CIE A Level Physics (9702) 2019-2021

Revision Notes

24.2.2 Relays


  • A relay in an electronic circuit is:

A device which switches the mains supply on or off using a low voltage/current output

  • It consists of an electromagnetic switch operated by a small current in the coil to convert a small electrical input to a higher current output
  • A relay can act as either a switch or an amplifier for the electric circuit
  • A single relay can activate multiple circuits, allowing one input to initiate many other outputs
  • They are also commonly used in combination to perform Boolean logic functions such as AND or NOT gates
  • Relay’s come in two types:
    • Normally open (NO)
    • Normally closed (NC)
  • Most relays are type NO
  • When the relay is activated, the contacts connect the circuit (like closing a switch) and the circuit is disconnected when the relay is inactive
  • NC relays disconnect the circuit when the relay is activated, and the circuit is connected when the relay is inactive

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