What is a solid state relay

 What is a solid state relay

 Solid State Relays (SSRs) are non-contact switches composed of microelectronic circuits, discrete electronic devices and power electronic power devices. The isolation between the control terminal and the load terminal is achieved by an isolation device. The input of the solid state relay is controlled by a tiny control signal, which directly drives the high current load.

SSR uses the switching characteristics of electronic components, such as semiconductor devices such as switching transistors and triacs, to achieve the purpose of connecting and disconnecting circuits without contact and spark, so it is also called "contactless switch".

A solid state relay is a four-terminal active device, in which two ends are the input control end and the other end is the output control end. It has both amplification and isolation functions. It is suitable for driving high-power switch actuators, more reliable than electromagnetic relays, non-contact, long life, fast, and free from external interference. Because of its small size, it is widely used.

 Working principle of solid state relay

 SSR can be divided into AC type and DC type according to the application.

They can switch loads on either AC or DC sources and cannot be mixed. The working principle of the AC-type SSR is described below as an example. The figure below is a block diagram of how it works. The components in the diagram form the bulk of the AC SSR. On the whole, the SSR has only two input terminals (A and B) and two output terminals (C and D), and is a four-terminal device.

When a certain control signal is added at both ends of A and B, the "switch" and "break" between the two ends of C and D can be controlled to realize the function of "switch". The role of the coupling circuit is to provide a channel between the input/output of the control signal input from the A and B terminals, but it disconnects the SSR input and output in the circuit.

In order to prevent the influence of the output end on the input end, the coupling circuit adopts an "optical coupler", which has high sensitivity, sensitive response, and high input and output insulation (voltage resistance) level; because the input end load is a light-emitting diode, which makes the input end of the SSR very easy. match the input signal level. When in use, it can be directly connected to the output interface of the computer, that is, the logic level control of "1" and "0".

The function of the trigger circuit is to generate the required trigger signal to drive the switch circuit 4, but since the switch circuit does not have a special control circuit, it will cause radio frequency interference and pollute the high-order power grid. harmonics or peaks, so set up a zero-crossing control circuit. "Zero-crossing" means that when the control signal is added and the AC voltage crosses zero, the SSR is in the pass state. When the control signal is disconnected, the SSR waits for the positive half cycle and the negative half cycle (zero potential) of the AC power at the connection point (zero potential), SSR just broken.

This design can prevent the interference and pollution of high-order harmonics to the power grid. The absorption circuit is to prevent the peak value, surge (voltage) of the power supply in the switching device from being transmitted to the triac, and the impact and interference (even malfunction), usually using "R-C" series absorption circuit or nonlinear resistor (varistor).

Solid State Relay Appliances

 Dedicated solid state relays are available with short circuit protection, overload protection and overheat protection. Through the combination logic curing package, users can realize intelligent modules. It is used directly in the control system.

Solid state relays have been widely used in computer peripheral interface equipment, constant temperature systems, temperature adjustment, electric furnace heating control, motor control, CNC machine tools, remote control systems, industrial automation devices, signal lights, dimming, scintillators, lighting stage lighting control systems, instruments, Medical equipment, copiers, automatic laundry. Machinery, automatic fire protection, security systems, power capacitor switching switches are used as power factor compensation for power grids, etc., in addition to chemical industry, coal mines, explosion-proof, anti-corrosion, anti-corrosion, etc. The logic curing package can realize the intelligent module required by the user, which can be directly used in the control system.

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