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What is a solid state relay?

A solid state relay (SSR) is a non-contact switch composed of microelectronic circuits, discrete electronic devices and power electronic devices. It is an integral part of the whole electronic circuit combination. It depends on the electromagnetic and optical characteristics of semiconductor devices and electronic components. Its isolation and relay switching function.

Compared with traditional electromagnetic relays, solid state relays are relays without machinery and moving parts, but their functions are basically the same as electromagnetic relays.

Solid state relays are widely used in industrial automation control, such as electric furnace heating systems, familiar control machinery, remote control machinery, motors, solenoid valves and signal lights, flashers, stage lighting control systems, medical equipment, photocopiers, washing machines, fire fighting systems, etc., and work reliably , No contact, no spark, long life, no noise, no electromagnetic interference, fast switching speed, realizing the purpose of directly driving a large current load with a tiny control signal.

Solid State Relay

Structure of solid state relay

The solid state relay is composed of three parts: input circuit, isolation (coupling) and output circuit.

1. Input circuit

According to the different types of input voltage, the input circuit can be divided into three types: DC input circuit, AC input circuit and AC/DC input circuit. Some input control circuits are also compatible with TTL/CMOS, positive and negative logic control and inverter functions, and can be easily connected with TTL and MOS logic circuits.

For a control signal with a fixed control voltage, a resistance input circuit is used. The control current is guaranteed to be greater than 5mA. For control signals with a large variation range (such as 3~32V), a constant current circuit is used to ensure reliable operation with a current greater than 5mA within the entire voltage variation range.

2. Isolation and coupling

The input and output circuits of solid state relays can be isolated and coupled in two ways: photoelectric coupling and transformer coupling: photoelectric coupling usually uses photodiode phototransistor, photodiode bidirectional light-controlled thyristor, photovoltaic cell, and realizes control side and load side isolation control; High-frequency transformer coupling is a self-excited high-frequency signal, generated by an input control signal, coupled to the secondary, detected and rectified, and processed by a logic circuit to form a drive signal.

3. Output circuit

The power switch of the SSR is directly connected to the power source and the load side to realize the switching of the load power source. Mainly use high-power transistors, unidirectional thyristors (or SCR), bidirectional thyristors (Triac), power field effect transistors (MOSFET) and insulated gate bipolar transistors (IGBT).

The output circuit of the solid state relay can also be divided into a DC output circuit, an AC output circuit and an AC and DC output circuit. According to the load type, it can be divided into DC solid state relay and AC solid state relay. DC output can use bipolar devices or power field effect transistors, and AC output usually uses two thyristors or one bidirectional thyristor. AC solid-state relays can be divided into single-phase AC solid-state relays and three-phase AC solid-state relays. AC solid-state relays can be divided into random AC solid-state relays and zero-crossing AC solid-state relays according to the switching time.

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