Relay is a kind of automatic and remote control for appliances, widely used in automatic control system, remote control and telemetry system, power protection system and communication system of controls, detection, protection and regulation, is in the most basic components of modern electrical apparatus.
Relay is defined as: when the input (or motivation) to meet the requirements of certain provisions, can produce a predetermined jump a device in a or a plurality of electrical output circuit. The relay according to the amount of electrical (voltage, current, etc.) or non electrical quantities (heat, time, speed, pressure, etc.) changes in the closed or disconnect control circuit, to complete control and protection of electrical appliances, electrical relays is when the input energizing quantity for a relay of electric parameters such as voltage or current.