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  • I had a quick look at the data for the two different solid state relays just because I have looked at several relay boards off the internet and found them unsafe.
    The smaller relay mentioned here (2A) could perhaps work for a max 300W load but I would not expect it to last very long because of voltage transients on the mains voltage. It really needs some overvoltage protection circuitry.

    The 25A relay should be able to handle larger loads but may need a heatsink as it drops about 1.6V. It needs 7.5mA drive current at 12V. There is no details on how to drive it from 5V. The safest approach would be to use 7.5mA constant current drive.

    With both these relays it is important to remember that when they are not ON, there can be a substantial leakage current through the "contacts", so it is not safe to touch the wire coming from this type of relay if the other wire is connected to the mains voltage. There can be enough current flow to create a danger. Also, voltage transients can cause the relay to turn on briefly (a few ms) even if there is no drive current. So if one is used to mechanical relays it can be a surprise.

    Also, both of these relays have relatively low isolation voltage between power side and drive side, and the driving circuitry should be grounded to avoid electrical shock if there is a large voltage transient on the mains voltage that is large enough to jump the isolation barrier. The most common path for a transient is usually across the surface of the relay or the circuit board. Where many of the internet relay boards fail is that there is not enough creepage distance between power traces and drive traces, and the most common type of relay has the pins placed so that it is necessary to mill a minimum 1mm wide slot in the circuit board between power traces and drive traces, to extend the creepage path and meet safety requirements.
    Also, some of the relay boards that are offered cheaply on the internet contain relays that do not have enough isolation voltage between coil and contacts. Most users may not bother or simply are not aware of the danger, and that may be fine if all you do with the relay board is tinker in your lab. But if the board is installed and perhaps forgotten and somebody happens to touch the low voltage electronics circuitry during a thunderstorm and gets hurt then it is no fun.

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