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A relay is an electrically controlled switch. Electromechanical relays use an electromagnet to activate mechanical contacts, solid-state relays use semiconductor switches.

A latching relay stays in its current state when power is removed. A non-latching relay reverts to a known state when power is removed.

Relay contacts come in several varieties. The most common are:

  • A: normally open contact (NO)
  • B: normally closed contact (NC)
  • C: one normally-open and one normally-closed contact, both actuated by a single coil
  • X: normally open, double make
  • Y: normally closed, double break
  • Z: one X and one Y, both actuated by a single coil