relæ

Relè is an electromechanical device used to control electrical circuits by means of a separate control signal. It consists of a coil that, when energized, creates a magnetic field which moves an armature to open or close one or more contacts. The coil side and the contact side are galvanically isolated.

The most common form is the electromagnetic relay, which uses an electromagnet, a movable iron armature, and fixed contact sets. When the coil is energized, the armature is attracted, causing NO contacts to close (and NC contacts to open) depending on configuration. Release returns the contacts to their original state. Relays may exhibit contact bounce during switching.

Other types include reed relays (miniature, sealed reed switch actuated by a coil field), latching relays (which hold state after coil is de-energized), and solid-state relays (SSR) that switch with semiconductor devices rather than moving parts.

Relays are specified by coil voltage, contact rating (current, voltage), and contact configuration (SPST, SPDT, DPDT, etc.). They provide galvanic isolation, allow control of high-power loads from microcontrollers or low-power circuits, and can be used for switching, protection, sequencing, and signal conditioning.

Limitations include mechanical wear, contact erosion, slower switching speeds compared to solid-state devices, and contact bounce. Relay selection balances coil power, isolation, speed, and durability; maintenance often focuses on contact wear and coil fatigue.

The relay concept emerged in the 19th century in telegraph networks and later became a staple of electrical and electronic control, automation, and control systems.