Red Seal Electrician Practice
Motor controlsQuestion 107 of 1506

Why is a relay needed to invert the output of a contact logic circuit?

  1. A switch contact cannot carry the load current of the output device or load
  2. A relay contact acts as a diode and blocks the reverse current
  3. A relay turns AC into DC, so the logic is inverted at the output
  4. A normally closed relay contact driven by the coil gives the opposite result
Show answer

Correct answer: A normally closed relay contact driven by the coil gives the opposite result

Inverting an input only needs an NC switch contact. Inverting an output needs a relay with an NC contact.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 6.2: Digital Logic Functions

Practice motor controls

Questions last reviewed: 18 October 2026