The working principle of the solid state relay explains the characteristics of the solid state relay

Solid State Relay (SSR) is a non-contact switch composed of microelectronic circuits, discrete electronic devices, and power electronic power devices. The isolation between the control terminal and the load terminal is realized by the isolation device. The input of the solid state relay uses a small control signal to directly drive a large current load.

1. What is a solid state relay?

Solid state relay is a new type of non-contact switching device composed entirely of solid electronic components. It utilizes the switching characteristics of electronic components (such as switching transistors, bidirectional thyristors, etc.) to achieve contactless and non-sparking connection. And the purpose of breaking the circuit, so it is also called "contactless switch". The solid state relay is a four-terminal active device in which two terminals are input control terminals and the other ends are output control terminals. It has both amplifying drive function and isolation function. It is suitable for driving high-power switch-type actuators. It has higher reliability than Electromagnetic Relays, and has no contact, long life and fast speed. It has little interference to the outside world. It is widely used.

Second, solid state relay structure and principle

The commonly used solid state relays are almost all modular four-terminal active devices, with the two ends being the input control terminals and the other ends being the output controlled terminals. The basic structure is shown in the following figure. Photocouplers are often used in devices to achieve electrical isolation between input and output. The output controlled terminal utilizes the switching characteristics of a semiconductor device such as a switching transistor or a bidirectional thyristor to achieve the purpose of turning on and off the external control circuit without contact or spark. The entire device has no moving parts and contacts, and can achieve the same function as a common Electromagnetic Relay.

Solid-state relays can be divided into two types, DC and AC, depending on the polarity of the output. The DC solid-state relay (DC-SSR) control voltage is input from the input terminal IN, and the control signal is coupled to the receiving circuit through the photocoupler, and the switching transistor VT is turned on after being amplified. Obviously, when the output terminal OUT of the DC solid-state relay is connected to the circuit of the controlled circuit, there are positive and negative poles. The circuit principle of AC solid-state relay (AC-SSR) is different from that of DC solid-state relay. Its switching element uses bidirectional thyristor VS or other AC switch, so its output terminal OUT has no positive and negative poles, which can control the AC circuit. On and off.

Because the mature and reliable photoelectric isolation technology is adopted between the input end and the output end of the solid state relay, the control weak current and the controlled high voltage are completely electrically isolated, so the signals output from various weak electric devices can be directly added. On the input control side of the solid state relay, no additional protection circuits are required.

Third, the characteristics of solid state relay

The solid state relay is a non-contact electronic switch with isolation function. There is no mechanical contact part during the switching process. Therefore, the solid state relay has the same functions as the electromagnetic relay, and also has logic circuit compatibility, vibration resistance and mechanical shock resistance, and the installation position is unlimited. It has good moisture, mildew and anti-corrosion performance, excellent performance in explosion protection and ozone pollution prevention, low input power, high sensitivity, low control power, good electromagnetic compatibility, low noise and high working frequency.

(1) There are no mechanical parts inside the SSR, and the structure is filled with a full-filling method. Therefore, the SSR has the advantages of vibration resistance, corrosion resistance, long life and high reliability, and its switching life is as high as 10.1 million times.

(2) Low noise: The AC SSR adopts the zero-crossing trigger technology, so the voltage rise rate dv/dt and the current rise rate di/dt value are effectively reduced on the line, so that the SSR has little interference to the mains when working for a long time. .

(3) The switching time is short, about 10ms, which can be applied to high frequency applications.

(4) The input circuit and the output circuit are optically isolated, and the insulation voltage is 2500V or more.

(5) Input power consumption is very low, compatible with TTL, COMS circuits.

(6) The output has a protection circuit.

(7) Strong load capacity.