The working principle of automotive relays

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 core, the yoke, the armature, and the working air gap of the magnetic circuit. Under the action of the magnetic field, the armature is attracted to the pole surface of the 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, the armature returns to the initial state and the normally open contact opens. The normally closed contact is connected.
Then, the automotive relay can be regarded as a collection of two parts: the control circuit where the coil works and the main circuit where the contacts work. In the control circuit of the relay, there is only a small operating current. This is because the contact capacity of the control switch is small and cannot be used to directly control loads with large power consumption. It can only be controlled through the contacts of the relay. On and off.
The relay is both a control switch and a control object (actuator). Take the fuel pump relay as an example. It is the control switch of the fuel pump. However, the coil of the fuel pump relay can only form a loop through the ground point of the electronic control unit when the transistor driven in the electronic control unit is turned on.