Solid state relay is a semiconductor device instead of the traditional electric contact switching device of the contactless switching device, with relay characteristics of single-phase active SSR for four-terminal devices, two of the input control side, the two outputs, for light isolation between input and output, the input and dc or pulse signal to a certain value, the output end can shift from off state to state. So how to choose solid state relay (SSR) model specifications?
Selection method of solid state relay
1. Select small current specifications
In the selection of small current specifications printed circuit boards using solid state relay, for the lead terminal is of high thermal conductive materials, welding should be in the temperature less than 250 ℃, the time less than 10 s conditions, such as considering the cause of the surrounding temperature, can consider derating use when necessary, to control the load current rating of the commonly used within 1/2.
2. Selection of SSR for solid-state relays with various load surge characteristics
The controlled load will produce a large surge current at the moment of switching on because the heat is not enough to dissipate, it is likely to damage the SSR internal SCR, so the user should analyze the surge characteristics of the controlled load when selecting the relay, and then select the relay. To ensure that the relay can withstand the inrush current under the premise of steady state operation, the selection can refer to Table 2 for derating coefficients under various loads (at room temperature).
If the relay selected needs to work in occasions with frequent operation and high life and reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable operation.
Generally follow the above principles in the selection, in the low voltage requirements of the signal distortion can be selected using the DC solid-state relay mosFETS as output devices; For example, for ac resistive load and most inductive load, the zero-crossing relay can be selected, so as to prolong the life of the load and relay, but also reduce their rf interference. If used as phase output control, solid-state relay of the random model should be selected.
3. The influence of ambient temperature
The load capacity of solid state relay is greatly affected by the ambient temperature and its own temperature rise. In the process of installation and use, it should be ensured that it has good heat dissipation conditions. Products with a rated working current above 10A should be equipped with a radiator, and products with a rated working current above 100A should be equipped with a radiator and fan strong cooling. When installing the relay, pay attention to the good contact between the bottom of the relay and the radiator, and consider coating an appropriate amount of thermal grease to achieve the best heat dissipation effect.
For example, when the relay works at a high temperature (40℃~80℃) for a long time, users can consider derating it according to the curve data of maximum output current and ambient temperature provided by the manufacturer to ensure normal operation.
4. Over current and over-voltage protection measures
When used in the relay, because the flow and load internal short circuit will cause the SSR solid state relay output SCR permanent damage, can consider adding the quick fuse in the control circuit and air switch to be protected (select relay should choose products output protection, built-in varistor absorbing circuit and RC buffer, can absorb the surge voltage and improve the dv/dt tolerance); RC absorption loop and varistor (MOV) can also be connected at the output end of relay to achieve output protection. The selection principle is 220V choose 500V-600V varistor, 380V can choose 800V-900V varistor.
5. Relay input circuit signal
When the input voltage is too high or the input current is too high, it is considered to connect the divider resistor in series at the input end or the shunt resistor at the input port to ensure that the input signal does not exceed the rated parameter value.
6 In the specific use, the control signal and load power supply should be stable, fluctuation should not be more than 10%, otherwise, voltage stabilizing measures should be taken.
7. During installation and use, it should be far away from electromagnetic interference and RADIO frequency interference sources to prevent the misoperation of the relay.
8. When the solid-state relay is open and the load end has a voltage, the output end will have a certain leakage current, which should be paid attention to in use or design.
9. When the solid-state relay fails to be replaced, the original model or the product with exactly the same technical parameters should be selected as far as possible to match the original application circuit and ensure the reliable operation of the system.
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