Why is it said that a relay is not only a control switch, but also a control object

Update:2020/09/29

A relay is an automatic control device whose output will change leaps and bounds when the input (electricity, magnetism, sound, light, heat) reaches a certain value. Automotive relays are relays used in automobiles. This type of Automotive relay has high switching load power, high shock and vibration resistance. Most power supplies in automobiles use 12V, and most of the coil voltages are designed to be 12V. Due to battery power supply and unstable voltage; harsh environmental conditions, suction voltage V≤60%VH (rated working voltage); coil overvoltage allowed to reach 1.5VH. The power consumption of the coil is relatively large, generally 1.6~2W, and the temperature rise is relatively high. The environmental requirements are quite harsh: in the engine compartment, the ambient temperature range is -40℃~125℃, and the ambient temperature range of other locations is -40℃~85℃; the relay used in the engine compartment must withstand sand, dust, water, and salt , Oil damage; vibration and shock are quite harsh.

15A Continuous Current Capacity ZX117 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, yoke, armature, and 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. Thus, the normally closed contact of the contact is pushed to open, and the normally open contact is closed; 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 is disconnected. The normally closed contact is turned on.

Then, the automobile relay can be regarded as an assembly composed of two parts: the control circuit of the coil and the main circuit of the contact. In the control circuit of the relay, there is only a small working current. This is because the contact capacity of the control switch is small, and it cannot be used to directly control the load with a large power consumption. It can only be controlled by the contact of the relay. On and off.

The relay is not only a control switch, but also a control object (actuator). Take 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 loop through the grounding point of the electronic control unit when the driving transistor in the electronic control unit is turned on.

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