter resistance, multiply the sensitivity by the full scale voltage for that particular range. For example for a 10 volt range, $R_T$ \= (20k Ω/V)(10V)=200kΩ. The total resistance for the 1 volt range is 20k Ω, so $R_M$ for a 10 V range is 200k Ω - 20k Ω = 180k Ω. *[[FIGURE 12-153.png|[Figure 12-153] ]]* ### Voltmeter Circuit Connections When voltmeters are used, they are connected in parallel with a circuit. If unsure about the voltage to be measured, take the first reading at the high value on the meter and then progressively move down through the range until a suitable read is obtained. Observe that the polarity is correct before connecting the meter to the circuit or damage occurs by driving the movement backwards. ### Influence of the Voltmeter in the Circuit When a voltmeter is connected across two points in a circuit, current is shunted. If the voltmeter has low resistance, it draws off a significant amount of current. This lowers the effective resistance of the circuit and change the voltage readings. When making a voltage measurement, use a high resistance voltmeter to prevent shunting of the circuit. ## The Ohmmeter The meter movement used for the ammeter and the voltmeter can also be used for the ohmmeter. The function of the ohmmeter is to measure resistance. A simplified one-stage ohmmeter is illustrated in *[[FIGURE 12-154.png|Figure 12-154]]*, which shows that the basic ohmmeter contains a battery and a variable resistor in series with the meter movement. To measure resistance, the leads of the meter are connected across an external resistance, which is to be measured. By doing this, the ohmmeter circuit is completed. This connection allows the internal battery to produce a current through the movement coil, causing a deflection of the pointer proportional to the value of the external resistance being measured. ### Zero Adjustment When the ohmmeter leads are open, the meter is at a full scale deflection, indicating an infinite (∞) resistance or an open circuit.*[[FIGURE 12-155.png|[Figure 12-155] ]]* When the leads are shorted as shown in figure “zero adjust,” the pointer is at the full right-hand position, indicating a short circuit or zero resistance. The purpose of the variable resistor in this figure is to adjust the current so that the pointer is at exactly zero when the leads are shorted. This is used to compensate for changes in the internal battery voltage due to aging. ![[Pasted image 20210302061916.png|350]] ### Ohmmeter Scale ![[FIGURE 12-156.png|350]] *[[FIGURE 12-156.png|Figure 12-156]]* shows a typical analog ohmmeter scale. Between zero and infinity (∞), the scale is marked to indicate various resistor values. Because the values decrease from left to right, this scale is often called a back-off scale. In the case of the example given, assume that a certain ohmmeter uses a 50 μA, 1,000 Ω meter movement and has an internal 1.5 volt battery. A current of 50 µA produces a full-scale deflection when the test leads are shorted. To have 50 μA, the total ohmmeter resistance is 1.5 V/50 μA = 30k Ω. Therefore, since the coil resistance is 1k Ω, the variable zero adjustment resistor must be set to 30k Ω – 1k Ω = 29k Ω. Now consider that a 120k Ω resistor is connected to the ohmmeter leads. Combined with the 30k Ω internal resistance, the total R is 150k Ω. The current is 1.5 V/150k Ω = 10 μA, which is 20 percent of the full scale current and appears on the scale shown in *[[FIGURE 12-156.png|Figure 12-156]]*. Now consider further that a 120k Ω resistor is connected to the ohmmeter leads. This results in a current of 1.5 V/75k Ω = 10 µA, which is 40 percent of the full scale current and marked on the scale. Additional calculations of this type show that the scale is nonlinear. It is more compressed toward the left side than the right side. The center scale point corresponds to the internal meter resistance of 30k Ω. The reason is as follows: With 30k Ω connected to the leads, the current is 1.5 V/60k Ω = 25 µA, which is half of the full scale current of 50 µA. ### The Multirange Ohmmeter A practical ohmmeter has several operational ranges. These typically are indicated by R × 1, R × 10, R × 100, R × 1k, R × 100k and R × 1M. These range selections are interpreted in a different manner than that of an ammeter or voltmeter. The reading on the ohmmeter scale is multiplied by the factor indicated by the range setting. For example, if the pointer is set on the scale and the range switch is set at R × 100, the actual resistance measurement is 20 × 100 or 2k Ω. ![[FIGURE 12-157.png|350]] To measure small resistance values, the technician must use a higher ohmmeter current than is needed for measuring large resistance values. Shunt resistors are needed to provide multiple ranges on the ohmmeter to measure a range of resistance values from the very small to very large. For each range, a different value of shunt resistance is switched in. The shunt resistance increases for higher ohm ranges and is always equal to the center scale reading on any selected range. In some meters, a higher battery voltage is used for the highest ohm range. *[[FIGURE 12-157.png|[Figure 12-157] ]]* ### Megger (Megohmmeter) The megger, or megohmmeter, is a high range ohmmeter containing a hand-operated generator. It is used to measure insulation resistance and other high-resistance values. It is also used for ground, continuity, and short-circuit testing of electrical power systems. The chief advantage of the megger over an ohmmeter is its capacity to measure resistance with a high potential, or “breakdown” voltage. This type of testing ensures that insulation or a dielectric material will not short or leak under potential electrical stress. ![[FIGURE 12-158.png|350]] The megger consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: a hand-driven DC generator, G, which supplies the necessary current for making the measurement; and the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of the opposed coil type. Coils A and B are mounted on the movable member with a fixed angular relationship to each other and are free to turn as a unit in a magnetic field. Coil B tends to move the pointer counterclockwise and coil A, clockwise. The coils are mounted on a light, movable frame that is pivoted in jewel bearings and free to move about axis 0. *[[FIGURE 12-158.png|[Figure 12-158] ]]* Coil A is connected in series with R3 and the unknown resistance, $R_X$, to be measured. The series combination of coil A, R3, and $R_X$ is connected between the + and - brushes of the DC generator. Coil B is connected in series with R2, and this combination is also connected across the generator. There are no restraining springs on the movable member of the instrument portion of the megger. When the generator is not in operation, the pointer floats freely and may come to rest at any position on the scale. If the terminals are open circuited, no current flows in coil A, and the current in coil B alone controls the movement of the moving element. Coil B takes a position opposite the gap in the core (since the core cannot move and coil B can), and the pointer indicates infinity on the scale. When a resistance is connected between the terminals, current flows in coil A, tending to move the pointer clockwise. At the same time, coil B tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position at which the two forces are balanced. This position depends upon the value of the external resistance, which controls the relative magnitude of current of coil A. Because changes in voltage affect both coils A and B in the same proportion, the position of the moving element is independent of the voltage. If the terminals are short circuited, the pointer rests at zero because the current in A is relatively large. The instrument is not damaged under these circumstances because the current is limited by R3. There are two types of hand-driven meggers: the variable type and the constant pressure type. The speed of the variable pressure megger is dependent on how fast the hand crank is turned. The constant pressure megger uses a centrifugal governor, or slip clutch. The governor becomes effective only when the megger is operated at a speed above its slip speed, at which speed its voltage remains constant. ## AC Measuring Instruments A DC meter, such as an ammeter, connected in an AC circuit indicates zero, because the meter movements used in a d’Arsonval type movement is restricted to DC. Since the field of a permanent magnet in the d’Arsonval type meter remains constant and in the same direction at all times, the moving coil follows the polarity of the current. The coil attempts to move in one direction during half of the AC cycle and in the reverse direction during the other half when the current reverses. The current reverses direction too rapidly for the coil to follow, causing the coil to assume an average position. Since the current is equal and opposite during each half of the AC cycle, the DC meter indicates zero, which is the average value. Thus, a meter with a permanent magnet cannot be used to measure alternating voltage and current. For AC measurements of current and voltage, additional circuitry is required. The additional circuitry has a rectifier, which converts AC to DC. There are two basic types of rectifiers: the half-wave rectifier and the full-wave rectifier. *[[FIGURE 12-159.png|[Figure 12-159] ]]* ![[FIGURE 12-159.png|350]] *[[FIGURE 12-159.png|Figure 12-159]]* also shows a simplified block diagram of an AC meter. In this depiction, the full-wave rectifier precedes the meter movement. The movement responds to the average value of the pulsating DC. The scale can then be calibrated to show anything the designer wants. In most cases, it is the root mean square (RMS) value or peak value. ### Electrodynamometer Meter Movement ![[FIGURE 12-160.png|350]] The electrodynamometer can be used to measure alternating or direct voltage and current. It operates on the same principles as the permanent magnet moving coil meter, except that the permanent magnet is replaced by an air core electromagnet. The field of the electrodynamometer is developed by the same current that flows through the moving coil. *[[FIGURE 12-160.png|[Figure 12-160] ]]* Because this movement contains no iron, the electrodynamometer can be used as a movement for both AC and DC instruments. AC can be measured by connecting the stationary and moving coils in series. Whenever the current in the moving coil reverses, the magnetic field produced by the stationary coil reverses. Regardless of the direction of the current, the needle moves in a clockwise direction. However, for either voltmeter or ammeter applications, the electrodynamometer is too expensive to economically compete with the d’Arsonval-type movement. ### Moving Iron Vane Meter ![[FIGURE 12-161.png|350]] The moving iron vane meter is another basic type of meter. It can be used to measure either AC or DC. Unlike the d’Arsonval meter, which employs permanent magnets, it depends on induced magnetism for its operation. It utilizes the principle of repulsion between two concentric iron vanes, one fixed and one movable, placed inside a solenoid. A pointer is attached to the movable vane. *[[FIGURE 12-161.png|[Figure 12-161] ]]* When current flows through the coil, the two iron vanes become magnetized with north poles at their upper ends and south poles at their lower ends for one direction of current through the coil. Because like poles repel, the unbalanced component of force, tangent to the movable element, causes it to turn against the force exerted by the springs. The movable vane is rectangular in shape and the fixed vane is tapered. This design permits the use of a relatively uniform scale. When no current flows through the coil, the movable vane is positioned so that it is opposite the larger portion of the tapered fixed vane, and the scale reading is zero. The amount of magnetization of the vanes depends on the strength of the field, which, in turn, depends on the amount of current flowing through the coil. The force of repulsion is greater opposite the larger end of the fixed vane than it is nearer the smaller end. Therefore, the movable vane moves toward the smaller end through an angle that is proportional to the magnitude of the coil current. The movement ceases when the force of repulsion is balanced by the restraining force of the spring. Because the repulsion is always in the same direction (toward the smaller end of the fixed vane), regardless of the direction of current flow through the coil, the moving iron vane instrument operates on either DC or AC circuits. Mechanical damping in this type of instrument can be obtained by the use of an aluminum vane attached to the shaft so that, as the shaft moves, the vane moves in a restricted air space. When the moving iron vane meter is used as an ammeter, the coil is wound with relatively few turns of large wire in order to carry the rated current. When the moving iron vane meter is used as a volt-meter, the solenoid is wound with many turns of small wire. Portable voltmeters are made with self-contained series resistance for ranges up to 750 volts. Higher ranges are obtained by the use of additional external multipliers. The moving iron vane instrument may be used to measure DC but has an error due to residual magnetism in the vanes. Reversing the meter connections and averaging the readings may minimize the error. When used on AC circuits, the instrument has an accuracy of 0.5 percent. Because of its simplicity, relatively low cost, and the fact that no current is conducted to the moving element, this type of movement is used extensively to measure current and voltage in AC power circuits. However, because the reluctance of the magnetic circuit is high, the moving iron vane meter requires much more power to produce full-scale deflection than is required by a d’Arsonval meter of the same range. Therefore, the moving iron vane meter is seldom used in high-resistance low-power circuits. ### Inclined Coil Iron Vane Meter The principle of the moving iron vane mechanism is applied to the inclined coil type of meter, which can be used to measure both AC and DC. The inclined coil, iron vane meter has a coil mounted at an angle to the shaft. Attached obliquely to the shaft, and located inside the coil, are two soft iron vanes. When no current flows through the coil, a control spring holds the pointer at zero, and the iron vanes lie in planes parallel to the plane of the coil. When current flows through the coil, the vanes tend to line up with magnetic lines passing through the center of the coil at right angles to the plane of the coil. Thus, the vanes rotate against the spring action to move the pointer over the scale. The iron vanes tend to line up with the magnetic lines regardless of the direction of current flow through the coil. Therefore, the inclined coil, iron vane meter can be used to measure either AC or DC. The aluminum disk and the drag magnets provide electromagnetic damping. Like the moving iron vane meter, the inclined coil type requires a relatively large amount of current for full-scale deflection and is seldom used in high-resistance low-power circuits. As in the moving iron vane instruments, the inclined coil instrument is wound with few turns of relatively large wire when used as an ammeter and with many turns of small wire when used as a voltmeter. ### Varmeters ![[FIGURE 12-162.png|350]] Multiplying the volts by the amperes in an AC circuit gives the apparent power: the combination of the [[true power]] (which does the work) and the reactive power (which does no work and is returned to the line). Reactive power is measured in units of vars (volt-amperes reactive) or kilovars (kilovoltamperes reactive (kVAR). When properly connected, wattmeters measure the reactive power. As such, they are called varmeters. *[[FIGURE 12-162.png|[Figure 12-162] ]]* ### Wattmeter ![[FIGURE 12-163.png|350]] Electric power is measured by means of a wattmeter. Because electric power is the product of current and voltage, a wattmeter must have two elements, one for current and the other for voltage. For this reason, wattmeters are usually of the electrodynamometer type. *[[FIGURE 12-163.png|[Figure 12-163] ]]* The movable coil with a series resistance forms the voltage element, and the stationary coils constitute the current element. The strength of the field around the potential coil depends on the amount of current that flows through it. The current, in turn, depends on the load voltage applied across the coil and the high resistance in series with it. The strength of the field around the current coils depends on the amount of current flowing through the load. Thus, the meter deflection is proportional to the product of the voltage across the potential coil and the current through the current coils. The effect is almost the same (if the scale is properly calibrated) as if the voltage applied across the load and the current through the load were multiplied together. If the current in the line is reversed, the direction of current in both coils and the potential coil is reversed, the net result is that the pointer continues to read up scale. Therefore, this type of wattmeter can be used to measure either AC or DC power. ### Frequency Measurement/Oscilloscope The oscilloscope is by far one of the more useful electronic measurements available. The viewing capabilities of the oscilloscope make it possible to see and quantify various waveform characteristics, such as phase relationships, amplitudes, and durations. While oscilloscopes come in a variety of configurations and presentations, the basic operation is typically the same. Most oscilloscopes in general bench or shop applications use a cathode-ray tube (CRT), which is the device or screen that displays the waveforms. The CRT is a vacuum instrument that contains an electron gun, which emits a very narrow and focused beam of electrons. A phosphorescent coat applied to the back of the screen forms the screen. The beam is electronically aimed and accelerated so that the electron beam strikes the screen. When the electron beam strikes the screen, light is emitted at the point of impact. *[[FIGURE 12-164.png|Figure 12-164]]* shows the basic components of the CRT with a block diagram. The heated cathode emits electrons. The magnitude of voltage on the control grid determines the actual flow of electrons and thus controls the intensity of the electron beam. The acceleration anodes increase the speed of the electrons, and the focusing anode narrows the beam down to a fine point. The surface of the screen is also an anode and assists in the acceleration of the electron beam. ![[FIGURE 12-164.png|350]] The purpose of the vertical and horizontal deflection plates is to bend the electron beam and position it to a specific point of the screen. *[[FIGURE 12-165.png|[Figure 12-165] ]]* By providing a neutral or zero voltage to a deflection plate, the electron beam is unaffected. By applying a negative voltage to a plate, the electron beam is repelled and driven away from the plate. Finally, by applying a positive voltage, the electron beam is drawing to the plate. *[[FIGURE 12-165.png|Figure 12-165]]* provides a few possible plate voltage combinations and the resultant beam position. ![[FIGURE 12-165.png|350]] ### Horizontal Deflection ![[FIGURE 12-166.png|350]] To get a visual representation of the input signal, an internally generated saw-tooth voltage is generated and then applied to the horizontal deflection plates. *[[FIGURE 12-166.png|Figure 12-166]]* illustrates that the saw-tooth is a pattern of voltage applied, which begins at a negative voltage and increases at a constant rate to a positive voltage. This applied varying voltage draws or traces the electron beam from the far left of the screen to the far right side of the screen. The resulting display is a straight line, if the sweep rate is fast enough. This saw-tooth applied voltage is a repetitive signal so that the beam is repeatedly swept across the tube. The rate at which the saw-tooth voltage goes from negative to positive is determined by the frequency. This rate then establishes the sweep rate of the beam. When the saw-tooth reaches the end of its sweep from left to right, the beam then rapidly returns to the left side and is ready to make another sweep. During this time, the electron beam is stopped or blanked out and does not produce any kind of a trace. This period of time is called flyback. ### Vertical Deflection If this same signal were applied to the vertical plates, it would also produce a vertical line by causing the beam to trace from the down position to the up position. ### Tracing a Sine Wave Reproducing the sine wave on the oscilloscope combines both the vertical and horizontal deflection patterns. *[[FIGURE 12-167.png|[Figure 12-167] ]]* If the sine wave voltage signal is applied across the vertical deflection plates, the result will be the vertical beam oscillation up and down on the screen. The amount that the beam moves above the centerline depends on the peak value of the voltage. ![[FIGURE 12-167.png|350]] While the beam is being swept from the left to the right by the horizontal plates, the sine wave voltage is being applied to the vertical plates, causing the form of the input signal to be traced out on the screen. ### Control Features on an Oscilloscope While there are many different styles of oscilloscopes, which range from the simple to the complex, they all have some controls in common. Apart from the screen and the ON/OFF switch, some of these controls are listed as follows: - Horizontal Position—allows for the adjustment of the neutral horizontal position of the beam. Use this control to reposition the waveform display in order to have a better view of the wave or to take measurements. - Vertical Position—moves the traced image up or down allowing better observations and measurements. - Focus—controls the electron beam as it is aimed and converges on the screen. When the beam is in sharp focus, it is narrowed down to a very fine point and does not have a fuzzy appearance. - Intensity—essentially the brightness of the trace. Controlling the flow of electrons onto the screen varies the intensity. Do not keep the intensity too high for extended testing or when the beam is motionless and forms a dot on the screen. This can damage the screen. - Seconds/Division—a time-based control that sets the horizontal sweep rate. Basically, the switch is used to select the time interval that each division on the horizontal scale represents. These divisions can be seconds, milliseconds, or even microseconds. A simple example would be if the technician had the seconds/division control set to 10 µS. If this technician is viewing a waveform that has a period of 4 divisions on the screen, then the period would be 40 µS. The frequency of this waveform can then be determined by taking the inverse of the period. In this case, 1⁄40 µS equals a frequency of 25 kHz. - Volts/Division—used to select the voltage interval that each division on the vertical scale represents. For example, suppose each vertical division was set to equal 10 mV. If a waveform was measured and had a peak value of 4 divisions, then the peak value in voltage would be 40 mV. - Trigger—The trigger control provides synchronization between the saw-tooth horizontal sweep and the applied signal on the vertical plates. The benefit is that the waveform on the screen appears to be stationary and fixed and not drifting across the screen. A triggering circuit is used to initiate the start of a sweep rather than the fixed saw-tooth sweep rate. In a typical oscilloscope, this triggering signal comes from the input signal itself at a selected point during the signal’s cycle. The horizontal signal goes through one sweep, retraces back to the left side and waits there until it is triggered again by the input signal to start another sweep. ### Flat Panel Color Displays for Oscilloscopes ![[Pasted image 20210117002310.png|350]] While the standard CRT design of oscilloscope is still in service, the technology of display and control has evolved into use of the flat panel monitors. Furthermore, the newer oscilloscopes can even be integrated with the common personal computer (PC). *[[FIGURE 12-168.png|[Figure 12-168] ]]* Some of the features of this technology include easy data capture, data transfer, documentation, and data analysis. Hand-held oscilloscopes are now available that can perform the functions of larger bench type equipment but are mobile and great tools for trouble shooting. *[[FIGURE 12-169.png|[Figure 12-169] ]]* ### Digital Multimeter Traditionally, the meters that technicians have used have been the analog voltmeter, ammeter, and the ohmmeter. These have usually been combined into the same instrument and called a multimeter or a VOM (volt-ohm-milliammeter). This approach has been both convenient and economical. Digital multimeters (DMM) and digital voltmeters (DVM) are more common due to their ease of use. These meters are easier to read and provide greater accuracy when compared to the older analog units with needle movement. The multimeter’s single-coil movement requires a number of scales, which are not always easy to read accurately. In addition, the loading characteristics due to the internal resistance sometimes affect the circuit and the measurements. Not only does the DVM offer greater accuracy and less ambiguity, but also higher input resistance, which has less of a loading effect and influence on a circuit. ## Basic Circuit Analysis and Troubleshooting Troubleshooting is the systematic process of recognizing the symptoms of a problem, identifying the possible cause, and locating the failed component or conductor in the circuit. To be proficient at troubleshooting, the technician must understand how the circuit operates and know how to properly use the test equipment. There are many ways in which a system can fail and to cover all of the possibilities is beyond the scope of this text. However, there are some basic concepts that enable the technician to handle many of the common faults encountered in the aircraft. Before starting a discussion on basic circuits and troubleshooting, the following definitions are given. - Short circuit—an unintentional low resistance path between two components in a circuit or between a component/conductor and ground. It usually creates high current flow, which burns out or causes damage to the circuit conductor or components. - Open circuit—a circuit that is not a complete or continuous path. An open circuit represents an infinitely large resistance. Switches are common devices used to open and close a circuit. Sometimes a circuit opens due to a component failure, such as a light bulb or a burned out resistor. - Continuity—the state of being continuous, uninterrupted or connected together; the opposite of a circuit that is not broken or does not have an open. - Discontinuity—the opposite of continuity, indicating that a circuit is broken or not continuous. ### Voltage Measurement ![[FIGURE 12-170.png|350]] Voltage is measured across a component with a voltmeter or the voltmeter position on a multimeter. Usually, there is a DC and an AC selection on the meter. Before the meter is used for measurements, make sure that the meter is selected for the correct type of voltage. When placing the probes across a component to take a measurement, take care to ensure that the polarity is correct. *[[FIGURE 12-170.png|[Figure 12-170] ]]* Standard practice is for the red meter lead to be installed in the positive (+) jack and the black meter lead to be installed in the negative meter jack (-). Then when placing the probes across or in parallel with a component to measure the voltage, the leads should match the polarity of the component. The red lead is on the positive side of the component and the black is on the negative side, which prevents damage to the meter or incorrect readings. All meters have some resistance and will shunt some of the current. This has the effect of changing the characteristic of the circuit because of this change in current. This is typically more of a concern with older analog type meters. If there are any questions about the magnitude of the voltage across a component, then the meter should be set to measure on the highest voltage range. This prevents the meter from “pegging” and possible damage. The range should then be selected to low values until the measured voltage is read at the mid-scale deflection. Readings taken at mid-scale are the most accurate. ### Current Measurement Current is measured with the ammeter connected in the current path by opening or breaking the circuit and inserting the meter in series. *[[FIGURE 12-170.png|[Figure 12-170] ]]* Standard practice is for the red meter lead to be installed in the positive (+) jack and the black meter lead to be installed in the negative meter jack (-). The positive side of the meter is connected towards the positive voltage source. Ideally, the meter should not alter the current and influence the circuit and the measurements. However, the meter does have some effect because of its internal resistance that is connected with the rest of the circuit in series. The resistance is rather small and for most practical purposes, this can be neglected. ### Checking Resistance in a Circuit ![[Pasted image 20210302062208.png|350]] The ohmmeter is used to measure the resistance. In its more basic form, the ohmmeter consists of a variable resistor in series with a meter movement and a voltage source. The meter must first be adjusted before use. Refer to *[[FIGURE 12-171.png|Figure 12-171]]* for meter configurations during adjustments. When the meter leads are not connected (open), the needle points to the full left-hand position, indicating infinite resistance or and open circuit. With the lead placed together, the circuit is shorted as shown with the meter needle to the full right-hand position. When a connection is made, the internal battery is allowed to produce a current through the movement coil, causing a deflection of the needle in proportion to the value of the external resistance. In this case, the resistance is zero because the leads are shorted. The purpose of the variable resistor in the meter is to adjust the current so that the pointer reads exactly zero when the leads are shorted. This is needed because as the battery continues to be used, the voltage changes, thus requiring an adjustment. The meter should be “zeroed” before each use. To check the value of a resistor, the resistor must be disconnected from the circuit. This prevents any possible damage to the ohmmeter, and it prevents the possibility of any inaccurate readings due to the circuit being in parallel with the resistor in question. *[[FIGURE 12-172.png|[Figure 12-172] ]]* ### Continuity Checks In many cases, the ohmmeter is not used for measuring the resistance of a component but to simply check the integrity of a connection from one portion of a circuit to another. If there is a good connection, then the ohmmeter reads a near zero resistance or a short. If the circuit is open or has a very poor connection at some point like an over-crimped pin in a connector, then the ohmmeter reads infinity or some very high resistance. Keep in mind that while any measurement is being taken, contact with the circuit or probes should be avoided. Contact can introduce another parallel path and provide misleading indications. ### Capacitance Measurement ![[FIGURE 12-173.png|350]] *[[FIGURE 12-173.png|Figure 12-173]]* illustrates a basic test of a capacitor with an ohmmeter. There are usually two common modes of failure for a capacitor. One is a complete failure characterized by short circuit through the capacitor due to the dielectric breaking down or an open circuit. The more insidious failure occurs due to degradation, which is a gradual deterioration of the capacitor’s characteristics. If a problem is suspected, remove the capacitor from the circuit and check with an ohmmeter. The first step is to short the two leads of the capacitor to ensure that it is entirely discharged. Next, connect the two leads as shown in *[[FIGURE 12-173.png|Figure 12-173]]* across the capacitor and observe the needle movement. At first, the needle should indicate a short circuit. Then as the capacitor begins to charge, the needle should move to the left or infinity and eventually indicate an open circuit. The capacitor takes its charge from the internal battery of the ohmmeter. The greater the capacitance, the longer it takes to charge. If the capacitor is shorted, then the needle remains at a very low or shorted resistance. If there is some internal deterioration of the dielectric, then the needle never reaches a high resistance but some intermediate value, indicating a current. ### Inductance Measurement The common mode of failure in an inductor is an open. To check the integrity of an inductor, it must be removed from the circuit and tested as an isolated component just like the capacitor. If there is an open in the inductor, a simple check with an ohmmeter shows it as an open circuit with infinite resistance. If in fact the inductor is in good condition, then the ohmmeter indicates the resistance of the coil. On occasions, the inductor fails due to overheating. When the inductor is overheated, it is possible for the insulation covering the wire in the coil to melt, causing a short. The effects of a shorted coil are that of reducing the number of turns. At this point, further testing of the inductor must be done with test equipment not covered in this text. ### Troubleshooting Open Faults in a Series Circuit One of the most common modes of failure is the “open” circuit. A component, such as a resistor, can overheat due to the power rating being exceeded. Other more frustrating problems can happen when a “cold” solder joint cracks leaving a wire disconnected from a relay or connector. This type of damage can occur during routine maintenance after a technician has accessed an area for inspections. In many cases, there is no visual indication that a failure has occurred, and the soon-to-be-frustrated technician is unaware that there is a problem until power is reapplied to the aircraft in the final days leading up to aircraft delivery and scheduled operations. ![[FIGURE 12-174.png|350]] The first example is a simplified diagram shown in Figures 12-174 through 12-176. The circuit depicted in *[[FIGURE 12-174.png|Figure 12-174]]* is designed to cause current to flow through a lamp, but because of the open resistor, the lamp will not light. To locate this open, a voltmeter or an ohmmeter should be used. ### Tracing Opens with the Voltmeter A general procedure to follow in this case is to measure the voltage drop across each component in the circuit, keeping in mind the following points. If there is an open in a series circuit, then the voltage drops on sides of the component. In this case, the total voltage must appear across the open resistor as per Kirchhoff’s Voltage Law. ![[Pasted image 20210302062326.png|350]] If a voltmeter is connected across the lamp, as shown in *[[FIGURE 12-175.png|Figure 12-175]]* , the voltmeter reads zero. Since no current can flow in the circuit because of the open resistor, there is no voltage drop across the lamp indicating that the lamp is good. Next, the voltmeter is connected across the open resistor, as shown in *[[FIGURE 12-176.png|Figure 12-176]]* . The voltmeter has closed the circuit by shunting (paralleling) the burned out resistor, allowing current to flow. Current flows from the negative terminal of the battery, through the switch, through the voltmeter and the lamp, back to the positive terminal of the battery. However, the resistance of the voltmeter is so high that only a very small current flows in the circuit. The current is too small to light the lamp, but the voltmeter reads the battery voltage. ### Tracing Opens with the Ohmmeter A simplified circuit, as shown in Figures 12-177 and 12-178, illustrates how to locate an open in a series circuit using the ohmmeter. A general rule to keep in mind when troubleshooting with an ohmmeter is: when an ohmmeter is properly connected across a circuit component and a resistance reading is obtained, the component has continuity and is not open. ![[FIGURE 12-177.png|350]] When an ohmmeter is used, the circuit component to be tested must be isolated and the power source removed from the circuit. In this case, these requirements can be met by opening the circuit switch as shown in *[[FIGURE 12-177.png|Figure 12-177]]* . The ohmmeter is zeroed and across all good components is zero. The voltage drop across the open component equals the total voltage across the series combination. This condition happens because the open component prevents current to pass through the series circuit. With there being no current, there can be no voltage drop across any of the good components. Because the current is zero, it can be determined by Ohm’s Law that E = IR = 0 volts across a component. The voltage is the same on both places across (in parallel with) the lamp. In this testing configuration, some value of resistance is read indicating that the lamp is in good condition and is not the source of the open in the circuit. ![[FIGURE 12-178.png|350]] Now the technician should move to the resistor and place the ohmmeter probe across it as shown in *[[FIGURE 12-178.png|Figure 12-178]]* . When the ohmmeter is connected across the open resistor, it indicates infinite resistance, or a discontinuity. Thus, the circuit open has now been located. ### Troubleshooting Shorting Faults in a Series Circuit An open fault can cause a component or system not to work, which can be critical and hazardous. A shorting fault can potentially be more of a severe nature than the open type of fault. A short circuit, or “short,” causes the opposite effect. A short across a series circuit produces a greater than normal current flow. Faults of this type can develop slowly when a wire bundle is not properly secured and is allowed to chafe against the airframe structure or other systems, such as hydraulic lines. Shorts can also occur due to a careless technician using incorrect hardware when installing an interior. If screws that are too long are used to install trim, it is possible to penetrate a wire bundle immediately causing numerous shorts. Worse yet, are the shorts that are not immediately seen but “latent” and do not show symptoms until the aircraft is in service. Another point to keep in mind is when closing panels. Wires can become pinched between the panel and the airframe causing either a short or a latent, intermittent short. The simplified circuit, shown in Figures 12-179 through 12-182 is used to illustrate troubleshooting a short in a series circuit. ![[FIGURE 12-179.png|350]] In *[[FIGURE 12-179.png|Figure 12-179]]* , a circuit is designed to light a lamp. A resistor is connected in the circuit to limit current flow. If the resistor is shorted, as shown in the illustration, the current flow increases and the lamp becomes brighter. If the applied voltage were high enough, the lamp would burn out, but in this case the fuse would protect the lamp by opening first. ![[FIGURE 12-180.png|350]] Usually a short circuit produces an open circuit by either blowing (opening) the fuse or burning out a circuit component. But in some circuits, there may be additional resistors which do not allow one shorted resistor to increase the current flow enough to blow the fuse or burn out a component. *[[FIGURE 12-180.png|[Figure 12-180] ]]* Thus, with one resistor shorted out, the circuit still functions since the power dissipated by the other resistors does not exceed the rating of the fuse. ### Tracing Shorts with the Ohmmeter ![[FIGURE 12-181.png|350]] The shorted resistor can be located with an ohmmeter. *[[FIGURE 12-181.png|[Figure 12-181] ]]* First the switch is opened to isolate the circuit components. In *[[FIGURE 12-181.png|Figure 12-181]]* , this circuit is shown with an ohmmeter connected across each of the resistors. Only the ohmmeter connected across the shorted resistor shows a zero reading, indicating that this resistor is shorted. ### Tracing Shorts with the Voltmeter ![[FIGURE 12-182.png|350]] To locate the shorted resistor while the circuit is functioning, a voltmeter can be used. *[[FIGURE 12-182.png|Figure 12-182]]* illustrates that when a voltmeter is connected across any of the resistors that are not shorted, a portion of the applied voltage is indicated on the voltmeter scale. When it is connected across the shorted resistor, the voltmeter reads zero. ### Troubleshooting Open Faults in a Parallel Circuit The procedures used in troubleshooting a parallel circuit are sometimes different from those used in a series circuit. Unlike a series circuit, a parallel circuit has more than one path in which current flows. A voltmeter cannot be used, since, when it is placed across an open resistor, it reads the voltage drop in a parallel branch. But an ammeter or the modified use of an ohmmeter can be employed to detect an open branch in a parallel circuit. ![[FIGURE 12-183.png|350]] If the open resistor shown in *[[FIGURE 12-183.png|Figure 12-183]]* was not visually apparent, the circuit might appear to be functioning properly, because current would continue to flow in the other two branches of the circuit. To determine that the circuit is not operating properly, a determination must be made as to how the circuit should behave when working properly. First, the total resistance, total current, and the branch currents of the circuit should be calculated as if there were no open in the circuit. In this case, the total resistance can be simply determined by: $\begin{align} &R_T = \frac{R}{N}\\ \text{Where} \quad &R_T\text{ is the total circuit resistance}\\ &N \text{ is the number of resistors}\\ &R \text{ is the resistor value }\\ &R_T = \frac{30Ω}{3} = 10Ω \end{align}$ The total current of the circuit can now be determined by using Ohm’s Law: $\begin{align} &I_T = \frac{E_S}{R_T}\\ \text{Where} \quad &I_T\text{ is the total current}\\ &E_S\text{ is the source voltage across the parallel branch}\\ &R_T\text{ is the resistor value }\\ &I_T = \frac{30v}{10Ω} = 3 \text{ amperes (total current)} \end{align}$ Each branch current should be determined in a similar manner. For the first branch, the current is: $\begin{align} &I_1 = \frac{E_S}{R_1}\\ \text{Where} \quad &I_1\text{ is the current in the first branch}\\ \qquad &E_S\text{ is the source voltage across the parallel branch}\\ &R_1\text{ is the resistance of the first branch}\\ &I_T = \frac{30v}{30Ω} = 1 \text{ amperes}\\ \end{align}$ Because the other two branches are of the same resistive value, then the current in each of those branches is 1 ampere also. Adding up the amperes in each branch confirms the initial calculation of total current being 3 amperes. ### Tracing an Open with an Ammeter If the technician now places an ammeter in the circuit, the total current would be indicated as 2 amperes as shown in *[[FIGURE 12-183.png|Figure 12-183]]* instead of the calculated 3 amperes. Since 1 ampere of current should be flowing through each branch, it is obvious that one branch is open. If the ammeter is then connected into the branches, one after another, the open branch is eventually located by a zero ammeter reading. ### Tracing an Open with an Ohmmeter ![[FIGURE 12-184.png|350]] A modified use of the ohmmeter can also locate this type of open. If the ohmmeter is connected across the open resistor, as shown in *[[FIGURE 12-184.png|Figure 12-184]]*, an erroneous reading of continuity would be obtained. Even though the circuit switch is open, the open resistor is still in parallel with $R_1$ and $R_2$, and the ohmmeter would indicate the open resistor had a resistance of 15 ohms, the equivalent resistance of the parallel combination of $R_1$ and $R_2$. ![[FIGURE 12-185.png|350]] Therefore, it is necessary to open the circuit as shown in *[[FIGURE 12-185.png|Figure 12-185]]* in order to check the resistance of $R_3$. In this way, the resistor is not shunted (paralleled) by $R_1$ and $R_2$. The reading on the ohmmeter now indicates infinite resistance, which means the open component has been isolated. ### Troubleshooting Shorting Faults in Parallel Circuits ![[FIGURE 12-186.png|350]] As in a series circuit, a short in a parallel circuit usually causes an open circuit by blowing the fuse. But, unlike a series circuit, one shorted component in a parallel circuit stops current flow by causing the fuse to open. Refer to the circuit in *[[FIGURE 12-186.png|Figure 12-186]]*. If resistor $R_3$ is shorted, a path of almost zero resistance is offered the current, and all the circuit current flows through the branch containing the shorted resistor. Since this is practically the same as connecting a wire between the terminals of the battery, the current rises to an excessive value, and the fuse opens. Since the fuse opens almost as soon as a resistor shorts out, there is no time to perform a current or voltage check. Thus, troubleshooting a parallel DC circuit for a shorted component should be accomplished with an ohmmeter. But, as in the case of checking for an open resistor in a parallel circuit, a shorted resistor can be detected with an ohmmeter only if one end of the shorted resistor is disconnected and isolated from the rest of the circuit. ### Troubleshooting Shorting Faults in Series-Parallel Circuits #### Logic in Tracing an Open Troubleshooting a series-parallel resistive circuit involves locating malfunctions similar to those found in a series or a parallel circuit. Figures 12-187 through 12-189 illustrate three points of failure in a series-parallel circuit and their generalized effects. 1. In the circuit shown in *[[FIGURE 12-187.png|Figure 12-187]]*, an open has occurred in the series portion of the circuit. When the open occurs anywhere in the series portion of a series-parallel circuit, current flow in the entire circuit stops. In this case, the circuit does not function, and the lamp, L1, is not lit. ![[FIGURE 12-187.png|350]] 2. If the open occurs in the parallel portion of a seriesparallel circuit, as shown in *[[FIGURE 12-188.png|Figure 12-188]]*, part of the circuit continues to function. In this case, the lamp continues to burn, but its brightness diminishes, since the total resistance of the circuit has increased and the total current has decreased. ![[FIGURE 12-188.png|350]] 3. If the open occurs in the branch containing the lamp, as shown in *[[FIGURE 12-189.png|Figure 12-189]]*, the circuit continues to function with increased resistance and decreased current, but the lamp does not light. ![[FIGURE 12-189.png|350]] ### Tracing Opens with the Voltmeter ![[FIGURE 12-190.png|350]] To explain how the voltmeter and ohmmeter can be used to troubleshoot series-parallel circuits, the circuit shown in *[[FIGURE 12-190.png|Figure 12-190]]* has been labeled at various points. A pointto-point description is listed below with expected results: 1. By connecting a voltmeter between points A and D, the battery and switch can be checked for opens. 2. By connecting the voltmeter between points A and B, the voltage drop across $R_1$ can be checked. This voltage drop is a portion of the applied voltage. 3. If $R_1$ is open, the reading between B and D is zero. 4. By connecting a voltmeter between A and E, the continuity of the conductor between the positive terminal of the battery and point E, as well as the fuse, can be checked. If the conductor or fuse is open, the voltmeter reads zero. 5. If the lamp is burning, it is obvious that no open exists in the branch containing the lamp, and the voltmeter could be used to detect an open in the branch containing $R_2$ by removing lamp, $L_1$, from the circuit. Troubleshooting the series portion of a series-parallel circuit presents no difficulties, but in the parallel portion of the circuit, misleading readings can be obtained. ## Batteries ### Primary Cell The dry cell is the most common type of primary-cell battery and is similar in its characteristics to that of an electrolytic cell. This type of a battery is basically designed with a metal electrode or graphite rod acting as the cathode (+) terminal, immersed in an electrolytic paste. This electrode⁄electrolytic build-up is then encased in a metal container, usually made of zinc, which itself acts as the anode (-) terminal. When the battery is in a discharge condition an electrochemical reaction takes place resulting in one of the metals being consumed. Because of this consumption, the charging process is not reversible. Attempting to reverse the chemical reaction in a primary cell by way of recharging is usually dangerous and can lead to a battery explosion. These batteries are commonly used to power items such as flashlights. The most common primary cells today are found in alkaline batteries, silver-oxide, and lithium batteries. The earlier carbon-zinc cells, with a carbon post as cathode and a zinc shell as anode were once prevalent but are not as common. ### Secondary Cell A secondary cell is any kind of electrolytic cell in which the electrochemical reaction that releases energy is reversible. The lead-acid car battery is a secondary-cell battery. The electrolyte is sulfuric acid (battery acid), the positive electrode is lead peroxide, and the negative electrode is lead. A typical lead-acid battery consists of six lead-acid cells in a case. Each cell produces 2 volts, so the whole battery produces a total of 12 volts. Other commonly used secondary cell chemistry types are nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and Lithium ion polymer (Li-ion polymer). ![[FIGURE 12-191.png|350]] Lead-acid batteries used in aircraft are similar to automobile batteries. The lead acid battery is made up of a series of identical cells each containing sets of positive and negative plates. *[[FIGURE 12-191.png|Figure 12-191]]*, illustrates each cell contains positive plates of lead dioxide ($PbO_2$), negative plates of spongy lead, and electrolyte (sulfuric acid and water). A practical cell is constructed with many more plates than just two in order to get the required current output. All positive plates are connected together as well as all the negatives. Because each positive plate is always positioned between two negative plates, there are always one or more negative plates than positive plates. Between the plates are porous separators that keep the positive and negative plates from touching each other 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. ![[FIGURE 12-192.png|350]] Each cell is seated in a hard rubber casing through the top of which are terminal posts and a hole into which a nonspill vent cap is screwed. 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. *[[FIGURE 12-192.png|[Figure 12-192] ]]* 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. ![[FIGURE 12-193.png|350]] The individual cells of the battery are connected in series by means of cell straps. *[[FIGURE 12-193.png|[Figure 12-193] ]]* 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, 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. ![[FIGURE 12-194.png|350]] To facilitate installation and removal of the battery in some aircraft, a quick disconnect assembly is used to connect the power leads to the battery. This assembly attaches the battery leads in the aircraft to a receptacle mounted on the side of the battery. *[[FIGURE 12-194.png|[Figure 12-194] ]]* 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. The voltage of lead acid cell is approximately two volts in order to attain the voltage required for the application. Each cell is then connected in series with heavy gauge metal straps to form a battery. In a typical battery, such as that used in an aircraft for starting, the voltage required is 12 or 24 volts. This voltage is achieved by connecting six cells or twelve cells respectively together in series and enclosing them in one plastic box. Each cell containing the plates are filled with an electrolyte composed of sulfuric acid and distilled water with a specific gravity of 1.270 at 60 °F. This solution contains positive hydrogen ions and negative sulfate ($SO_4$) ions that are free to combine with other ions and form a new chemical compound. When the cell is discharged, electrons leave the negative plate and flow to the positive plates where they cause the lead dioxide ($PbO_2$) to break down into negative oxygen ions and positive lead ions. The negative oxygen ions join with positive hydrogen ions from the sulfuric acid and form water ($H_2O$). The negative sulfate ions join with the lead ions in both plates and form lead sulfate ($PbSO_4$). After the discharge, the specific gravity changes to about 1.150. ### 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 approximately 2 volts. 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 most common battery rating is the amp-hour rating. This is a unit of measurement for battery capacity. It is determined by multiplying a current flow in amperes by the time in hours that the battery is being discharged. A battery with a capacity of 1 amp-hour should be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for 1⁄2 hour, or 1⁄3 amp for 3 hours, etc., before becoming completely discharged. 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. ### Life Cycle of a Battery Battery life cycle is defined as the number of complete charge/discharge cycles a battery can perform before its normal charge capacity falls below 80 percent of its initial rated capacity. Battery life can vary anywhere from 500 to 1,300 cycles. Various factors can cause deterioration of a battery and shorten its service life. The first is over-discharging, which causes excess sulfation; second, too-rapid charging or discharging that results in overheating of the plates and shedding of active material. The accumulation of shed 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. The deterioration can continue to a point where cell capacity can drop to 80 percent after 1,000 cycles. In many cases, the cell can continue working to nearly 2,000 cycles but with a diminished capacity of 60 percent of its original state. ### 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 that measures the specific gravity (weight as compared with water) of liquids. ![[FIGURE 12-195.png|350]] The most commonly used hydrometer consists of a small sealed glass tube weighted at its lower end so it floats upright. *[[FIGURE 12-195.png|[Figure 12-195] ]]* 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 floats; 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) becomes 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 that wouldoon 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 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 consider. 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. Always take hydrometer readings 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 that does not represent the true strength of the solution. Exercise extreme care when making the hydrometer test of a lead-acid cell. Handle the electrolyte carefully because sulfuric acid burns clothing and skin. If the acid does contact the skin, wash the area thoroughly with water and then apply bicarbonate of soda. ### Lead-Acid Battery Charging Methods Passing direct current through the battery in a direction opposite to that of the discharge current may charge a storage battery. Because of the internal resistance (IR) in the battery, the 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 × 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). ![[FIGURE 12-196.png|350]] Batteries are charged by either the constant voltage or constant current method. In the constant voltage method *[[FIGURE 12-196.png|[Figure 12-196A] ]]* a motor generator 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 *[[FIGURE 12-196.png|[Figure 12-196B] ]]*, 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 exercised. In the aircraft, the storage battery is charged by direct current from the aircraft generator system. This method of charging is the constant voltage method, since the generator voltage is held constant by use of a voltage regulator. ![[FIGURE 12-197.png|350]] When a storage battery is being charged, it generates a certain amount of hydrogen and oxygen. Since this is an explosive mixture, it is important to take steps to prevent ignition of the gas mixture. Loosen the vent caps and leave in place. Do not permit open flames, sparks, or other sources of ignition in the vicinity. Before disconnecting or connecting a battery to the charge, always turn off the power by means of a remote switch. *[[FIGURE 12-197.png|Figure 12-197]]* shows battery charging equipment. ## Nickel-Cadmium Batteries ### Chemistry and Construction Active materials in nickel-cadmium cells (Ni-Cad) are nickel hydrate (NiOOH) in the charged positive plate (Anode) and sponge cadmium (Cd) in the charged negative plate (Cathode). The electrolyte is a potassium hydroxide (KOH) solution in concentration of 20–34 percent by weight pure KOH in distilled water. ![[FIGURE 12-198.png|350]] Sintered nickel-cadmium cells have relatively thin sintered nickel matrices forming a plate grid structure. The grid structure is highly porous and is impregnated with the active positive material (nickel-hydroxide) and the negative material (cadmium-hydroxide). The plates are then formed by sintering nickel powder to fine-mesh wire screen. In other variations of the process, the active material in the sintered matrix is converted chemically, or thermally, to an active state and then formed. In general, there are many steps to these cycles of impregnation and formation. Thin sintered plate cells are ideally suited for very high rate charge and discharge service. Pocket plate nickel-cadmium cells have the positive or negative active material, pressed into pockets of perforated nickel-plated steel plates or into tubes. The active material is trapped securely in contact with a metal current collector so active material shedding is largely eliminated. Plate designs vary in thickness depending upon cycling service requirements. The typical open circuit cell voltage of a nickel-cadmium battery is about 1.25 volts. *[[FIGURE 12-198.png|Figure 12-198]]* shows a nickel cadmium aircraft battery. ### Operation of Nickel-Cadmium Cells When a charging current is applied to a nickel-cadmium 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 also occurs 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 occurs. To completely charge a nickel-cadmium battery, some gassing, however slight, must take place; thus, some water is 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 is 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. #### General Maintenance and Safety Precautions Refer to the battery manufacturer for detailed service instructions. Below are general recommendations for maintenance and safety precautions. For vented nickel-cadmium cells, the general maintenance requirements are: 1. Hydrate cells to supply water lost during overcharging. 2. Maintain inter-cell connectors at proper torque values. 3. Keep cell tops and exposed sides clean and dry. Electrolyte spillage can form grounding paths. White moss around vent cap seals is potassium carbonate ($K_2CO_3$). Clean up these surfaces with distilled water and dry. While handling the caustic potassium hydroxide electrolyte, wear safety goggles to protect the eyes. The technician should also wear plastic gloves and an apron to protect skin and clothes. In case of spillage on hands or clothes, neutralize the alkali immediately with vinegar or dilute boric acid solution (one pound per gallon of water); then rinse with clear water. During overcharging conditions, explosive mixtures of hydrogen and oxygen develop in nickel-cadmium cells. When this occurs, the cell relief valves vent these gases to the atmosphere, creating a potentially explosive hazard. Additionally, room ventilation should be such as to prevent a hydrogen build up in closed spaces from exceeding one percent by volume. Explosions can occur at concentrations above four percent by volume in air. ### Sealed Lead Acid (SLA) Batteries In many applications, sealed lead acid (SLA) batteries are gaining in use over flooded lead acid and Ni-Cad batteries. One leading characteristic of Ni-Cad batteries is that they perform well in low voltage, full-discharge, high cycle applications. However, they do not perform as well in extended standby applications, such as auxiliary or as emergency battery packs used to power inertial reference units or stand-by equipment (attitude gyro). It is typical during the servicing of a Ni-Cad battery to match as many as twenty individual cells in order to prevent unbalance and thus cell reversal during end of discharge. When a Ni-Cad does reverse, very high pressure and heat can result. The result is often pressure seal rupture, and in the worst case, a cell explosion. With SLA batteries, cell matching is inherent in each battery. Ni-Cads also have an undesirable characteristic caused by constant overcharge and infrequent discharges, as in standby applications. It is technically known as “voltage depression” and commonly but erroneously called “memory effect.” This characteristic is only detectable when a full discharge is attempted. Thus, it is possible to believe a full charge exists, while in fact it does not. SLA batteries do not have this characteristic voltage depression (memory) phenomenon, and therefore do not require scheduled deep cycle maintenance as do Ni-Cads. The Ni-Cad emergency battery pack requires relatively complicated test equipment due to the complex characteristics of the Ni-Cad. Sealed lead acid batteries do not have these temperamental characteristics and therefore it is not necessary to purchase special battery maintenance equipment. Some manufacturers of SLA batteries have included in the battery packs a means by which the battery can be tested while still installed on the aircraft. Ni-Cads must have a scheduled energy test performed on the bench due to the inability to measure their energy level on the aircraft, and because of their notable “memory” shortcoming. ![[FIGURE 12-199.png|350]] The SLA battery can be designed to alert the technician if a battery is failing. Furthermore, it may be possible to test the failure detection circuits by activating a Built in Test (BITE) button. This practice significantly reduces FAA paperwork and maintenance workload. *[[FIGURE 12-199.png|Figure 12-199]]* shows a SLA battery. ## Lithium Ion Batteries ![[FIGURE 12-200.png|350]] Lithium ion batteries are the primary type of battery for many consumer type of equipment, such as cell phones, battery-powered tools, and computers, but now they are also being used in commercial and military aircraft. The FAA has certified lithium ion batteries to be used on aircraft and one of the first aircraft to utilize the lithium ion battery is the Boeing 787. The three primary functional components of a lithium-ion battery are the positive and negative electrodes and electrolyte. Generally, the negative electrode of a conventional lithium-ion cell is made from carbon. The positive electrode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent. The electrochemical roles of the electrodes reverse between anode and cathode, depending on the direction of current flow through the cell. Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard since they contain, unlike other rechargeable batteries, a flammable electrolyte and are also kept pressurized. Under certain conditions, they can overheat and a fire can occur. The Boeing 787 aircraft utilizes two large 32V 8 cell lithiumion batteries. These batteries are much lighter and more powerful than Ni-Cad batteries used in similar-sized aircraft. These batteries can produce 150 A for airplane power up. *[[FIGURE 12-200.png|Figure 12-200]]* shows a B787 battery. [[AMT General Handbook Ch12_5|➡️]]