I plan to use a small 220v to 5v buck converter I bought, to power a Wemos D1 mini from 220v mains power. I am not sure how to safely hook it up to the mains power.

It seems that because the system will not draw a lot of current I could use a relatively thin wire, but somehow it feels unsafe to use thin wires for 220v. The power converter I have has small holes in the PCB for the input, so I can not solder 'normal' 220v wires directly to them.

The project is a power monitor which will be installed into a box with a 220v socket. So I can easily branch off the 220v from there. This also means there will not be a lot of strain on the wire as it is all contained in the same enclosure. Another concern however is that I want to keep it small so all can fit into the existing enclosure. To summarize, my question breaks down to:

  • What type of wire is safe for the 220v input side of the converter
  • Can I solder the wires directly to the converter or should I use some sort of screw terminals (which will take up more space)
  • Should I use a fuse between the mains power and the converter?
  • Should I be concerned about heat beeing generated by the converter in a small enclosure
  • \$\begingroup\$ What does the data sheet for the converter tell you about the things you ask? \$\endgroup\$
    – Andy aka
    Commented May 30, 2019 at 11:21

1 Answer 1


I don't know all the regulations, especially since you live in a different country than I do, but the general goal is that you don't want a downstream short to be able to melt wires and cause a fire. Using large wires to the converter doesn't solve the problem, you don't know what wire/trace sizes are inside the converter.

So, you want a fuse in your device. You want the wires before the fuse to be large enough to trip the mains breaker without melting if they short. After the fuse, you want the wires large enough so they can withstand the fuse current plus some margin. Use the converter rating and add some margin to determine the fuse rating.

Remember that a fuse rating is the current at which it is guaranteed not to blow, not the current at which it is supposed to blow. The guaranteed to blow value is usually much higher than the fuse rating.

If your device will likely be used by non-technical people, you may want to use a non-replaceable fuse. Otherwise, it is too easy for the user to defeat any safety provisions that you have designed into your device by installing a fuse with a much larger rating.


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