# How can I get 3.7V 100mA from the 220V phase line? [duplicate]

Possible Duplicate:
Transformless power supply without ground

If I have access only to the 220V phase line, and I would like to charge a 3.7V battery, is it possible, without null line?

For example:

220V
------------------ 3.7 charger --------------
Source                                      |
light
|
Null line

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 Ah yes this is almost the same, and there is an example. Do I have to delete this post? – iUngi Jun 11 '12 at 14:05 You can't since the answer has votes. Don't worry, a moderator will close it, then there can't be any new answers. End of story. – stevenvh Jun 11 '12 at 14:07 @ungibungi, no worries. Glad we could be of help. The site has a large wealth of existing questions, if we have one we will get it cleaned up for you. – Kortuk♦ Jun 11 '12 at 14:55

## marked as duplicate by stevenvh, Kevin VermeerJun 11 '12 at 14:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

## 1 Answer

Yes, it's possible. Many moons ago Siemens had a touch dimmer IC which got its power supply from the voltage drop across the triac:

So 4 of diodes in both directions in series with the load will give you a voltage drop of $\pm$ 3V as a square wave. You can rectify this with Schottky diodes and still have more than 2V left. There are switchers (SMPS, Switched Mode Power Supply) which can boost 1.2V to whatever your charger needs.

WARNING The diodes and everything connected to them carry a lethal voltage!

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Is it possible that there is a heating problem(overheat)? – iUngi Jun 11 '12 at 13:39
It might. Each diode will see half of the loads current (one half cycle), and that times 1V drop can be 1W per diode if your load is 500W at 230V AC. The switcher won't heat up much. – stevenvh Jun 11 '12 at 13:44
Is it possible to get 100mA? Is there any power limitation? – iUngi Jun 11 '12 at 13:53
The limit is in your load. If that's a 60W bulb you'll have 250mA RMS through (diodes + charger). You can't get more than that. – stevenvh Jun 11 '12 at 14:05
it seams that Siemens stop manufacturing this IC, does it has and equivalent IC in production? – iUngi Jun 11 '12 at 15:09
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