Basic Circuit Analysis and Troubleshooting (Part Two)

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. Using an ohmmeter to check a circuit component.

Figure 12-178. Using an ohmmeter to locate an open in a circuit component.

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. 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. Now the technician should move to the resistor and place the ohmmeter probe across it as shown in 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.

In 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.

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] 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.

Figure 12-180. A short that does not open the circuit.

Tracing Shorts with the Ohmmeter

The shorted resistor can be located with an ohmmeter. [Figure 12-181] First the switch is opened to isolate the 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.

Figure 12-181. Using an ohmmeter to locate a shorted resistor.

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.

Figure 12-182. Voltmeter connected across resistors.

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.

Figure 12-183. Finding an open branch in a parallel 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:

The total current of the circuit can now be determined by using Ohm’s Law:

Each branch current should be determined in a similar manner. For the first branch, the current is:

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 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

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 R₁ and R₂, and the ohmmeter would indicate the open resistor had a resistance of 15 ohms, the equivalent resistance of the parallel combination of R₁ and R₂.

Figure 12-184. A misleading ohmmeter indication.

Therefore, it is necessary to open the circuit as shown in Figure 12-185 in order to check the resistance of R₃. In this 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.

Figure 12-185. Opening a branch circuit to obtain an accurate ohmmeter reading.

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.

Figure 12-186. A shorted component causes the fuse to open.

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.

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.
Figure 12-187. An open in the series portion of a series-parallel circuit.
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.
Figure 12-188. An open in the parallel portion of a series-parallel circuit.
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.
Figure 12-189. An open lamp in a series-parallel circuit.

Tracing Opens with the Voltmeter

To explain how the voltmeter and ohmmeter can be used to troubleshoot series-parallel circuits, the circuit shown in Figure 12-190 has been labeled at various points.

Figure 12-190. Using the voltmeter to troubleshoot a series-parallel circuit.

A point-to-point description is listed below with expected results:

By connecting a voltmeter between points A and D, the battery and switch can be checked for opens.
By connecting the voltmeter between points A and B, the voltage drop across R₁ can be checked. This voltage drop is a portion of the applied voltage.
If R₁ is open, the reading between B and D is zero.
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.
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 R₂ by removing lamp, L₁, 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.

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