Relays

Relays are electromechanical devices that use an electromagnet to open or close one or more sets of contacts in another circuit. They provide electrical isolation between a control circuit and a load circuit and allow a low-power signal to switch a higher-power circuit, enabling control, protection, and amplification in automation systems.

A relay typically consists of a coil, a magnetic core, an armature, and one or more contact sets. When the coil is energized, the magnetic field pulls the armature, changing the state of the contacts. Contacts can be normally open (NO) or normally closed (NC), and common configurations include single pole single throw (SPST), single pole double throw (SPDT), and double pole double throw (DPDT). Relays can also be designed as latching devices that hold their state after a pulse.

Relays come in several main types. Electromechanical relays (EMRs) use moving parts and can switch both AC and DC loads. Reed relays use a small magnetic reed switch inside a sealed tube for compact, fast operation. Solid-state relays (SSRs) use semiconductor devices to switch without moving parts, offering long life and quiet operation but with different voltage drop and surge characteristics. Automotive relays and high-power relays are specialized EMRs designed for vehicles and industrial power applications, respectively.

Key specifications include coil voltage and current (for energizing the relay), contact ratings (current, voltage, AC/DC compatibility), switching speed, and insulation and dielectric strength. Relays are widely used for control logic, motor and light control, protection schemes, and interfaces between low-level electronics and high-power systems, where isolation and amplification are advantageous. History traces their use from early telegraph and telephone systems to modern automation and digital circuits.