Tracing Opens with the Ohmmeter

A simplified circuit as shown in Figures 168 and 169 illustrates how to locate an open in a series circuit using the ohmmeter. A general rule to keep in mind when trouble shooting 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, as shown in Figure 168, these requirements can be met by open opening the circuit switch. The ohmmeter is zeroed and across all good components will be zero. The voltage drop across the open component will equal the total voltage across the series combination. This condition happens because the open component will prevent 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 169. When the ohmmeter is connected across the open resistor, it indicates infinite resistance, or a discontinuity. Thus, the circuit open has now been located.