Solid state relay with two input terminals (A and B) and two output terminals (C and D).

By adding a certain control signal to A and B during work, the "on" and "off" between the ends of C and D can be controlled, achieving the function of a "switch".

The function of the coupling circuit is to provide a channel between the input/output terminals for the control signals of terminals A and B.

But it also electrically disconnects the (electrical) connection between the input and output terminals in SSR to prevent the impact of the output terminal on the input terminal. The component used in the coupling circuit is an "optocoupler", which has sensitive action, high response speed, and high insulation (withstand voltage) level between the input/output terminals; Due to the fact that the load on the input end is a light-emitting diode, it is easy for the input end of SSR to match the input signal level. When in use, it can be directly connected to the computer output interface, which is controlled by the logic levels of "1" and "0".

The function of the trigger circuit is to generate a triggering signal that meets the requirements and drive the switch circuit ④ to work. However, due to the fact that the switch circuit will generate radio frequency interference and pollute the power grid with high-order harmonics or spikes without a special control circuit, a "zero crossing control circuit" is specially designed for this purpose.

The so-called 'zero crossing' refers to when a control signal is added and the AC voltage crosses zero, the SSR is in the on state; And when the control signal is disconnected, the SSR needs to wait for the junction point (zero potential) of the positive and negative half cycles of the AC power before it becomes disconnected. This design can prevent interference from high-order harmonics and pollution to the power grid. Absorption circuit is designed to prevent the impact and interference (even misoperation) of bidirectional thyristors caused by spikes and surges (voltage) transmitted from the power supply. Generally, "R-C" series absorption circuit or nonlinear resistor (varistor) is used.