By Kim Lewis | Updated Mar 24, 2022

What an Electric Relay Is

An electric relay is an electromechanical switch that uses an electric current to control a separate circuit. It can be energized by either AC or DC power, allowing it to handle a wide range of applications.

Construction and Operation

At its core, a relay is a solenoid—a coil of wire that generates a magnetic field when energized. A steel core or armature sits inside the coil, and when the field is strong enough, the armature moves to close or open contacts. The movement is controlled by a small input signal, providing isolation between the control and load circuits.

Key Advantages

Relays enable remote switching and the handling of high voltage or current loads with a low‑power control signal. They are indispensable in AC power systems, automotive circuits, and industrial automation because they keep the control side electrically isolated from the high‑power side.

Common Relay Types

• Latching (Bistable) Relays – retain their position after the control signal is removed.
• Step (Rotary) Relays – allow a contact arm to rotate through multiple positions.
• Reed Relays – use magnetic reeds sealed in glass for fast, silent operation.
• Mercury‑Wetted Relays – feature contacts wetted with mercury for low contact resistance.
• Solid‑State Relays – employ semiconductor devices, eliminating moving parts for faster switching.

Typical Applications

Relays are found in diverse settings: pinball machines, telephone exchanges, automotive control modules, industrial PLCs, and home appliances. Their ability to isolate circuits while handling high power makes them a staple in any system that requires safe, reliable switching.