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.

In addition, automotive relays typically consist of two parts: the control circuit for coil operation and the main circuit for contact operation. The current in the control circuit is relatively small because the contact capacity of the operating switch is small and cannot be used to directly control loads with high electricity consumption. It can only be controlled through the contacts of the relay to make and break.