I want to build a small information display panel which gets mounted to a wall at about eye level. It seems to me the logical choice is to power it from the 110 AC line which goes to the lightswitch, and to make it fit the “decora” style lightswitch so I can use the available wall plates for a nice appearance.
Does anyone know of an off-the-shelf lightswitch form-factor AC-DC (5V or 3.3V, either one) power supply? They can be found in the various smart light switches like those made by Levitron or Smarthome, but my searching hasn’t found just the power supply portion available anywhere.
To be clear, I’m talking about powering a 5V DC device from the lightswitch AC power, and NOT switching the AC power with the 5V controller. The only interaction with the AC is the input power.
The CUI VOF 25 series power supplies (available from Digikey, etc.) are 2.0x3.5x0.8 inches. I think they should fit. I have used the 12 volt version (CUI VOF-25-12) for a few years and they are great.
The 5V version puts out 4.4A and they cost between $16-$20 in single quantities.
Before you start, make sure you have a neutral in the switch box. Many are just a switch loop (ie, they have unswitched hot coming in, and switched hot going out). Yes, 2014 NEC requires either the neutral be there or be easy to add, but that’s only for new construction in areas that have adopted it…
You are NOT allowed to leak more than 0.5mA into the safety ground, so you can NOT use it instead of a neutral if the neutral isn’t present.
Most of the existing devices don’t use a power supply module; they make their own power supply, including winding their own transformers. For a homebrew project, howerver, I would use a completed module for safety and simplicity. You don’t say how much 5V you need; something like these should work.
aren’t there some (US?) code restrictions on mixing low voltage and mains voltage in the same box? Or perhaps only if the low voltage passes on a wire going outside of the box.
There are restrictions on running low voltage dc and 120 vac wires together. There you have to make sure the least insulation value is greater than the highest required insulation value, and it can be a bad idea for interference reasons. Your low voltage dc wiring may pick up noise from the ac wire right next to it. Hence twisted pair, etc. If you can’t get dc wire with a high enough insulation value (for instance Ethernet wire), then you have to run it separately.
Inside an electrical box, you just have to make sure of proper separation. Based on the smart light switches I’ve taken apart, that seems to mean putting the ac and dc on separate sides of a pcb. The separation between ac hot and ac neutral has to be maintained as well. The comments on the Sparkfun Big Relay Kit are illuminating.
That’s my understanding from googling, anyway. I’m not an electrician. Always double-check information where lethal electricity is concerned. Panda bears look cuddly, but will mess you up.