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.

When a certain voltage or current is applied to both ends of the electromagnetic relay coil, the magnetic flux generated by the coil passes through the magnetic circuit composed of the iron core, yoke, armature, and magnetic circuit working air gap. Under the action of the magnetic field, the armature is attracted to the pole surface of the iron core, thereby pushing the normally closed contact of the contact to open and the normally open contact to close; When the voltage or current at both ends of the coil is less than a certain value, and the mechanical reaction force is greater than the electromagnetic attraction force, the armature returns to its initial state, the normally open contact opens, and the normally closed contact closes.

The working principle of automotive relays mainly utilizes the principle of electromagnetic induction, which generates a magnetic field through the electrification of the coil, thereby controlling the closure and disconnection of the contacts. Specifically, when the coil is energized, it generates a magnetic field, and the moving iron is attracted to drive the contact spring, causing the static contact point to separate and the moving contact point to close. When the current of the electromagnetic coil is cut off, the iron core loses magnetism, the moving iron core resets under the action of spring force, the moving contact opens, and the stationary contact closes.

The main function of automotive relays is to control high currents, as directly controlling high currents through switches may generate electrical sparks and cause danger. Automotive relays belong to switches that control high current with low current, and are a type of electrical component in automotive electrical systems.