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

[Wimpie <wimtel@xxxxxxxxx>]

On 29 mayo, 18:01, and...@xxxxxxxxxxxxxxxx wrote:
> 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

Hello Andrea,

When it is a hobby project, and even a professional one, I would drop
both options.  100mA at 12 volt is 1.2W and that is not low power in
my opinion.

Many designs that does not use a transformer, use a capacitor directly
connected to mains. The capacitor serves as a dissipation less current
limiter. The capacitor feeds a 2 diode rectifier with storage
capacitor and zener (regulator) diode.

The rectifier must be loaded, otherwise the voltage rises to several
100 V, therefore the regulator diode is added. Note that when your
circuit doesn't consume the designed current, all power is dissipated
by the regulator diode (in your case about 17V to feed the 7812).

A resistor is in series with the capacitor to absorb transients.
Mostly a varistor across the mains is added to absorb large
transients. The design of such a capacitor/rectifier circuit is not
easy if you want that you circuit will survive over voltage category 3
or 4 transients.

When it comes to space saving, the capacitor will be in the 2..4 uF
range and therefore will have a large size.  I would consider a
transformer for that power level. Even professional energy meters do
use a small mains transformer rather then the transformerless approach

Hope this will help you a bit.

Best regards.
Wim