Hi Jinx,

I've posted a schematic at:

http://www.pacificsun.ca/john/www/pdf-files/UVPower.pdf

That has a switching regulator constructed with a 555 timer.  The stack
of zener diodes limits the maximum voltage.  I needed this high voltage
supply to create a bias voltage for a UV flame detector for my foundry
furnace.  That's why the high power transistor.  The math for
determining component values was in a circuit cellar inc article I
think.  I'll see if I can find it and post the pertinant info.

A switching supply will be far more efficent and use less components.
I'm not sure that the capacitor multiplier circuit would have any less
noise but perhaps some other expert can voice an opinion.

I'd still run the output into 7815 regulators or perhaps even two stage
it and first run LM317 devices at 18V and then run LM7815/ML7915  after
that to make the voltage more stable.  Obviously filtering is important.
You could use the PIC isntead of the 555 to create the PWM signal.

Regards,

John Dammeyer


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jinx
> Sent: Friday, April 05, 2002 7:04 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC[:Dual voltage rails from PIC
>
>
> I've been playing around with this circuit
>
> http://home.clear.net.nz/pages/joecolquitt/15vx2.html
>
> to get 2 x 15VDC supplies or 1 x +/-15VDC supply for
> op amps and sensors. I'm hoping someone can help
> me optimise it. The hurdle at the moment is the choice
> of capacitors
>
> There's calculation info here but I don't think it says, in
> so many words, what to use
>
http://home.earthlink.net/~jimlux/hv/cw1.htm

At present I've got 4u7 (top) and 1u (bottom), but that's purely
trial-and-error. 20kHz (a 50us TMR0 interrupt) seems about
right for those values. Anything higher makes no difference
and the voltage goes down with lower frequency

Output before the 317 is 14.4V with hardly any ripple. There's
dropout because of the 317, so the 15V is just an ideal target.
I'd rather not add another stage, but wonder if a small separate
multiplier to inject > 5V (rather than the available 5V Vcc) would
make any significant difference. The 1N914s could be replaced
by Schottkys to minimise diode losses.

In some applications higher DC voltages would be available
with the PIC creating the frequency via a transistor, although
with this circuit in particular 5V produces 15V yet 12V produces
only 26V

Any help appreciated, TIA

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