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.
12-82 Ammeter symbol Voltmeter symbol Meter symbols Simplified use of meters Ohmmeter symbol R1 + − 3k Ω Open the circuit and insert ammeter in series with the load A R1 + + − −3k Ω Voltage is measured across a component V R1 + + − −3k Ω Resistance is measured across a component Ω A V Ω done with test equipment not covered in this handbook. 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. The first example is a simplified diagram shown in Figures 12-174 through 12-176. The circuit depicted in 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 V oltage Law. If a voltmeter is connected across the lamp, as shown in 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. 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. 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. 12-83 + − Black lead Open circuit Red lead 0 Simplified ohmmeter ∞ + − Black lead Shorted circuit Red lead 0 Simplified ohmmeter ∞ + − Black lead R1 Red lead Isolate from circuit connect across resistor 0 Simplified ohmmeter ∞ R2 R3 + − 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.
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