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.
12-84 Break V + − Black lead Needle moves full-scale (short) Conventional current Initial charging of capacitor behaves like a shorted circuit. Red lead 0 Simplified ohmmeter ∞ + − +− Black lead Needle moves full-scale (open) Full charge no current Once charged, the capacitor behaves like an open circuit. Red lead 0 Simplified ohmmeter ∞ circuit components. In 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 To locate the shorted resistor while the circuit is functioning, a voltmeter can be used. 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. If the open resistor shown in 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: RT = R N Where RT is the total circuit resistance N is the number of resistors R is the resistor value 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