I would like to make a transformerless power supply, I found some description on the Internet like this pdf provided by Microchip. My question is it is possible to use only the phase line(without neutral line) to build a transformerless power supply and power up a microcontroller, for example in the wall mounted light switch I have only the phase.

This is what I found on the Internet, there is only two wire.


I just order one of this bulb dimmer, here is a few picture:

enter image description here

And inside the cover:

enter image description here enter image description here

It seams that there aren't a battery, how is it possible that this circuit is working?

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    \$\begingroup\$ Probably has a built-in PIC10F200 as a timer to signal it when it should fall apart. Can also be a 555 :) \$\endgroup\$ Jun 10, 2012 at 17:31
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    \$\begingroup\$ @Federico: Don't laugh, that's more real than you think. I once did a product that was rated for one month. The PIC 10F202 in it deliberately made the product die after 35 days so it wouldn't compete with the longer life products the company was selling, even though the battery would have run it for 3-6 months. \$\endgroup\$ Jun 10, 2012 at 17:41
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    \$\begingroup\$ @Olin - must be great for your conscience, working for such a company! :-/ \$\endgroup\$
    – stevenvh
    Jun 10, 2012 at 17:53
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    \$\begingroup\$ @OlinLathrop wow, that is disgusting! stevenh: same as the laptop batteries as I've heard.. \$\endgroup\$ Jun 10, 2012 at 17:55
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    \$\begingroup\$ @stevenvh: There's nothing wrong such such designs if buyer and seller both agree that the customer is effectively paying to rent the product for a month, and will have no property interest in it after that time aside from any possible scrap value. \$\endgroup\$
    – supercat
    Aug 29, 2013 at 18:16

3 Answers 3


You need them both, otherwise you have no voltage difference and no power. You may have to pull wires from a wall outlet instead of the switch. The wall outlet has both phase and neutral.

I had a look at the product you refer to in your edit, and you probably mean this:

enter image description here

I'm not sure, but my guess is that it parasites on the load by placing a small load in series which gives it enough voltage drop to power it. But that would mean it has to switch a load on from time to time to keep going.
In any case, if the \$In\$ comes from a switch, and the \$Out1\$ goes to the neutral, the only thing to make it work is a battery.

  • \$\begingroup\$ I thought, I edited my question, in the last link there is only two wire, both are phase. \$\endgroup\$
    – Kicsi Mano
    Jun 10, 2012 at 17:15
  • \$\begingroup\$ After some research I found the following site:electroschematics.com/6604/touch-light-dimmer-circuit It seams that there is only the phase line. \$\endgroup\$
    – Kicsi Mano
    Jun 10, 2012 at 18:14
  • \$\begingroup\$ @Kicsi - That one uses the voltage drop across the triac for its power supply. It's a dimmer, and so has no function if you switch the light off. It couldn't because it wouldn't have power. The Chinese junk seems to suggest it does switch power, and it can switch it off, but how will it switch it on again if there's no two different voltages? \$\endgroup\$
    – stevenvh
    Jun 10, 2012 at 18:20
  • \$\begingroup\$ I just bough one of this switch, you can find some picture above. \$\endgroup\$
    – Kicsi Mano
    Jun 23, 2012 at 8:10

You cannot break the rules of Physics.

IF it has Phase and load in only, as shown, and IF it operates a wireless receiver when it is off, to allow it to be turned on remotely, then it MUST receive energy from somewhere to operate. Available choices are as below - ij the order of decreasing probablility, although the first two are about equal.

  • Line power using trickle current through load with operation from small voltage drop when on.

  • Internal (supercap or battery) - charged by small drop when on.

  • External (eg signal from wireless sender, energy harvesting from heat or vibration etc)

The first is potentially dangerous and will be illegal in some countries.

The second is doable but "has issues" [tm] if left off for long enough.

The third is "nice" but unlikely. Note however that most RFID systems work as per this method. If a receiver can obtain enough energy to identify a turn-on signal it can then use operating voltage drop to operate. So, possible but unlikely.


These circuits fets only for incandescents lamps because they use the return wire as the neutral. Incandescent lamp has low resistence but require lot of current to turn on. So, if the circuit current consumption is too low compared to incandescent lamp, you can supply power to the circuit but not turns on the lamp.

Explaning based on circuit:

enter image description here

Once SW1 (original wall switch) has been removed and replaced by the proposed circuit, as in circuit above, according to the low resistence incandescent lamp properties, the circuit "sees" the return wire as a Neutral (original one, no extra wire). Allowing to supply power necessary to the circuit (around 200mA), without turning the lamp (L1) on. By this way, you may have your circuit powered up and control of lamp state.

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    \$\begingroup\$ -1. It fails to address any of the issues in the question. \$\endgroup\$
    – user3624
    Aug 29, 2013 at 18:06
  • \$\begingroup\$ Maybe I didn´t explained properly. I edited the comment adding an image that explain what I pointed. \$\endgroup\$ Sep 5, 2013 at 18:22

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