Relay

A relay is an electrically operated switching device that uses a low-power control signal to open or close one or more high-power circuits. It provides isolation between control and load circuits, allowing a PLC, DCS, or microcontroller to safely drive motors, heaters, solenoids, or lamps.

The classic electromechanical relay (EMR) consists of an electromagnet (coil) and a moving armature with contacts. When the coil is energized, the magnetic field pulls the armature, changing contact states—normally open (NO) closes, normally closed (NC) opens. EMRs are robust, handle high inrush currents, and provide physical galvanic isolation.

For faster, silent, and maintenance-free operation, solid-state relays (SSRs) use semiconductors (thyristors, triacs, or MOSFETs) with optical isolation. SSRs have no moving parts, offer faster switching, and resist vibration and shock, though they may leak small currents when off.