I've done one, some time ago, see
http://www.geocities.com/vsurducan/c520.htm
at the bottom of the page, up to 150mA, but I don't recommend such power
supplies for production devices ( maybe on 120V mains, only )
Note that in my design, the supply don't like to remain without load...
Vasile

On Thu, 26 Jul 2001, Roman Black wrote:

> Alexandre Domingos F. Souza wrote:
> >
> >         I designed a circuit with a PIC (12C508) that turns on and off a
> >relay, controlled by an external source. The power to this circuit (around
> >120mA) is a small transformer. I'd like to build a transformerless PSU into
> >this circuit. But EVERY scheme I found on the internet was unusable. I saw
> >something on the Embeeded Control Handbook from Microchip, but it only goes
> >to (if I'm not mistaken) 45 mA. Maybe someone can help me to develop (or at
> >least learn to) a simple transformerless PSU that 120mA.
>
>
> Hi Alexandre! This can be done if you REALLY want
> to do it. The Mchip app note shows 45mA, and that
> is with a half-wave rectifier (two diodes).
>
> Just change to a proper rectifier bridge, this will
> double the current to 90mA. Then using a 30% larger
> value of the main Xc cap will give you 120mA.
> This web page has Traffic.PDF sent by Richard Ottosen
> which shows a larger current Xc supply with a
> full-wave bridge:
> http://centauri.ezy.net.au/~fastvid/tube4w.htm
>
> IMPORTANT! using a full wave rectifier will give
> lethal voltages at ALL points of your 24v circuit.
> You no longer have a reference ground! I know you
> said you were using an optocoupler but remember at
> all times this circuit is dangerous...
>
> Some handy points, you can get reliable relay
> operation at half the hold current usng the old RC
> trick, which gives it full current for pull-in
> but only half current under hold. Costs one resistor
> and one 470uF electro. Maybe this would get you
> down to 60mA circuit current. I have used this trick
> down to about 25% hold current.
>
> Remember the Xc supply ALWAYS runs full current,
> so if your 120mA relay is OFF, something else
> (large zener?) must be drawing the 120mA. Have a
> serious think about this. Xc supply is best for
> circuits that always draw the same current.
> :o)
> -Roman
>
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>

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