Ned, we are on the same wavelenght !

Cheers, Vasile

On Fri, 27 Jul 2001, Ned Seith wrote:

> Vasile,
>
> Your series capacitance voltage divider circuit has certainly received
> criticism.
> So I hope that I'm not kicking a dead horse.
> While a transformer would definitely be the safest approach for the typical
> hobbyist, I'm glad to see that there is someone who is willing to be more
> intimate with the electrons.
> The concept and configuration of your series capacitance voltage divider
> circuit is fine.
> I have designed industrial applications which have provided several amps of
> current at low voltages from 220 Vac and 277 Vac mains using a series
> capacitor. Industrial equipment and machines frequently use a series
> capacitor instead of a transformer for providing lower voltages.
> A few points:
> I would not add a resistor in series with the capacitor (C9) as this will
> vary the effective capacitance reactance which is providing the desirable
> voltage drop and will also dissipate heat.
> I would not add a resistor in parallel to the capacitor (C9) as this will
> vary the effective capacitance reactance which is providing the desirable
> voltage drop. This resistor would also dissipate heat.
> I would not be too concerned with the capacitor (C9) remaining charged as
> the approximate 6 mA current consumption from the voltage regulator will
> quickly discharge the capacitor. UL and TUV stipulate that the voltage
> potential on an unplugged power cord must diminish to 1/3 of the mains
> voltage within 10 seconds. So, If I wanted to increase the rate at which
> the capacitor discharges, I would place a resistor in parallel to (C5) on
> the low voltage side of the circuit.
> I would want a 1/4 amp fuse between the 220 Vac main and the series
> capacitor (C9).
> I would also want the series capacitor (C9) rated for 800 V to 1000 V.
> If the series capacitor (C9) is getting warm at all, then the voltage
> rating should be increased.
> I would want the diode rectifiers voltage rating increased to 800 V to 1000
> V (1N4006, 1N4007) to assure that there is not any brief reverse conduction
> during the delay of the capacitors charge cycle.
> Otherwise, looks good to me.
> Just a few comments from an old engineer.
>
> Sincerely,
> Ned Seith
> Nedtronics
> 59 3rd Street
> Gilroy, CA 95020
> (408) 842-0858
> G O T    G A R L I C ?
> GILROY GARLIC FESTIVAL 2001 !
>  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>
> At 12:10 PM 7/26/01 +0300, you wrote:
> >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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