I'm doing a home automation system which uses a RF transceiver(nRF24L01+) paired with an arduino to control the relays. Since these require low power DC to run I was researching on ac to dc when I came across capacitative power supplies. This type of supply is needed as there isn't enough of space to throw in a transformer in a back box.

My only concern here is if such a power supply is safe, cause I don't want my house burning down.

This is the circuit I've found so far : http://www.circuitsgallery.com/2012/07/transformer-less-ac-to-dc-capacitor-power-supply-circuit2.html

  • \$\begingroup\$ You should also be asking whether connecting it to the mains is legal. \$\endgroup\$
    – user207421
    Commented Apr 26, 2016 at 23:27
  • \$\begingroup\$ @EJP Are you aware of any jurisdictions where it wouldn't be legal for personal use? Because I'm not. \$\endgroup\$
    – user39382
    Commented Apr 26, 2016 at 23:52
  • \$\begingroup\$ @EJP I honestly don't see how there's a legal aspect to this. As far as I know, the mains are within the confines of my house for which I pay a monthly electricity bill. OR do you mean the mains being tampered (for lack of a better word) by someone who is not a professional electrician? \$\endgroup\$
    – azhamn
    Commented Apr 27, 2016 at 0:05
  • 1
    \$\begingroup\$ @nappyboi -- most countries have regulations as to what can be plugged into a mains outlet -- these are intended to make sure that gizmos don't fry people with mains AC to the face, go on a rash of burning down folks' houses behind their backs, or spew RF trash all over the powerlines that hoses up your neighbor's radio. \$\endgroup\$ Commented Apr 27, 2016 at 0:31
  • 1
    \$\begingroup\$ You certainly can build such a thing (I have), but you would never want to sell it. It would be unlikely to get UL approval, which is a legal issue only in that it helps protect you from liability. \$\endgroup\$
    – Mark
    Commented Apr 27, 2016 at 4:35

2 Answers 2


The capacitive PSU is common in many devices like LED lamps,etc..The device shall not have exposed metallic parts, because it is a shock hazard as the part have mains live voltage. As for burning the house: you need to properly fuse the input and there is no big difference between transformer and transformerless PSU regarding fire hazzard.


There are several safety issues with this circuit:

  1. A "400 V rated capacitor" may be very misleading. A lot of capacitors can withstand 400 V DC, but are not suitable for prolonged operation at 50/60 Hz 230 V AC. You should use a special X or Y rated capacitor.
  2. You must use a fuze even with X/Y rated capacitor.
  3. Equipment connected to the circuit is under the line voltage with respect to Earth. So you are exposed to the electrical shock hazard.
  • \$\begingroup\$ 1. Exactly where to I add the fuse? on the life or neutral side? 2. Is there any way to ground this circuit/the arduino? \$\endgroup\$
    – azhamn
    Commented Apr 28, 2016 at 1:29
  • \$\begingroup\$ I've just found this. Seems better doesn't ? microcontrollerslab.com/… \$\endgroup\$
    – azhamn
    Commented Apr 28, 2016 at 1:45
  • \$\begingroup\$ A fuze in the line wire is slightly more preferable, since it will provide protection from line to ground fault (but not from the electrical shock hazard). \$\endgroup\$
    – dmitryvm
    Commented Apr 28, 2016 at 19:21
  • \$\begingroup\$ There is no way to ground this circuit or the arduino. It is possible to ground (connect to an "earth" wire) an isolated metallic casing around the circuit. A better solution was already mentioned by Marko Buršič: "the device shall not have exposed metallic parts". As for other circuit, it uses half-wave rectifier, so a larger capacitor is required for a given power. Also, the current-limiting 47Ω resistor may be a good thing, but such resistor should be specially designed to act like a fuze (otherwise it will pose the fire hazard) and should withstand high current pulses. \$\endgroup\$
    – dmitryvm
    Commented Apr 28, 2016 at 19:38

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