Transformerless PSU using a zener/transil to drop voltage

[andrea@xxxxxxxxxxxxxxxx]

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.

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?

Thanks!
Andrea