I have implemented a "Capacitive transformerless power supply" to control a microcontroller-based circuit that contains capacitive touch sensors, and I had an "AC polarity problem"

Surprisingly, the circuit behavior became affected by the polarity of the input AC (Line & Neutral), such that:

  • When the "Line" of the AC is connected to the 820nF capacitor and the "Neutral" is connected to the 100 ohm resistor --> behavior of the capacitive touch differs from connecting them in vice versa.

I have no idea how could the AC polarity affects the behavior of the supplied circuit?

Is there any reasonable explanation?

This is the schematic of the Capacitive power supply circuit I implemented:

Note that, the ground symbol in the circuit indicates the common point in the DC part, the AC source is only 2 pins (Line & Neutral)

enter image description here

Thanks in advance.

  • 2
    \$\begingroup\$ You should be careful with AC. The issue is with the grounded rectifier dc- \$\endgroup\$
    – user16222
    Dec 14, 2014 at 21:17
  • 1
    \$\begingroup\$ I see "transformerless" and "touch" in one question. I close the browser tab. \$\endgroup\$
    – PlasmaHH
    Dec 15, 2014 at 12:27
  • \$\begingroup\$ I'm with PlasmaHH on this - "Capacitive transformerless power supply" and "capacitive touch sensors" do not belong in the same project. And if you really had grounded the bottom end of your 1000uF cap as your schematic indicates then you'd have seen smoke & flames (depending on how quickly your circuit breaker trips) \$\endgroup\$
    – brhans
    Dec 15, 2014 at 14:28
  • \$\begingroup\$ The ground here is just the common line in the circuit, not the earth coming from the source, the source is only 2 lines (live and neutral) \$\endgroup\$
    – mina_g
    Dec 15, 2014 at 22:05
  • \$\begingroup\$ You're using a bridge rectifier. This changes the 50Hz factor to 100Hz since it is working both half-cycles. And also your neutral does not seem to be soo "neutral". \$\endgroup\$ Sep 9, 2015 at 10:37

1 Answer 1


You appear to be in Egypt. As far as I know the electrical system there (and in Europe) uses either a two-pin ungrounded or Schuko grounded plug. One pin is "hot" (roughly 240VAC wrt earth) and the other is "neutral" (roughly 0VAC wrt earth). It's not clear to me what the ground symbol in your schematic is intended to mean- an earthed connection or simply a common point in your circuit. I'll assume it's the latter since the power supply would more-or-less explode (R4 and likely the bridge would fry) if the point shown as grounded was earthed and the plug connected the 'wrong' way.

If the common in your circuit is connected to the 'hot' then the result is similar to attaching the person to the hot (240VAC wrt ground) with the circuit grounded.

This is potentially an extremely dangerous situation- if the person comes into direct contact with 240VAC hot and there is another path to earth (say they touch a water tap), then they could easily die.

I strongly suggest that you use a galvanically isolated wall-wart supply, approved and from a reputable manufacturer (people have died from cheap crappy ones- but even the worst failed one is no worse than your circuit). You may still run into issues with 'polarity' because of the Y capacitor internal to the supply, but at least it won't result in unnecessary corpses.

  • 2
    \$\begingroup\$ Agree 100%. This sort of power supply is acceptable if completely isolated from the user. In conjunction with touch sensing, it is just too inherently dangerous if anything goes wrong - such as reversed live/neutral, or component failure. I tried writing an answer last night but had to delete it - it was just too snarky. \$\endgroup\$
    – user16324
    Dec 15, 2014 at 11:47
  • \$\begingroup\$ The source is 2 pins ungrounded as you said (Line & Neutral), and the ground symbol is the common point. \$\endgroup\$
    – mina_g
    Dec 15, 2014 at 22:25
  • \$\begingroup\$ The circuit is working good with no hazard problems. The only problem is that, interchanging the Line & Neutral pins affects the behavior of the DC part. \$\endgroup\$
    – mina_g
    Dec 15, 2014 at 22:50

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