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Prusa says the MK3S+ draws up to 120 W. So let's say 150 W of inrush current.

$$150 W \div 230 V ≈ 0.65 A$$

An order of magnitude less than a typical Raspberry Pi optically isolated relay shield can do.

Is there anything I should be aware of when choosing the board? Anything I'm missing here?

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  • $\begingroup$ I doubt the printer will do that in idle when you turn on the printer, the power will be much less (no heating of the bed or the hot end). $\endgroup$
    – 0scar
    Commented Dec 7, 2023 at 15:55
  • $\begingroup$ @0scar I also doubt it'll be a problem, but I'd much rather know it won't. I hope someone here already knows and can share. $\endgroup$
    – Mołot
    Commented Dec 7, 2023 at 16:23

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Yes it is safe to use such relay modules. Note that the printer will use less power than the specified power, when in idle it nearly doesn't use any power at all, the inrush current will be ver low when switching the relay. Furthermore, the relay is capable of handling the current according to its specification. Do note that you are using mains power, which can be dangerous, please make precautions working on mains (pull the plug from the socket) and shield the wires and use proper crimping tools, no solder.

I have done this with 2 printers, one where I cut the 230 V, but the latest one uses a computer 12 V PSU which has a separate wire (I think this is 5 V) which you can short to start the PSU; this is much safer. I used and use OctoPrint with custom shell scripts to turn on the printer and the hot end cooler fan as described in this answer.

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