Re: Transformerless PSU using a zener/transil to drop voltage

[Fred Bartoli <fred._canxxxel_this_bartoli@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>]

andrea@xxxxxxxxxxxxxxxx a écrit :
> Hi all,
>
> For a hobby home automation system I need to make a very small, non
> isolated, direct-mains power supply unit regulated via a 7812 IC or such.
> Thus it has to be transformer-less, and since I need to draw something
> like 100mA, even a capacitor (of the kind that do not explode on mains!)
> would be too big. I have two ideas so far to lower the voltage to a range
> acceptable by the 7812:
>
> 1) a rectifier in serie with a zener (even better a transil), the latter
> used to drop the voltage so that only the peaks of the AC top waveform
> come through, and lessed by ~300 volts. Care must be taken because if the
> (230VAC case) RMS voltage can be anywhere from 210V to 250V, then it means
> that the DC coming out of this circuit will have the ~same offset range
> (eventually a fuse will be blown or, better, I'll use a polyswitch).
> Another thing that worries me is power dissipation: if I draw 100mA, then
> of course those 100mA will have to be multiplied by the zener/transil drop
> voltage, which (again, 230VAC case) will be, say, 300V: 30W! But anyway
> this will only happen for a fraction of the 50Hz waveform, when the diode
> conducts (probably much, charging a low voltage capacitor), so the average
> power dissipation of this diode hopefully should still be acceptable.
>

No, it still will be 30W (average current in = average current out).

> 2) if the above is not feasible, I was thinking about working on the low
> part of the AC waveform rather than on the peaks: maybe a high voltage
> MOSFET which conducts only when the serie voltage is under e.g. 20V? For
> the rest of the AC waveform the MOSFET would be turned off. I should use
> a 20V zener (in a high impedance way, something zeners really don't like)
> and a P-MOSFET, intuitively, or a depletion-mode N-MOSFET.
>
> Any suggestions/comments please?
>

I think it's Linear that has a circuit doing exactly this but I can't
find the part# now.

You can also adapt this: http://www.edn.com/contents/images/21700di.pdf


--
Thanks,
Fred.