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Difficulty: MediumSwitches and Relays

A technician is designing a control circuit to reverse the operating direction of a direct current (DC) motor by reversing the polarity of both supply leads simultaneously using a single manual switch. Which switch configuration is required to achieve this operation?

  1. A
    Single-Pole, Single-Throw (SPST)
  2. B
    Single-Pole, Double-Throw (SPDT)
  3. Double-Pole, Double-Throw (DPDT)Answer
  4. D
    Double-Pole, Single-Throw (DPST)

Answer

Double-Pole, Double-Throw (DPDT)
A Double-Pole, Double-Throw (DPDT) switch contains two electrically isolated common terminals (poles) that simultaneously connect to one of two sets of output contacts (throws). By cross-wiring the two throw sets, toggling the switch swaps the positive and negative supply leads connected to the motor, reversing its rotational direction.

Step-by-Step Solution

1
Determine the number of isolated poles required
Two poles (Double-Pole) are needed
Reversing DC motor polarity requires controlling both the positive and negative supply lines simultaneously within isolated circuit paths.
2
Determine the number of positions/throws required per pole
Two throws (Double-Throw) are needed
Each pole must alternate between two different electrical connections (forward polarity and reversed polarity) rather than simply opening or closing.
3
Combine pole and throw specifications
A Double-Pole, Double-Throw (DPDT) switch is identified
When the outer terminals of a DPDT switch are cross-wired, flipping the switch reverses the voltage polarity applied across the motor terminals.

Key Concept

Switch nomenclature regarding poles (number of separate circuits controlled) and throws (number of output paths per pole) for polarity-reversal applications.
Estimated Time:1m 0s
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Topics

Switches and Relays

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