Re: Mariss' power supply circuit

Les,

I appreciate you taking the time to understand the circuit. The PWM
modulation is actually a linear function of the over-voltage seen by
the supply. The amplitude of the 120 Hz sawtooth (R2,C1) is set to
about 2V peak-to-peak, this being the "regulation" range of the
circuit. The "trough" of the full-wave rectified reference clamps the
negative excursion of the sawtooth to the difference voltage.

The IRF540 has an Rds of about .07 ohms. It always operates as a
switch, the current being limited by the 10 to 20W shunt resistor.
The resistor must be wire-wound because wire-wound resistors have a
superb overload capability, typically 10 times their rated wattage
for 5 seconds, 50 times rated for 200mS.

In an undervoltage situation the circuit would attempt to adjust
itself to the new, lower voltage. It would be abley assisted by the
load in this endevor.

Like all silicon, MOSFETs invariably fail shorted if abused. The
consequence would be the shunt resistor would rapidly overheat and
fuse open, clearing the circuit. Everything else would fail
harmlessly and would leave the basic supply uneffected.

Another thing about the circuit is the component value choice for the
+12VDC supply. The size of C3 is chosen so that the circuit remains
functional for .5 second after AC power removal to fully discharge
the supply.

Thanks again for your comments; my pleasure is designing minimalist
circuits the way others might enjoy working crossword puzzles.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "Les Watts" <leswatts@r...> wrote:
> I finally found the .pdf file and had a look.
> As I understand it the circuit will perform shunt regulation
> using PWM at 120 hz. Duty cycle will vary from 0-100% over a
> couple volts diff between the filter cap and transformer secondary
> peak. I think it will modulate with a raised cosine law. Right?
>
> Question: what is the on resistance of the Fet you selected? Just
> want to calculate heat sink thermal resistance. Also, what happens
> in a line power undervoltage scenario?
>
> And as a little FMEA thing, what would be the most likely part to
fail?
> Would it be the fet if say the primary mains went very high? Would
it
> fail open or shorted? (I currently have a floating neutral argument
with
> my Georgia Power supplier and their electrical feed)
>
> I want to publicly thank you for publishing this as open source.
> I appreciate your weekend efforts.
> These things cost about $300 from other suppliers.
>
> Les
>
> Leslie Watts
> L M Watts Furniture
> Tiger, Georgia USA
> http://www.rabun.net/~leswatts/wattsfurniturewp.html
> engineering page:
> http://www.rabun.net/~leswatts/shop.html
> ----- Original Message -----
> From: "mariss92705" <mariss92705@y...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Sunday, June 16, 2002 3:54 PM
> Subject: [CAD_CAM_EDM_DRO] Re: Power Supply Circuit
>
>
> > Hi,
> >
> > OK, here's my Saturday's project. I posted a "Power supply
> > circuit.pdf" in the files section of this list.
> >
> > It wound up being different than what I started with. Sensing
current
> > was too crude. Instead this circuit compares the power supply
voltage
> > against the xformer secondary voltage. It pulse width modulates
the
> > dump resistor and has a very nice proportional response. The PWM
goes
> > from 0% to 100% as the power supply voltage climbs to 2V above
the
> > nominal voltage. It has been tested and works very well.
> >
> > Mariss