Red Seal Electrician Practice
Motor controlsQuestion 28 of 1506

Inside a PLC, what isolates each input terminal from the computer circuits?

  1. A step-down transformer wired to each input channel
  2. A normally closed relay contact
  3. A fuse and varistor
  4. An opto-isolator with an LED and phototransistor
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Correct answer: An opto-isolator with an LED and phototransistor

The input LED lights a phototransistor, giving an electrically isolated logic signal.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 6.6: Programmable Logic Controllers (PLC)

Practice motor controls

Questions last reviewed: 18 October 2026