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

["B Fuhrmann" <b-fuhrmann-usenet@xxxxxxxxxxxxxxxxxxxxxxxxx>]

>> 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?


"MooseFET"  wrote in message ...

> Use a comparitor to switch the MOSFET off if the output voltage
> exceeds 12V or the drop on the MOSFET exceeds, lets say, 6V.
>
> The mains spends only a smallish time inthe 12V to 18V span so the
> MOSFET needs to be able to conduct way more than 100mA when on.

I think that the Origonal Poster should consider using a transformer since
the need for the larger capacitor is a hint that it isn't really a small
supply.

In the idea of switching the power:
About 15 years ago, I used an app note from Linear Technology to create a
very high efficiency transformerless power supply (it was going into a well
insulated case that plugged into the utility meter socket on industrial
buildings).

The first stage of the supply was a simple and dirty switching supply.
Basically (there were more parts involved) a capacitor with a transistor to
the voltage input that turns on whenever the capacitor is < x volts.  As
soon as the capacitor is charged, the transistor shuts off.
As the load draws power, the capacitor drops voltage slightly and the
transistor drops more energy in from the supply.  The hysterisis in the
control of the transistor becomes ripple in the supply.
In the system they were pushing in the app note, the output of the cheap and
dirty switching supply went into a low dropout linear regulator that they
had recently come out with.

--
Bill Fuhrmann