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Difficulty: MediumCircuit Faults and Troubleshooting

A 12 V12\text{ V} direct-current (DC) circuit consists of a power source, a single-pole toggle switch, and an indicator lamp wired in series. When the switch is flipped to the ON (closed) position, the lamp fails to illuminate. A technician connects a digital multimeter across the two terminals of the closed switch and reads 12 V12\text{ V}. Which of the following faults is indicated by this measurement?

  1. A
    The indicator lamp has a short circuit across its filament.
  2. B
    The circuit is operating normally and drawing maximum rated current.
  3. The switch contacts have failed open internally.Answer
  4. D
    The indicator lamp filament is burned open.

Answer

The switch contacts have failed open internally.
In a series circuit, a closed switch should exhibit continuity (0 Ω0\ \Omega) and produce a 0 V0\text{ V} drop. When a digital multimeter measures the entire source voltage (12 V12\text{ V}) across a switch in the ON position, it proves that the switch contacts have failed to close electrically, acting as an open circuit that drops the entire available potential.

Step-by-Step Solution

1
Analyze the expected behavior of a properly closed switch in a series circuit.
An ideal closed switch provides continuity with approximately 0 Ω0\ \Omega resistance and a voltage drop of nearly 0 V0\text{ V}.
According to Ohm's Law (V=I×RV = I \times R), current flowing through near-zero resistance produces negligible voltage drop.
2
Evaluate the meter reading across the switch terminals.
The meter measures 12 V12\text{ V}, which equals the full source voltage.
In a series circuit, an open point has infinite resistance and drops the entire supply voltage because no current can flow through the rest of the circuit.
3
Determine the exact fault location based on the voltage drop.
Because the full 12 V12\text{ V} appears across the switch itself, the break in continuity is inside the switch.
If the lamp were open instead, the voltmeter across the intact closed switch would read 0 V0\text{ V}.

Key Concept

Voltage Drop Across Open vs. Closed Series Components
Estimated Time:1m 0s
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Topics

Circuit Faults and Troubleshooting

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