Solid state relay principle and application circuit

Solid state relay (SOLIDSTATE RELAYS), abbreviated as "SSR", is a new type of non-contact switching device composed entirely of solid-state electronic components. It utilizes the switching characteristics of electronic components (such as switching transistors, triacs and other semiconductor devices) , can achieve the purpose of connecting and disconnecting circuits without contacts and sparks, so it is also called "contactless switch". It came out in the 1970s. Due to its non-contact working characteristics, it has been widely used in many fields. Electronic control and computer control are increasingly widely used. 1. Principle and structure of solid state relay SSR can be divided into two categories: AC type and DC type according to the use occasion. They are used as load switches on AC or DC power supply respectively. They cannot be mixed. The following takes AC type SSR as an example to illustrate its function. Working principle, Figure 1 is a block diagram of its working principle. Components ①-④ in Figure 1 constitute the main body of the AC SSR. Overall, the SSR has only two input terminals (A and B) and two output terminals (C and D), is a four-terminal device. When working, as long as certain control signals are added to A and B, the "on" and "off" between ends C and D can be controlled to realize the "switch" function. The function of the coupling circuit is to provide A and B. The control signal input at terminal B provides a channel between the input and output terminals, but electrically disconnects the (electrical) connection between the input terminal and the output terminal in the SSR to prevent the output terminal from affecting the input terminal. Coupling circuit The component used is an "optocoupler", which has sensitive action, high response speed, and high insulation (withstand voltage) level between the input/output terminals; since the load at the input terminal is a light-emitting diode, this makes it easy for the input terminal of the SSR to It matches the input signal level and can be directly connected to the computer output interface during use, that is, it is controlled by the logic levels of "1" and "0". The function of the trigger circuit is to generate a trigger signal that meets the requirements and drive the switch circuit ④ to work. However, since the switch circuit without a special control circuit will produce radio frequency interference and pollute the power grid with high-order harmonics or spikes, a specially designed trigger circuit is designed for this purpose. "Zero Crossing Control Circuit". The so-called "zero crossing" means that when the control signal is added and the AC voltage crosses zero, the SSR is in the on state; and when the control signal is disconnected, the SSR has to wait for the intersection point (zero potential) of the positive half cycle and the negative half cycle of the alternating current. , the SSR is in the off state. This design can prevent high-order harmonic interference and pollution to the power grid. The absorption circuit is designed to prevent the impact and interference (even malfunction) of the peaks and surges (voltage) coming from the power supply on the switching device triac (or even malfunction). Generally, an "RC" series absorption circuit or non- Linear resistor (varistor). Figure 2 is the electrical schematic diagram of a typical AC SSR. Compared with AC SSR, DC type SSR does not have a zero-crossing control circuit, and there is no need to set up an absorption circuit. The switching device generally uses a high-power switching transistor, and the other working principles are the same. However, when using DC SSR, you should pay attention to: ① When the load is an inductive load, such as a DC solenoid valve or electromagnet, a diode should be connected in parallel at both ends of the load. The polarity is as shown in Figure 3. The current of the diode should be equal to the working Current and voltage should be greater than 4 times of the working voltage. ② When the SSR is working, it should be placed as close to the load as possible, and its output lead should meet the load current needs. ③The power supply used is obtained by AC step-down rectification, and its filter electrolytic capacitor should be large enough. Figure 4 shows the appearance of several common SSRs at home and abroad. 2. Characteristics of Solid State Relay SSR successfully realizes the control of weak signal (Vsr) to strong current (output load voltage). Due to the application of optocouplers, the power required for control signals is extremely low (about ten milliwatts can work normally), and the working level required for Vsr is compatible with commonly used integrated circuits such as TTL, HTL, CMOS, etc., which can be achieved Direct connection. This makes SSR widely used in CNC and automatic control equipment. It can replace the traditional "coil-reed contact type" relay ("MER" for short) to a considerable extent. Since SSR is composed of all solid-state electronic components, compared with MER, it does not have any movable mechanical parts and does not have any mechanical action during work; SSR realizes the "on" and "off" switching functions by changing the working state of the circuit. There are no electrical contacts, so it has a series of advantages that MER does not have, namely high reliability and long life (data show that the number of switching times of SSR can reach 108-109 times, which is hundreds of times higher than the 106 of general MER); no action Noise; resistant to vibration and mechanical impact; no restrictions on the installation location; it is easy to be potted with insulating and waterproof materials to make a fully sealed form, and has good moisture-proof, mildew-proof and anti-corrosion properties; it also has excellent performance in explosion-proof and ozone pollution prevention. These characteristics allow SSR to be used in the military (such as aircraft, artillery, ships, vehicle-mounted weapon systems), chemical industry, underground coal mining and various industrial and civilian electronic control equipment applications, and has technical advantages beyond MER. Since the AC SSR uses zero-crossing trigger technology, the SSR can be safely used on the computer output interface without having to worry about a series of interferences to the computer caused by using MER on the interface. In addition, SSR has the characteristic of being able to withstand surge currents that can reach approximately ten times the rated current. Table 1 Parameter name (unit) Parameter value Minimum typical maximum input DC control voltage (V) 3.2 14 Input current (mA) 20 Turn-on voltage (V) 3.2 Off