March 29, 2024

Classification and operation principle of solid state relays

Solid State Relays (SSR) is a non-contact electronic switch that consists of discrete components, fixed-resistor networks, and chips. It uses a hybrid process assembly to implement control loops (input circuits) and load loops (output circuits). The electrical isolation and signal coupling, the solid-state device to achieve load on-off switching function, the internal without any moving parts. Although there are many types of solid state relays on the market, their working principles are basically similar. Mainly by the input (control) circuit, drive circuit and output (load) circuit consists of three parts.

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. The input circuit of the solid state relay is mostly a DC input, and the individual is an AC input. DC input circuit is divided into resistive input and constant current input. The input control current of the resistive input circuit changes linearly with the input voltage. Constant-current input circuit, when the input voltage reaches a certain value, the current does not increase significantly with the increase of the voltage, this relay can be applied to a wide range of input voltage.

The drive circuit of the solid state relay may include three parts: an isolation coupling circuit, a function circuit, and a trigger circuit. Isolation coupling circuit, the current use of optocoupler and high frequency transformer two circuit forms. The commonly used optocouplers are light-triodes, light-triacs, photo-diode arrays (light-volts), and the like. The high-frequency transformer is coupled with a certain input voltage to form a self-oscillation of about 10 MHz, and the high-frequency signal is transmitted to the transformer secondary through the transformer core. The functional circuit may include various functional circuits such as detection, rectification, zero-crossing, acceleration, protection, and display. The function of the trigger circuit is to provide a trigger signal to the output device.

The output circuit of the solid state relay is under the control of the trigger signal to realize the on-off switching of the solid state relay. The output circuit is mainly composed of an output device (chip) and an absorption loop that acts as a transient suppressor, and sometimes includes a feedback circuit. At present, the output devices used in various solid state relays mainly include transistors, thyristors or SCRs, triacs, MOSFETs, insulated gate bipolars. Transistors (IGBTs), etc.

Solid state relay principle Solid state relay (SSR) is a new type of non-contact switch consisting of solid state electronic components. It utilizes the switching characteristics of electronic components (semiconductor components such as switching transistors, triacs, etc.) to achieve contactless , No spark, but can turn on and off the circuit, it is also known as "non-contact switch." Compared to the conventional "Coil-Reed contact" relay (EMR), SSR does not have any movable mechanical parts and does not have any mechanical action during operation. It has advantages over EMR such as fast response and reliability. High and long life (SSR can be switched 108 times (109 times), 106 times higher than the average EMR 106), no action noise, shock resistance, mechanical shock resistance, good moisture resistance, mildew and anti-corrosion properties. These features make SSR It is widely used in military, chemical, and various industrial civil electric control equipments.The power required for the control signals of solid state relays is extremely low, so it is possible to use weak signals to control strong currents.Also the AC type SSR uses zero-crossing triggering technology. The SSR can be safely used at the computer output interface, it will not produce a series of interference to the computer like EMR, and it may even cause serious crashes.The most commonly used is the DIP package type.Control voltage and load voltage can be divided into use occasions. There are two types of AC and DC, so there are four types of DC-AC, DC-DC, AC-AC, and AC-DC, which are respectively used for AC or DC power. Load switches cannot be mixed.

According to different types of load power supply, SSR can be divided into alternating current solid state relay (AC-SSR) and direct current solid state relay (DC-SSR). AC-SSR is a bidirectional thyristor as a switching device used to turn on or off the AC load power solid state relay. AC-SSR control trigger mode is different, it can be divided into zero-crossing trigger type and random conduction type two. The zero-crossing trigger type AC-SSR is turned on when the AC power supply passes near zero voltage when the control signal is input, so the interference is very small. Random conduction type AC-SSR is turned on or off at any phase of the AC power source, so it may generate large interference at the moment of conduction.

Working Principle Zero-crossing triggering AC-SSR is a four-terminal device. Its internal circuit is shown in Figure 1. 1, 2 for the input, 3,4 for the output. R0 is a current-limiting resistor. The optocoupler electrically separates the input and output circuits. V1 forms an inverter. R4, R5, V2 and thyristor V3 form a zero-crossing detection circuit. UR is a bidirectional rectifier bridge. It consists of V3 and UR. Used to obtain a bidirectional trigger pulse to turn on triac V4, R3 and R7 are shunt resistors to protect V3 and V4 respectively, and R8 and C form a surge absorption network to absorb the spike voltage or surge current in the power supply. To prevent impact or interference on the switching circuit.

When the input terminal does not apply a voltage signal, the phototransistor phototransistor is turned off due to non-receiving light, V1 is saturated, and V3 and V4 are turned off due to no trigger voltage, and the SSR is turned off at this time. When the input signal is added, the light emitting diode in the optocoupler emits light and the phototransistor saturates, turning V1 off. At this time, if the voltage across V3 is in the range of -(10~25)V or 10~25V, V2 can be turned off by properly selecting the voltage dividing resistors R4 and R5. This causes V3 to turn on, thus enabling V4 to turn on. On the control electrode, a trigger pulse from R6 → UR → V3 → UR → R7 or in the opposite direction is obtained, and V4 is turned on, so that the load is connected to the AC power supply. If the AC voltage waveform is in the III region in FIG. 2, V3 and V4 are turned on due to V2 saturation, and the SSR is suppressed, so that the zero-cross trigger control is realized. Because the amplitude of 10 ~ 25V compared with the power supply voltage amplitude can be approximated as "zero." Therefore, the zero-crossing voltage is generally defined as 0 ~ ± 25V, that is, in this area, as long as the input signal is added, the zero-crossing triggering AC-SSR can be turned on.

When the input voltage signal is removed, the phototransistor in the optocoupler turns off, V1 saturates, and V3 turns off, but V4 remains on until the load current decreases with the supply voltage less than the holding current of the triac. Only then turned to the deadline.

SSR output devices can be divided into bidirectional thyristors and anti-parallel two unidirectional thyristors. If the load is an inductive load such as a motor, the static voltage rise rate dv/dt is an important parameter. Because the static voltage rise rate of the unidirectional thyristor (200V/μs) is much higher than that of the triac (10V/μs), if two high-power unidirectional thyristors are used in parallel instead of the bidirectional thyristors, the output power can be increased on the one hand. On the other hand, surge resistance can also be improved. This type of SSR is called enhanced SSR.

Yuhai Company is engaged to research and development of piezoelectric products and related piezoelectric products,  the related piezoelectric products includes the piezo sensors and transducer. 


The Ultrasonic Transducer is a kind of transducer that converts the ultrasonic signal into electric signal, or vice versa. Ultrasound transmitter and receiver is a transducer that can transmit and receive ultrasound. Ultrasound sensor is a kind of sensor, in essence, it is also a transmitter and receiver. The working principle of this kind of equipment is similar to that of radar and sonar. Active ultrasonic sensors can emit and receive reflected waves, and determine the distance of the target by measuring the time interval between transmission and reception. Passive Ultrasonic Sensor is actually a microphone that can convert ultrasonic signal into electrical signal.

According to the working principle and materials used, the ultrasonic transducer has Piezoelectric Transducer, electrostatic transducer (capacitive transducer), magnetostrictive transducer, electromagnetic acoustic transducer, mechanical transducer and other types [1].

 

Ultrasonic Sensor

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Zibo Yuhai Electronic Ceramic Co., Ltd. , https://www.yhpiezo.com