When a certain voltage or current is applied across the coil of an electromagnetic relay, the magnetic flux generated by the coil passes through the magnetic circuit composed of the iron core, yoke, armature, and working air gap. Under the influence of the magnetic field, the armature is attracted to the pole face of the iron core, thereby pushing the normally closed contacts to open and the normally open contacts to close. When the voltage or current across the coil is less than a certain value, the mechanical reaction force is greater than the electromagnetic attraction force, the armature returns to its initial state, the normally open contacts open, and the normally closed contacts close.
Therefore, an automotive relay can be viewed as an assembly consisting of a control circuit that operates with the coil and a main circuit that operates with the contacts. In the relay's control circuit, only a small operating current is present. This is because the contact capacity of the operating switch is small and cannot be used to directly control loads with large electrical demands; its on/off state can only be controlled through the relay contacts.
A relay is both a control switch and a controlled object (actuator). Taking a fuel pump relay as an example, it is the control switch for 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 (ECU) when the driving transistor in the ECU is turned on.
