Relays

Relays are devices that open and close electrical circuits either electromechanically or electronically. They control one circuit by switching contacts in another. In relay diagrams, a normally open (NO) contact remains open when the relay is not energized, while a normally closed (NC) contact remains closed when the relay is not energized. When electrical current is applied to the relay coil, the contact state changes.

Relays and contactors are electrically operated switches that allow a low-power control signal to switch a higher-power load while maintaining galvanic isolation between the control and load circuits. In an electromechanical relay, energizing the coil creates a magnetic field that moves an armature, causing NO contacts to close and NC contacts to open. This design enables safe and reliable switching of higher voltages and currents without a direct electrical connection between the control and load sides.

Relay types are typically classified by construction and application. These include PCB signal relays for low-power switching, power and automotive relays for high-current and demanding environments, reed relays for fast and low-level signal switching in sealed contacts, and solid-state relays (SSRs), which use semiconductor components to provide silent, wear-free operation. Protection devices such as thermal overload relays are also used in motor control systems to disconnect circuits during sustained overcurrent conditions, extending relay technology into safety-critical applications.