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Aviation Maintenance Technician Handbook–General

FAA-H-8083-30B Version 2023
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Chapter 12

Fundamentals of Electricity & Electronics

Formula: ES – E1 – E2 – E3 = 0 volts

Substitute the known values:

Formula: 24v – 12v – 10v − E3 = 0 ; Collect known values: 2v – E3 = 0 ; Solve for the unknown: E3 = 2 volts ; Determine the value of E4 in Figure 12-88. For this example, ; I = 200mA.

First, the voltage drop across each of the individual resistors must be determined.

Formula: E1 = I (R1) ; E1 = (200 mA) (10 Ω)
Figure 12-84. Voltage sources in series add algebraically.
Figure 12-84. Voltage sources in series add algebraically.
Formula: E2 = (200 mA) (50 Ω) ; Voltage drop across R2 E2 = 10 volts ; E3 = I (R3) ; E3 = (200 mA) (100 Ω) ; Voltage drop across R3 E3 = 20 volts

Kirchhoff’s Voltage Law is now employed to determine the voltage drop across E4.

With four resistors in the circuit

Formula: ES – E1 – E2 – E3 – E4 = 0 volts ; Substituting values: ; 100v – 2v – 10v – 20v – E4 = 0 ; Combine: 68v – E4 = 0 ; Solve for unknown: E4 = 68v

Using Ohm’s Law and substituting in E4, the value for R4 can now be determined.

Figure 12-85. Voltage sources add algebraically; one source reversed.
Figure 12-85. Voltage sources add algebraically; one source reversed.
Formula: Ohm’s Law: R = E ; I ; Specific application: R4 = E ; 4 ; I ; 68 V ; Substitute values: R4 = ; 200 mA ; Value for R4: R4 = 340 Ω

Voltage Dividers

Voltage dividers are devices that make it possible to obtain more than one voltage from a single power source. A voltage divider usually consists of a resistor, or resistors connected in series, with fixed or movable contacts and two fixed terminal contacts. As current flows through the resistor, different voltages can be obtained between the contacts.

Series circuits are used for voltage dividers. The voltage divider rule allows the technician to calculate the voltage across one or a combination of series resistors without having to first calculate the current in the circuit. [Figure 12-89] Because the current flows through each resistor, the voltage drops are proportional to the ohmic values of the constituent resistors. To understand how a voltage divider works, examine Figure 12-90 carefully and observe the following:

Each load draws a given amount of current: I1, I2, I3. In addition to the load currents, some bleeder current (IB) flows. The current (IT) is drawn from the power source and is equal to the sum of all currents.

Figure 12-86. Kirchhoff’s Voltage Law.
Figure 12-86. Kirchhoff’s Voltage Law.
Figure 12-87. Determine the unknown voltage drop.
Figure 12-87. Determine the unknown voltage drop.

The voltage at each point is measured with respect to a common point. Note that the common point is the point at which the total current (IT) divides into separate currents (I1, I2, I3). Each part of the voltage divider has a different current flowing in it. The current distribution is as follows: