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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.

So, car relays can be seen as a collection of two parts: the control circuit for coil operation and the main circuit for contact operation. In the control circuit of a relay, there is only a small working current. This is 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.

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.

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.