Classification and working principle of solid state relays

Solid State Relay (SSR for short) is a non-contact electronic switch composed of discrete components, thin-film specific resistor networks and chips, assembled through a hybrid process to achieve electrical connection between the control loop (input circuit) and the load loop (output circuit). isolation and signal coupling. The solid-state device implements the switching function of the load, with no external moving parts. Although there are many types and sizes of solid state relays on the market, their working principles are basically similar. It is mainly composed of input (control) circuit, drive circuit and output (load) circuit.

The input circuit of the solid state relay provides a loop for the input control signal, making it a trigger signal source for the solid state relay. Most of the input circuits of solid state relays are DC input, some are AC input. The DC input circuit is divided into resistance input and constant current input. The input control current of the resistive input circuit is changing linearly with the input voltage. Constant current input circuit, when the input voltage reaches a certain value, the current no longer increases significantly with the increase of the voltage. This relay is suitable for a small range of input voltages.

The driving circuit of the solid state relay may include an isolation coupling circuit, a functional circuit and a trigger circuit. At present, optocouplers and high-frequency transformers are widely used in isolated coupling circuits. Common optocouplers include phototriodes, optical triacs, photodiode arrays (photovoltaics), etc. The high-frequency transformer coupling is to form a self-excited oscillation of about 10MHz under a certain input voltage, and transmit the low-frequency signal to the secondary side of the transformer through the transformer core. Functional circuits may include detection and rectification, zero-crossing, acceleration, protection, display and other functional circuits. The function of the trigger circuit is to provide a trigger signal to the output device.

The output circuit of the solid state relay realizes the on-off switching of the solid state relay under the control of the trigger signal. The output circuit is mainly composed of an output device (chip) and a transient suppression absorption circuit, and sometimes a feedback circuit is also included. At present, the output devices used in various solid state relays mainly include transistors, thyristors or thyristors, triacs, MOSFETs, IGBTs, etc.

Solid State Relay Principle Solid State Relay (SSR) is a new type of non-contact switch composed of solid state electronic components. It utilizes the switching characteristics of electronic components (such as switching transistors, triacs, and other semiconductor components) to achieve non-contact, non-sparking purposes, and to connect and disconnect circuits. Therefore, it is also called "non-contact switch". In contrast to previous "coil-reed contact" relays (EMRs), SSRs operate without moving mechanical parts and mechanical actions. Compared with electronic medical records, it has slow response speed, high reliability and short service life (the number of times of switching of solid state relays can reach 108"109 times, which is 100 times lower than the usual EMR 106), no action noise, shock resistance, mechanical shock resistance , Good moisture-proof, mildew-proof and rust-proof characteristics. These characteristics make solid-state relays widely used in military, chemical and various industrial and civil electronic control equipment. The power required for the control signals of solid-state relays is very high, so it can be Use tiny signals to control strong current. At the same time, the AC solid state relay adopts zero-crossing trigger technology, so that the solid state relay can be safely used for the computer output interface, and will not cause a series of interference to the computer like the electronic medical record, or even cause a slight crash. Dip package is common. Depending on the application, the control voltage and load voltage can be divided into AC and DC. Therefore, there will be DC-AC, DC-DC, AC-AC and AC-DC. They are used as AC or DC power supply respectively The load switches above can not be mixed.

According to the type of load power supply, solid state relays can be divided into AC solid state relays (AC SSR) and DC solid state relays (DC SSR). An AC solid state relay is a solid state relay that uses a triac as a switching device to turn on or off power to an AC load. The control triggering methods of AC solid state relays are different, which can be divided into zero-crossing triggering type and random conduction type. After the control signal is input, when the AC power supply is close to zero voltage, the zero-crossing trigger AC second harmonic recovery will be performed, so the interference is very large. Random conduction type AC secondary relays turn on or off in any phase of the AC mains, so there may be less disturbance at the moment of conduction.

The working principle and structure of thermal relay

Thermal relays are protective devices used for overload protection of motors or other electrical equipment and wires. During the actual operation of the motor, such as driving the production machinery, if the machine is abnormal or the circuit is abnormal, the motor is overloaded, the motor speed will decrease, the current in the winding will increase, and the motor winding temperature will increase. If the overload current is large, the overload time is long, and the motor winding does not exceed the allowable temperature rise, the overload is allowed. However, if the overload time is short and the overload current is small, the temperature rise of the motor windings will exceed the allowable value, resulting in the aging of the motor windings, shortening the service life of the motor, and even burning the motor windings. Therefore, the motor cannot withstand this overload. Thermal relay is a kind of protection device. When the motor cannot withstand overload, it uses the principle of thermal effect of current to cut off the motor circuit, thereby providing overload protection for the motor.