Red Seal Electrician Practice
Motor controlsQuestion 238 of 1506

How is a solid-state relay turned on?

  1. By a coil that pulls in an armature
  2. By lighting an internal LED
  3. By a tilting mercury tube
  4. By a heated bimetal strip
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Correct answer: By lighting an internal LED

The LED is optically coupled to the output device, which gives isolation between input and output.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 5.5: Solid-state Relays

Practice motor controls

Questions last reviewed: 18 October 2026