Solid State Relay SSR

Solid State Relay (SSR) is a new type of contactless electronic switch device developed by combining modern microelectronics technology with power electronics technology. It can control 0.1A to hundreds of A current load with weak control signal (several mA to tens of mA) and make contactless connection or disconnection. Solid State Relay is a four-terminal device with two input terminals and two output terminals. The input terminal is connected to the control signal, and the output terminal is connected in series with the load and power supply. SSR is actually a controlled power electronic switch, and its equivalent circuit is shown in the figure.

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Because solid-state relays have the advantages of high stability, high reliability, no contacts and long life, they are widely used in motor speed regulation, forward and reverse control, dimming, household appliances, oven heating and temperature control, construction and transformation of power transmission and transformation grids, electric traction, printing and dyeing, plastic processing, coal mining, steel, chemical industry and military use.

Working Principle of Solid State Relay Circuit

Solid state relay is different from common electromagnetic relay: no contact, optical (electrical) isolation between input circuit and output circuit, assembled from discrete components, semiconductor microelectronic circuit chips and power electronic devices, made of flame retardant epoxy resin, using potting technology to seal it in the shell and isolate it from the outside world, with good pressure resistance, corrosion resistance, moisture resistance and vibration resistance.

The solid-state relay consists of three parts: input circuit, drive circuit and output circuit.

Here we only take the AC zero-crossing solid-state relay, which is widely used, as an example to introduce its working principle. This circuit adopts zero-crossing trigger technology, which has the characteristics of turning on when the voltage passes through zero and turning off when the negative current passes through zero. A complete sinusoidal waveform can be obtained on the load, so the circuit has very little radio frequency interference.

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The circuit consists of signal input circuit, zero voltage detection control circuit, working indication circuit, bidirectional thyristor control circuit and absorption circuit. Photocoupler GD is used as the isolation element between input circuit and output circuit. VD is used to prevent Vin from being burned out due to reverse positive and negative connection.

Circuit working process: When there is no input signal, the phototransistor in GD is cut off, VT1 is an AC voltage zero point detector, and it is saturated and turned on by obtaining the base current through R3, clamping the gate of VTH at a low potential and in the off state. When there is an input signal, the phototransistor is turned on, and the state of VTH is determined by VT1. In this way, when the power supply voltage is greater than the zero-crossing voltage, the voltage at the voltage divider point P of the voltage divider R3 and R2 is greater than VBE1, VT1 is saturated and turned on, the SCR gate is cut off due to being clamped at a low potential, and the gate of TR is in the off state due to the lack of a trigger pulse. Only when the power supply voltage is less than the zero-crossing voltage and the voltage at point P is less than VBE1, G1 is cut off, and the SCR gate obtains a trigger signal and turns on. When a trigger pulse is obtained at the gate of TR, TR turns on, thereby connecting the load power supply.

When the input signal is turned off, the phototransistor in GD is cut off, and G1 is saturated and turned on, so that the SCR gate is clamped at a low potential and turned off. However, TR still remains on at this time, and current still flows through the load. It will not turn off automatically until the load current decreases with VAC to less than the holding current of the bidirectional thyristor TR, thus cutting off the load power supply.

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Due to different trigger signal modes, AC-SSR is also divided into two types: zero-crossing trigger (Z type) and non-zero-crossing or random trigger (P type), as shown in the figure. It can be seen that the difference between zero-crossing type and non-zero-crossing type lies mainly in the conditions for the conduction of load AC current. At the moment t1 when the input signal Vin is applied, the output end of the zero-crossing type is not turned on because the power supply voltage is in the non-zero-crossing area at this time. Only when the power supply voltage reaches the zero-crossing area t2, the current flows through the output load. At the moment t1 when the input signal Vin is applied, the non-zero-crossing type immediately turns on the load regardless of the state of the power supply voltage. The shut-off conditions of these two types of solid relays are the same.