Relay – Basic Operation

A Tale of Two Circuits

At it’s simplest a relay is a two circuit device, one comprised of a electromagnet that when energized either closes or opens a switch in the other, the load circuit.

  • An un-energized relay that passes current on the load circuit is referred to as normally closed (NC).
  • Conversely a relay that does not pass current when un-energized is known as normally open (NO).
Swich is open which prevents current from flowing in wire wound core (the electromagnet). De-energized, there is not enough magnetism to close the relay’s switch so preventing the relay from conducting current on the load circuit.
With the switch closed, current can flow through the core forming an energized electromagnet.  The magnetic field created pulls the switch closed position. This in turn allows current to flow to the load.