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

["Paul Hovnanian P.E." <paul@xxxxxxxxxxxxx>]

Fred Bartoli wrote:
>
> 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).

Exactly. So if the OP can afford the real estate to heat sink 30W, a
transformer shouldn't be an issue.

A switched mode supply shouldn't be too demanding at this power level
and will be lighter and smaller than one with a transformer. But if the
OP is challenged by the exploding capacitor problem, I'd suggest a quick
trip to the local electronics shop to buy a ready made wall wart.

100 mA @ 12V should be a cinch and will eliminate the safety risk. The
stuff on the 12V side will be safe and easier to tinker with.

--
Paul Hovnanian     mailto:Paul@xxxxxxxxxxxxx
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I have a very firm grasp on reality. I can reach out and strangle it any
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