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.
12-85 Break V Break Ω Break Ω component. RT = = 10 Ω30 Ω 3 The total current of the circuit can now be determined by using Ohm’s Law: IT = ES RT Where IT is the total current ES is the source voltage across the parallel branch RT is the total resistance of the parallel branch IT = = 3 amperes (total current)30 v 10 Ω Each branch current should be determined in a similar manner. For the first branch, the current is: I1 = ES R1 Where I1 is the current in the first branch ES is the source voltage across the parallel branch R1 is the resistance of the first branch I1 = = 1 ampere30 v 30 Ω 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 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 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, 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 R2, 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 R2.
Therefore, it is necessary to open the circuit as shown in way, the resistor is not shunted (paralleled) by R1 and R2. The reading on the ohmmeter now indicates infinite resistance, which means the open component has been isolated. Troubleshooting Shorting Faults in Parallel Circuits 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. If resistor R3 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 12-86 V Some voltage Short Zero voltage Ω Ω Ω 40 Ω 50 Ω R1 R3 R2 R4 10 Ω 100 V 0 Ω 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, 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.
2. If the open occurs in the parallel portion of a series- parallel circuit, as shown in 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. 3. If the open occurs in the branch containing the lamp, as shown in Figure 12-189, the circuit continues to function with increased resistance and decreased current, but the lamp does not light. Tracing Opens with the Voltmeter To explain how the voltmeter and ohmmeter can be used to troubleshoot series-parallel circuits, the circuit shown in to-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 R1 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,