Editing Relay (section)

=== Latching relay ===
[[File:Latching relay bistable permanent magnet.jpg|thumb|Latching relay with permanent magnet]]

A latching relay, also called ''impulse'', ''bistable'', ''keep'', or ''stay'' relay, or simply ''latch'', maintains either contact position indefinitely without power applied to the coil. The advantage is that one coil consumes power only for an instant while the relay is being switched, and the relay contacts retain this setting across a power outage. A latching relay allows remote control of building lighting without the hum that may be produced from a continuously (AC) energized coil.

In one mechanism, two opposing coils with an over-center spring or permanent magnet hold the contacts in position after the coil is de-energized. A pulse to one coil turns the relay on, and a pulse to the opposite coil turns the relay off. This type is widely used where control is from simple switches or single-ended outputs of a control system, and such relays are found in [[avionics]] and numerous industrial applications.

Another latching type has a [[remanence|remanent]] core that retains the contacts in the operated position by the remanent magnetism in the core. This type requires a current pulse of opposite polarity to release the contacts. A variation uses a permanent magnet that produces part of the force required to close the contact; the coil supplies sufficient force to move the contact open or closed by aiding or opposing the field of the permanent magnet.<ref name="IRS2001">{{citation |last=Sinclair |first=Ian R. |title=Sensors and Transducers |edition=3rd |publisher=Elsevier |date=2001 |isbn=978-0-7506-4932-2 |page=262}}</ref> A polarity controlled relay needs changeover switches or an [[H-bridge]] drive circuit to control it. The relay may be less expensive than other types, but this is partly offset by the increased costs in the external circuit.

In another type, a ''ratchet relay'' has a ratchet mechanism that holds the contacts closed after the coil is momentarily energized. A second impulse, in the same or a separate coil, releases the contacts.<ref name="IRS2001"/> This type may be found in certain cars, for [[headlamp]] dipping and other functions where alternating operation on each switch actuation is needed.

A [[Stepping switch|stepping relay]] is a specialized kind of multi-way latching relay designed for early automatic [[telephone exchange]]s.

An [[earth-leakage circuit breaker]] includes a specialized latching relay.{{Clarify|date=January 2025|reason=Is this meant to be a reference to the voltage-operated ELCB, or current-operated (RCD), or both?}}

Very [[Mechanical computer#Electro-mechanical computers|early computers]] often stored bits in a magnetically latching relay, such as [[ferreed]] or the later [[remreed]] in the [[1ESS switch]].

{{anchor|Holding circuit}}Some early computers used ordinary relays as a kind of [[latch (electronics)|latch]]—they store bits in ordinary wire-spring relays or reed relays by feeding an output wire back as an input, resulting in a feedback loop or [[sequential circuit]]. Such an electrically latching relay requires continuous power to maintain state, unlike magnetically latching relays or mechanically ratcheting relays. While ''[[self-holding circuit<!-- this circular link with possibilities is already linked to wikidata and other Wikipedias -->|(self-)holding circuit]]s'' are often realized with relays they can also be implemented by other means.

In computer memories, latching relays and other relays were replaced by [[delay-line memory]], which in turn was replaced by a series of ever faster and ever smaller memory technologies.