Relay (English name: relay) is a kind of electric control device, is when the input quantity (excitation quantity) changes to meet the specified requirements, in the electrical output circuit so that the controlled quantity to occur a predetermined step change of a kind of electrical appliances.
It has an interactive relationship between the control system (also known as input circuit) and the controlled system (also known as output circuit). Usually used in automated control circuits, it is actually a small current to control the operation of the large current of an "automatic switch.
Therefore, it plays the role of automatic adjustment, safety protection, and circuit conversion in the circuit.