Classification 1: According to the different working principles, time relays can be divided into air damping time relays, electric time relays, electromagnetic time relays, electronic time relays, etc. (1) The air damping time relay utilizes the principle of damping when air passes through a small hole to obtain a delay. Its structure consists of three parts: electromagnetic system, delay mechanism, and contact. The electromagnetic mechanism is a dual port direct acting type, the contact system is a microswitch, and the delay mechanism adopts an airbag type damper. (2) Electronic time relay uses the principle that the capacitance voltage in RC circuits cannot jump, but can only gradually change exponentially, that is, the electrical damping characteristics obtain delay. Features: Wide delay range, up to 3600 seconds, high accuracy, generally around 5%, small size, shock resistance, and convenient adjustment. (3) Electric motor type time relay uses a micro synchronous motor to drive a reduction gear train to obtain a delay. Features: Wide delay range, up to 72 hours, delay accuracy up to 1%, and the delay value is not affected by voltage fluctuations and environmental temperature changes. The delay range and accuracy of electric motor type time relays are incomparable to other time relays. Their disadvantages are complex structure, large volume, low lifespan, expensive price, and accuracy is affected by the power frequency. (4) Electromagnetic time relay. The electromagnetic time relay is made by using the principle of slow decay of magnetic flux after the power of the electromagnetic coil to delay the release of the armature of the magnetic system and obtain the delayed action principle of the contacts. Its characteristic is that the contact capacity is large, so the control capacity is large, but the delay time range is small and the accuracy is slightly poor. It is mainly used in the control of DC circuits. 2、 According to the different delay methods, time relays can be divided into two types: power on delay type and power off delay type. (1) The power-on delay type time relay starts to delay immediately after receiving the input signal, and only after the delay is completed can its execution part output a signal to manipulate the control circuit; When the input signal disappears, the relay immediately returns to its pre action state. (2) On the contrary, the power-off delay type time relay has an output signal immediately after receiving the input signal; After the input signal disappears, the relay needs to undergo a certain delay in order to return to the state before action&Nbsp