When working, adding a certain control signal to the input circuit can control the switching state between the two ends of the output circuit. In order to prevent the influence of the output end on the input end, the circuit between the input end and the output end of the electrical solid state relay (SSR) can be disconnected.

Composition of solid state relay

1. Input circuit

According to the type of input voltage, the input circuit is divided into three types: DC input circuit, AC input circuit and ac/dc input circuit. Some input control circuits have compatibility with ttl/cmos and equivalent functions of positive and negative logic control, and can be simply connected with TTL and MOS logic circuits.

2. Isolated coupling

The input and output circuits of solid state relays have two isolation coupling modes: photoelectric coupling and transformer coupling. Photoelectric coupling usually adopts photodiode phototransistor, photodiode bidirectional optocoupler thyristor and solar cell to realize the isolation control of control side and load side; The high-frequency transformer coupling carries out secondary coupling on the self-excited high-frequency signal generated by the input control signal, detects and rectifies, and forms the driving signal through logic circuit processing.

3. Output circuit

The power switch of solid state relay (SSR) is directly connected to the power supply and load side to realize the on-off switching of load power supply. The output circuits of high output transistors, three terminal bidirectional thyristors, three terminal bidirectional thyristors, power field effect transistors, MOSFETs, and absolute solid state relays mainly include DC output circuits, AC output circuits, AC and DC output circuits, etc. According to different load types, it is divided into DC solid-state relay and AC solid-state relay. Bipolar elements or power field effect transistors can be used for DC output, and two thyristors or a bidirectional thyristor are usually used for AC output. AC solid-state relay is divided into single-phase AC solid-state relay and three-phase AC solid-state relay. AC solid state relay can be divided into random AC solid state relay and zero crossing AC solid state relay according to on or off time.

Solid state relay is a contactless switch composed of solid components, which has the characteristics of safe operation, long service life, no contact, no spark, no pollution, high insulation, high withstand voltage (above 2.5KV), small trigger current, fast switching speed and so on. It can be combined with digital circuit. The flame retardant epoxy resin solid-state relay adopts the characteristics of conducting when the voltage is too low and cutting off when the load is too low. The load can get a complete sinusoidal waveform. Therefore, its advantages are that the RF noise of the circuit is small, the back EMF of inductive loads (such as fans, three-phase motors, etc.) is small, or the surge current can be significantly reduced when the resistive loads (such as incandescent lamps, heating wires, etc.) are driven.

Solid state relay (SSR) is a contactless switch composed of microelectronic circuits, discrete electronic devices and power electronic devices. The isolating element is used to isolate the control side and the load side. The input of solid-state relay is a tiny control signal, which directly drives the high current load.