1. Reasonable choice of solid state relay
When selecting solid state relays, the research manual, detailed specifications and product technical performance and parameters should be analyzed according to the application conditions. At the same time, it should be noted that the solid state relay performance parameters and load capacity are greatly affected by the ambient temperature and its own temperature, and its input has polarity. It is required that the output circuit is susceptible to the damage of the peaking voltage and the inrush current during the inductive and capacitive loads, and the power loss and heat dissipation caused by the on-state voltage drop during the conduction. Focusing on these issues will help to realize the scientific use of solid state relays to make them more reliable.
(1): Load type, output current and surge current: The steady-state current flowing through the output terminal of the solid-state relay in use shall not exceed the rated output current at the corresponding ambient temperature of the detailed specification of the product. The possible surge current shall not exceed the relay. Load capacity, generally leave a certain margin. The two-way thyristor output is mostly used for resistive loads, and the unidirectional thyristor anti-parallel output is mostly used for inductive and capacitive loads. Most loads can use the "zero control" type, but need to regulate voltage (such as dimming) and a few inductive loads (such as transformers) must use "random control" type of solid state relay load with almost no surge, even if the heating element Although they are purely resistive, they have a positive temperature coefficient and a low resistance at low temperatures, and thus generally exhibit a large starting current. For example, the on-state current of the electric furnace is usually 1.3-1.4 times of the steady-state current, and the incandescent lamp-on current is often 10 times of the steady-state current. The tungsten and tungsten currents can reach up to 25 times the steady state value. Some metal halide lamps require 10 minutes to turn on the lamp, and the lamp and its ballast may appear capacitive and inductive during this process. It may be accompanied by a current pulse up to 100 times the stable value.
Capacitive loads are potentially hazardous. Because it initially exhibits a short circuit when energized, a high inrush current occurs during charging, which is limited by the internal resistance of the power supply, circuit resistance, and circuit inductance. If the power capacitor is switched, not only the surge current but also the overvoltage at the time of "overcompensation" should be considered. It is recommended to use the capacitors produced by our company to cut special solid state relays or SGDF series solid state relays.
Inductive loads can generate large inrush currents, which can cause twice the overvoltage of the supply voltage when turned off. For example, the AC electromagnet and the contactor have a low input impedance in the non-energized state, and a surge current of 3-4 times the steady-state current occurs when the current is applied. A transformer with saturated remanence will continue to excite in the direction of remanence when it is turned on. Due to severe magnetic saturation, a surge current almost determined by the winding resistance will occur in the first half of the cycle, and it can even reach steady state. 30 times. The maximum value of the inrush current when the AC induction motor starts is 5-7 times of the steady-state rated current, and the surge current at the start of the AC gradually transitions from the initial stall current to the steady-state current, the duration of the transition and the motor. And the inertia of the load is very large, from a dozen power supply cycles to tens of seconds. Therefore, users are advised to carefully analyze or test the surge characteristics of the load before selecting a solid state relay, and then select the solid state relay. Solid state relays must withstand this inrush current while maintaining steady state operation.
The following is a recommended value for the derating factor of the rated current of various loads at room temperature against the rated output current of the solid state relay considering the load surge current and the solid state relay overload capacity. When the solid state relay is in frequent operation and requires long life and high reliability, the derating factor of the following table should be multiplied by 0.6, and the enhanced solid state relay with good thermal fatigue resistance should be used.
(2) Output voltage, transient voltage and dv/dt
DC solid state relays are only suitable for controlling DC power and load, and AC solid state relays are only suitable for controlling AC power and load. The voltage of the load power supply must not exceed the rated output voltage of the relay or the specified minimum output voltage. In use, the maximum voltage peak that may be added to the output of the solid state relay must be lower than the transient voltage of the solid state relay. When switching AC inductive loads, single-phase motors, and three-phase motor loads, or when these load circuits are powered up, the solid-state relay output may experience twice the voltage of the supply voltage peak. For such loads, it is best to use an AC solid state relay with a rated output voltage of 280Vac for single phase and 530VaC for three phases. Peak blocking voltages are up to 800V and 1400V, respectively. For inductive and capacitive loads, when the AC solid state relay is turned off at zero current, the supply voltage is not zero and is added to the solid state relay output with a large dv/dt value. Therefore, a relay with a high dv/dt should be used. Especially for solid state relays used for forward and reverse control, the company's SGR series products should be used.
(3) Input characteristics
The DC resistive input solid state relay typically has an input voltage of 4-16Vdc. This solid state relay has a resistor in series with the input circuit, and the current increases with increasing voltage, which is simple and durable. In the application, if the control voltage or current is not suitable, it can be solved by the method of controlling the series connection resistance of the circuit. It is recommended. In addition, it is a constant current input solid state relay with a wide input voltage range, ranging from 3-32Vdc. It has a constant current source circuit connected in series at the input end, and the current is basically unchanged when the voltage is increased to a certain value. Most of the AC input is used in the case of no DC power supply, paying attention to the maximum delay of 40mS when it is turned on.
(4) Other characteristics
This aspect includes analysis and consideration of the electrical characteristics such as the output voltage drop of the solid state relay, the output leakage current, the insulation withstand voltage, the thermal resistance, the climatic environment characteristics such as storage temperature, moisture resistance, and air tightness, and the mechanical environment characteristics such as vibration resistance and impact resistance. Whether the physical characteristics such as weight, size, and terminal mode meet the requirements for use.
The output leakage current is used to characterize the insulation state between the same output when the solid state relay is in the off state. It is not allowed to measure the insulation resistance and dielectric withstand voltage between instruments with a test voltage higher than the output voltage, nor to test the leakage current in an unspecified direction. For special applications, such as inductive, capacitive loads and frequent operations, it is recommended to contact the manufacturer's application engineer for their support.
2, the protection of solid state relay
(1) Overcurrent protection
Solid state relays use semiconductor switching devices as power output components, which are sensitive to temperature changes. Overcurrent can cause semiconductor chips to overheat and cause quality degradation, life reduction or even permanent damage. Although solid state relays can withstand 10 times of rated current in an instant. The above surge current, but exceeding this value is easy to cause permanent damage. Therefore, overcurrent protection is very important, there are many ways to protect the overcurrent, the key is to reflect the speed faster. For the thyristor as the output device AC solid state relays, because the thyristor requires current zero-crossing characteristics, the general protection circuit is ineffective for surge currents and short-circuit currents exceeding the SSR surge current withstand values ​​within 10Ms (50Hz). Fast fuses for semiconductor devices. The nominal fuse current of the fuse should not exceed the nominal current value of the SSR. There are many types of fast-acting fuses available on the market, but the quality varies greatly. Please pay attention when selecting.
(2) Overvoltage protection
When the load is inductive or capacitive, it is likely to produce a transient voltage (blocking voltage) and a voltage rise rate (dv/dt) that the solid state relay can withstand. If the protection measures are not correct or the response is not sensitive, not only will the solid state relay be out of control, but also the solid state relay or equipment may be burnt in severe cases. Therefore, overvoltage protection is a must. Common applications are external transient suppression (RC absorption) circuits and voltage clamping circuits (bidirectional Zener diodes, varistors). When designing products, most of the products have added RC absorption circuit or varistor inside, which can play a certain protective role. It is recommended that users carefully calculate and test the selection of RC circuits according to the relevant parameters and environmental conditions of the load during use. value. If it is not met, an RC loop and a voltage sensitive protection circuit should be connected in parallel. In general, the RC absorption loop can effectively suppress the transient voltage and voltage exponential rise rate (dv/dt) applied to the solid state relay, and the varistor protection circuit can absorb the overvoltage of the wide pulse. The RC value selection of the RC absorption loop is calculated and verified. The choice of experience (for reference only) is that the resistance is between 27-150 ohms, the power is between 2-5 watts, the capacitance is between 0.01-1 uf, and the withstand voltage is between 250-500 VAC (example: R=51 ohm/2W) , C = 0.2uF / 500VAC) The exact value should be determined after the experiment. Be careful not to oscillate. The choice of varistor should also be carefully calculated and verified. Under normal circumstances (for reference only), use the standard in 220VAC circuit. Weigh 470-680V, ​​ø12-16, and use 780-1000V, ø12-16 in 380VAC circuit.
(3) Overheat protection
If the solid state relay is overheated, it will be out of control, but it will cause permanent damage. It is recommended to install overheat protection measures. The usual practice is to ensure that the temperature of the bottom plate of the solid state relay does not exceed 75-80 °C. A general temperature protection circuit can achieve the goal. It is economical to install a temperature control switch on the heat sink near the SSR base plate. When the temperature rise reaches a limited temperature, the SSR input signal is cut off.
When a solid state relay is working, there is a certain power loss on its internal chip. This power loss is mainly determined by the product of the output voltage drop of the solid state relay and the load current, and is consumed in the form of heat. Therefore, the heat dissipation directly affects the reliability of the solid state relay operation, and the excellent thermal design can avoid failure and damage caused by poor heat dissipation.
In general, a solid state relay with a load current of less than 5A can be cooled by air convection. It is required to be installed in a good convection environment, and the distance between two solid state relays is greater than the width of one solid state relay. Solid-state relays with currents greater than 10A should be equipped with heat sinks. For solid-state relays with load currents greater than 40A, air-cooled or water-cooled should be used if necessary according to the design size.
3, the application of solid state relay thermal design and radiator selection
Solid-state relays have a certain power dissipation inside the work. This dissipation value is mainly determined by the product of the output voltage drop and the load current, which is reflected in the form of heat. The heat dissipation directly affects the maximum load current of the solid state relay and the maximum allowable working environment temperature value, which is one of the important factors affecting the reliable operation of the solid state relay. Therefore, we should pay attention to the application of thermal design of solid state relays and the choice of heat sinks, so that solid state relays can work stably and reliably, avoiding the failure and damage of solid state relays caused by poor heat dissipation.
In general, solid-state relays with an output current of less than 5A use natural convection of air to achieve cooling and heat dissipation. However, a good convection environment must be installed. The distance between solid-state relays must not be less than the width of a solid-state relay.
Solid-state relays with a load current greater than 10A are necessary to use a suitable heat sink, if necessary air-cooled (air-cooled wind speed 6 m / s) or water-cooled. Good heat dissipation conditions are important for the reliable operation of solid state relays. The product manual will generally provide the user with operating current, on-state voltage drop and thermal resistance parameters, and some will also give the working current / dissipated power and ambient temperature / substrate temperature curve for reference. Using these parameters, you can calculate the thermal resistance of the required heat sink, and then select the heat sink according to the thermal resistance parameters on the heat sink manufacturer's product manual.
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