Editing Relay (section)

{{Short description|Electrically operated switch}}
{{About|the electrical component}}
[[file:Delta Electronics DPS-350FB A - board 1 - OEG SDT-SS-112M - case removed-3045.jpg|thumb|A relay]]
[[File:Relay principle horizontal new.gif|thumb|Electromechanical relay principle]]
[[file:Kontakt.svg|thumb|Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts]]
[[file:Relay.jpg|thumb|An automotive-style miniature relay with the dust cover taken off]]

A '''relay''' is an [[electric]]ally operated [[switch]]. It has a set of input [[Terminal (electronics)|terminals]] for one or more control [[signals]], and a set of operating contact terminals. The switch may have any number of contacts in multiple [[Electrical contact#Contact form|contact forms]], such as make contacts, break contacts, or combinations thereof.

Relays are used to control a [[Electrical network|circuit]] by an independent low-power signal and to control several circuits by one signal. They were first used in long-distance [[Electrical telegraph|telegraph]] circuits as signal repeaters that transmit a refreshed copy of the incoming signal onto another circuit. Relays were used extensively in [[telephone exchange]]s and early [[Computer|computers]] to perform [[Boolean algebra|logical operations]].

The traditional [[electromechanical]] relay uses an [[electromagnet]] to close or open the contacts, but relays using other operating principles have also been invented, such as in [[solid-state relay]]s which use [[semiconductor]] properties for control without relying on [[moving parts]]. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect [[Electrical network|electrical circuits]] from overload or faults; in modern electric power systems these functions are performed by digital instruments still called ''[[protective relay]]s'' or ''[[safety relay]]s''.

Latching relays require only a single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling.