A relay is both a control switch and a control object (actuator). Taking the fuel pump relay as an example, it is the control switch of the fuel pump, but the coil of the fuel pump relay can only form a circuit through the grounding point of the electronic control unit when the driving transistor is conductive in the electronic control unit.

Automobile relay is an automatic control device, and when the input quantity (such as electricity, magnetism, sound, light, heat) reaches a certain value, the output quantity will undergo a jumping change. The relays used in cars are typically used to control circuits and implement power paths in switch control circuits, such as lighting and air conditioning systems, heaters, horn signals, and fuel pumps.