What are the selection methods of solid state relays

Update:10-11-2021
Selection method of solid state relay:
1. When selecting solid state relays used on printed circuit boards with low current specifications, since the lead terminals are made of high thermal conductivity materials, the soldering should be carried out under the conditions of a temperature less than 250℃ and a time less than 10S. If the surrounding temperature is considered, If necessary, derating can be considered. The industry recommends the selection method of solid state relays: generally control the resistive load current within 1/2 of the rated value for use, inductive load within 1/3 of the rated value, frequent startup or severe environmental conditions , It should be used in derating.
2. Selection of solid state relay SSR for various load surge characteristics
The controlled load will generate a large surge current at the moment of switching on. Because the heat is too late to dissipate, it is likely to damage the SSR's internal thyristor. Therefore, when selecting a relay, the user should analyze the surge characteristics of the controlled load, and then Select the relay. The relay can withstand this surge current under the premise of ensuring steady-state operation. When selecting, refer to Table 2 for the derating factor for various loads (under normal temperature).
If the selected relay needs to work in the occasions with more frequent work, high life and high reliability requirements, it should be multiplied by 0.6 on the basis of Table 2 to ensure reliable work.
Generally, follow the above principles when selecting. When low voltage requires low signal distortion, DC solid-state relays using field effect transistors as output devices can be used; for AC resistive loads and most inductive loads, zero-crossing relays can be selected. Extend the life of loads and relays, and also reduce their own radio frequency interference. For phase output control, random solid state relays should be used.
3. The influence of 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, ensure that it has good heat dissipation conditions. Products with a rated working current of 10A or more should be equipped with a radiator, and products with a rated operating current of more than 100A should be equipped with a radiator. Equipped with a radiator and a fan for forced cooling. When installing, pay attention to the good contact between the bottom of the relay and the radiator, and consider applying an appropriate amount of thermal grease to achieve the best heat dissipation effect.
For example, when the relay is working at high temperature for a long time (40℃~80℃), users can consider derating to ensure normal operation according to the curve data of the maximum output current and ambient temperature provided by the manufacturer.