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Difficulty: HardMultimeter: Resistance and Continuity

An electronics technician is bench-testing a de-energized two-position rotary selector switch using a digital multimeter set to the low-resistance continuity function. With the switch set to Position 1, the meter connected across the common terminal and output terminal 1 displays 0.2 Ω0.2\ \Omega with an audible tone. When the switch is turned to Position 2, the meter across the common terminal and output terminal 1 continues to display 0.2 Ω0.2\ \Omega, while testing across the common terminal and output terminal 2 displays O.L.\text{O.L.} (over limit). What specific switch fault do these multimeter readings indicate?

  1. A
    The common terminal insulator has broken down into a conductor, creating an open circuit on all switch positions.
  2. The internal contacts for Position 1 are welded or shorted closed, while the contacts for Position 2 have failed open.Answer
  3. C
    The meter is reading residual circuit voltage, which forces the resistance display to read zero ohms regardless of contact position.
  4. D
    The switch is functioning normally because a reading of O.L.\text{O.L.} indicates zero resistance and continuous current flow.

Answer

The internal contacts for Position 1 are welded or shorted closed, while the contacts for Position 2 have failed open.
A reading of 0.2 Ω0.2\ \Omega with an audible tone confirms a closed, continuous electrical path. When the rotary switch is moved to Position 2, output terminal 1 should disconnect and display O.L.\text{O.L.}; because it remains at 0.2 Ω0.2\ \Omega, the Position 1 contacts are welded or shorted closed. Simultaneously, output terminal 2 should establish continuity in Position 2; showing O.L.\text{O.L.} indicates an open circuit where contacts failed to close.

Step-by-Step Solution

1
Analyze the baseline continuity measurement in Position 1.
A reading of 0.2 Ω0.2\ \Omega with an audible tone indicates a normal, closed, continuous connection between the common terminal and output terminal 1.
Digital multimeters indicate continuity with near-zero resistance and an audible beeper.
2
Evaluate the resistance between common and terminal 1 after switching to Position 2.
The meter continues to read 0.2 Ω0.2\ \Omega, meaning the contacts remained closed instead of opening.
When rotated away from Position 1, the contacts should open and show infinite resistance (O.L.\text{O.L.}); failing to do so indicates contacts welded or mechanically shorted closed.
3
Evaluate the resistance between common and terminal 2 in Position 2.
The meter displays O.L.\text{O.L.}, indicating an open circuit.
In Position 2, terminal 2 should connect to common with near-zero ohms; an over-limit reading proves the contacts failed to make contact (open circuit defect).

Key Concept

Interpreting multimeter resistance and continuity values (0 Ω0\ \Omega vs. O.L.\text{O.L.}) to diagnose internal short-circuit and open-circuit defects in multi-position switches.
Estimated Time:1m 15s
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Topics

Multimeter: Resistance and Continuity

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