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Дома > Список технологий > Main problems and maintenance methods of solid state relays

Main problems and maintenance methods of solid state relays

Время публикации: 2019-08-13

The main problem of solid state relay

When the solid state relay is open and there is voltage at the load end, there will be a certain leakage current at the output end, and care should be taken to prevent electric shock during use or design. When the solid state relay fails to be replaced, the original model or the product with the same technical parameters should be selected as much as possible to match the original application line to ensure reliable operation of the system.

overheat

When the SSR is turned on, the component will withstand the dissipated power of P=V (tube voltage drop)×I (load), where the V rms value and the I rms value are the effective values of the saturation voltage drop and the operating current, respectively. The load capacity of solid state relays is greatly affected by the ambient temperature and its own temperature rise. It is necessary to strictly refer to the allowable case temperature rise (75 °C) according to the actual working environment conditions, and reasonably select the size of the heat sink or reduce the current use. During installation and use, it should be ensured that it has good heat dissipation conditions, otherwise it will be out of control due to overheating and even cause product damage.

Generally speaking, below 10A, the instrument board with good heat dissipation condition can be used. Products with rated working current above 10A should be equipped with radiator. Below 30A, natural air cooling is adopted. When continuous load current is greater than 30A, instrument fan forced air is required. Cold, products above 100A should be equipped with a radiator and a fan to cool. Pay attention to the good contact between the bottom of the relay and the heat sink during installation, and consider applying proper amount of thermal grease to achieve the best heat dissipation. If the relay is working in high temperature for a long time (40 °C ~ 80 °C), the user can consider the derating to ensure normal operation according to the maximum output current and ambient temperature curve data provided by the manufacturer.


Solid state relay heating reasons

Solid-state relays have a certain power loss on their internal chips during normal operation. This power loss is mainly determined by the product voltage drop of the solid-state relay and the load current, which 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.

Overcurrent and overpressure

When the relay is used, the overcurrent and load short circuit will cause permanent damage to the SCR solid state relay internal output thyristor. Consider adding a fast fuse and air switch to the control circuit to protect it. (Selecting the relay should select product output protection, built-in Varistor absorption loop and RC snubber to absorb surge voltage and increase dv/dt tolerance); fast fuses and air switches are common overcurrent protection methods. The quick-blow fuse can be selected at 1.2 times the rated operating current. Generally, a small fuse can be used. Pay special attention to load short circuit, which is the main cause of damage to SSR products.

Inductive and capacitive loads, in addition to internal RC circuit protection, it is recommended to use varistor in parallel at the output, as a combination of protection. The size of the metal zinc oxide varistor (MOV) determines the absorption power, and the thickness determines the protection voltage value. AC 220V SSR, MYH12-430V varistor; 380V select MYH12-750V varistor; larger capacity motor transformer should use MYH20 or MYH2024 varistor with large current capacity. The selection principle is that 500V selects 500V-600V varistor for 220V, and 800V-900V varistor for 380V.

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Maintenance method of solid state relay

1. When using the solid state relay used in the printed circuit board of small current specification, the lead terminal is made of high thermal conductive material, and the welding should be carried out under the condition of temperature less than 250 ° C and time less than 10 S, for example, considering the ambient temperature. If necessary, derating can be considered. Generally, the load current is controlled within 1/2 of the rated value.

2. Selection of solid state relay SSR for various load surge characteristics

The controlled load will generate a large inrush current at the moment of turning on. Due to the inability to dissipate heat, it is likely to damage the internal thyristor of the SSR. Therefore, the user should analyze the surge characteristics of the controlled load when selecting the relay, and then analyze Select the relay. The relay can withstand the surge current under the premise of ensuring steady state operation. Refer to Table 2 for the derating factor of various loads (at normal temperature).

If the selected relay needs to work in a place with more frequent work, long life and high reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.

Generally, the above principle is followed in the selection. In the low voltage requirement, the signal distortion is small, and the MOSFET can be used as the DC solid-state repeater of the output device; for the AC resistive load and most inductive loads, the zero-crossing relay can be selected. Extends load and relay life and also reduces its own RF interference. For phase output control, a random type solid state relay should be used.

3. Effects of using ambient temperature

The load capacity of solid state relays is greatly affected by the ambient temperature and its own temperature rise. During installation and use, it should be ensured that it has good heat dissipation conditions. Products with rated working current above 10A should be equipped with heat sinks. Products above 100A should be used. With radiator and fan strong cooling. Pay attention to the good contact between the bottom of the relay and the heat sink during installation, and consider applying proper amount of thermal grease to achieve the best heat dissipation.

If the relay is working in high temperature for a long time (40 °C ~ 80 °C), the user can consider the derating to ensure normal operation according to the maximum output current and ambient temperature curve data provided by the manufacturer.

4. Overcurrent and overvoltage protection measures

When the relay is used, the overcurrent and load short circuit will cause permanent damage to the SCR solid state relay internal output thyristor. Consider adding a fast fuse and air switch to the control circuit to protect it. (Selecting the relay should select product output protection, built-in The varistor absorption circuit and RC snubber can absorb surge voltage and increase dv/dt tolerance. It can also be connected to the RC absorption circuit and varistor (MOV) at the output of the relay for output protection. The selection principle is that 500V selects 500V-600V varistor for 220V, and 800V-900V varistor for 380V.

5. Relay input loop signal

When using the input voltage too high or the input current is too large to exceed its specified rated parameters, consider connecting the voltage divider resistor at the input end or connecting the shunt resistor at the input port so that the input signal does not exceed its rated value.

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6 In the specific use, the control signal and load power supply requirements are stable, the fluctuation should not exceed 10%, otherwise the voltage regulation measures should be taken.

7. Keep away from electromagnetic interference and RF interference sources during installation and use to prevent the relay from malfunctioning.

8. When the solid state relay is open and there is voltage at the load end, there will be a certain leakage current at the output end, so be careful when using or designing.

9. When the solid state relay fails to be replaced, the original model or the same technical parameters should be selected as much as possible to match the original application line to ensure reliable operation of the system.

 

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