I’ve wondered at the lack of ‘drop in solutions’ (module) for PCB’s. When doing appliance repair every board I worked with had a transformer for the mains. I guess in theory it is easy enough for the experienced, deep pocket companies to design their own solution with each board.
There are plenty of drop-in modules, they just cost a lot more than a custom design, so normally anyone who is worried about pennies of BOM cost will not use them.
e.g. https://www.cui.com/product/internal-ac-dc-power-supplies/board-mount/psk-s2c-series
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I guess I used the wrong search parameters when I looked before. It’s been several years. Clears that up for me. 
By “everyday exposed AC” I mean the copper wires you can easily see screwed into the poles of a light switch. A user actually has to go behind those to get to my box, where I plan to put it.
Trying to delete this
Looks like just what I need, craftyjon. Low voltage times low current equals low power. Might even fit in the metal box. Thank you, craftyjon.
I might be too new of a member to send PMs, would you PM me? I’d be interested to talk about our designs some more
Try now…
When I was 13 I built an amateur radio transceiver kit with 750V on the anodes of the transmitter tubes. My only reasonable explanation now is that my parents had no idea what I was doing.
Be careful about the thermal environment for the “wall wart.” Those things are intended to operate in ambient. I worry about the ones on my desk which tend to get buried under papers and debris.
I would otherwise try to comment intelligently on this discussion, but it seems to be going in too many different directions.
For Joe Johnson’s original request I have wondered whether there is a safety-approved something available which (packaging wise) resembles a laptop power supply (but may be bigger) and includes a mechanical or electronic relay, probably a power supply, IEC 320 AC input and AC output connections, and Safety Extra Low Voltage logic control input for the relay. Conceptually this seems straightforward but I have not found such thing.
Blockquote I have wondered whether there is a safety-approved something available which (packaging wise) resembles a laptop power supply (but may be bigger) and includes a mechanical or electronic relay, probably a power supply, IEC 320 AC input and AC output connections, and Safety Extra Low Voltage logic control input for the relay.
Would definitely be interested, but I’m at a loss for space. This is supposed to fit in the work box (blue box in the wall) behind a typical light switch.
Ahh OK I missed that point somehow. That might change quite a bit. In that situation, I think it is presumed to be inaccessible. We are supposed to know that we do not work in that box without shutting down the power first. I have not worked with those safety standards. Those standards are probably less “harmonized” internationally because once something is installed into the wall outlet it is not expected to move. So if I were designing for the USA I would be concerned about NEMA and UL. If you are concerned with countries in Europe then I am not sure. But if anything is accessible (your connection to the Arduino?) then safety isolation would be very important.
I think that in the USA you can now buy wall outlets which include a USB outlet for charging a phone for example. https://www.homedepot.com/p/TruePower-Electrical-Outlet-Receptacle-with-2-High-Power-USB-Ports-ACE-9511HD/305088059?source=shoppingads&locale=en-US&mtc=Shopping-B-F_Brand-G-Multi-NA-Multi-NA-Feed-PLA-NA-NA-Catchall_PLA&cm_mmc=Shopping-B-F_Brand-G-Multi-NA-Multi-NA-Feed-PLA-NA-NA-Catchall_PLA-71700000014585962-58700001236285396-92700010802552517&gclid=CjwKCAiAg8OBBhA8EiwAlKw3kq7AEtO8xSqPkmdKOHjNjk-wGYFjk5f7w5ybabtzDzvk1Js3GfDKtRoCSqcQAvD_BwE&gclsrc=aw.ds For your countries of interest it might be helpful to tear down something like that and see how it is built.
That is correct, nothing in the device is accessible at all.
In fact, you’d have to 1) unscrew the wall plate, 2) unscrew the switch assembly (which itself contains partially exposed AC copper), 3), unscrew my enclosed device from the back of the assembly, and 4) open the plastic case I want to put the PCB in - before you would get to anything.
I will take a look at NEMA and UL, I’m only concerned with the US
OK if your arduino and all else is not accessible, then it seems that it should be easier than the USB power outlet. I am pretty sure that the USB outlet is presumed to be accessible. A user may not be routinely touching the wires, but I don’t think that there is any safety isolation between a phone (that is plugged into USB) and the user. So basically your whole design is non-SELV.
What does SELV mean ?
Separated, Extra-Low-Voltage… the LVD defines “Extra-Low” as <50Vac (75Vdc)
My last information:
Safety Extra Low Voltage referring to voltage which is no more than 30 VAC or (42V peak or DC.) and adequately isolated from the mains. There may be an exception for telephone wires which can get up over 70 VDC in some circumstances.
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The limits of SELV have been modified over the years. Presumably in an attempt to make 48 V telecoms equipment “safe” and get rid of TNV
What does TNV mean ?
I was not familiar with it but found it here; googling “TNV voltage safety”. TNV by itself came up with a lot of unrelated stuff.
So maybe now getting cardiac fibrillation is acceptable, as long as your corpse does not smoke. (Sorry for my dark humor.)
Which is why I am surprised that 2014/35/EU sets this at 50Vac/75Vdc