Solid State Relay Basics & Working Principle

Solid state relay basics
What is a solid state relay(SSR)? A solid state relay is a non-contact switch fully composed of solid state electrical-element, which can drive high-current load with a small control signal. It can achieve switching on and off with no contact and no spark due to the switching characteristics of electrical element (i.e. semiconductor components like switch transistor, triac and so on). Solid state relay has such following advantages over electromagnetic relays: high reliability, no contact, no spark, long life, fast switching speed, strong anti-interference ability, and small size. It has widely used in the wide applications like CNC machine, remote control system, and industrial automation devices, chemical industry, medical equipment, security system etc.

Solid state relay features

1. SSRs have no internal mechanical elements and full-sealed perfusion in the structure. Therefore, SSRs have advantages such as vibration resistance, corrosion resistance, long life and high reliability.
2. Low noises. AC SSRs use zero-crossing triggering technology, thus effectively reducing the voltage rise rate dv/dt and current rise rate di/dt on the line, making SSR minimal interference to the power supply in the long-term working.
3. Switching time is short, so SSRs can be used in high frequency applications.
4. Optoelectronic isolation is used between input circuits and outputs circuits, and insulation voltage is more than 2500V.
5. Low input power consumption, compatible with TTL and COMS circuits.
6. Protection circuit is set in the output terminals.
7. High load capacity.

Solid state relay working principle
How does a solid state relay work? General solid state relays can be divided into AC SSR and DC SSR according to applications. Now take AC solid state relay as example to explain SSR working principle. As Figure 1 show, it is a AC SSR working principle diagram and the parts ①～④ form its main body. As a whole, SSR only has 2 input terminals (A & B) and 2 output terminals (C & D). It is a four-terminal active device.

When operating, only place a certain control signal to A&B, able to control on-off state between C and D, and then achieve the switching function. The coupling circuit plays a role to provide a channel between input and output terminals for control signal input from A and B, but cut off the electrical connection between input and output to prevent the output from affecting the input. Components used in the coupling circuits are "optical couplers", which has good action sensitivity, high response speed, high level of input/output insulation (withstand voltage). The load at the input terminal is a light-emitting diode, which makes the SSR input very easy to match input signal level. In the use, it can be directly connected with the computer output interface, that is, it is controlled by the logic level of "1" and "0". Trigger circuit function is to generate the desired trigger signal to drive switching circuit work. However, without special control circuit, the switching circuit will produce RFI (Radio Frequency Interference) and pollute power grid as high harmonics or peaks, so zero-crossing control circuit is set for this purpose. Zero crossing means, SSR is on-state when placing the control signal and AC voltage crossing zero; after switching off control signal, SSR is not off-state until AC current is at the junction the positive half cycle and the negative half cycle (zero potential). This design prevents the interference of higher harmonics and the pollution of the power grid. Snubber circuit is designed to prevent the impact and interference to switching component Triac from the surges and spikes (voltage) from the power supply. Normally RC series snubber circuit or non-linear resistance (MOV) is used. Compared with AC SSR, DC SSR has no zero-crossing control circuit and snubber circuit inside, and large power transistor is usually used for the switching component. In addition, other work principles are the same.

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