terry wrote:
Understand that inside a typical computer switching power supply there
is a higher multi kilohertz voltage (350v RMS????) produced by the
switching action of the rectified 115 or 230 volt AC input?

This high freq AC through various usually toroidal step down
transforner windings is then rectified to provided the plus and minus
5, 12 volt outputs etc. Correct?

We recently modified such a power supply to get a single 12 volt DC
output at about 20 amps for a particular, amateur radio application.
In order to do so we had to load one of the 5 volt ouputs with a
couple of amps in order to get the unit to work; but that's normal.

Question: Is it possible to get at that higher AC voltage inside and
directly rectify it as a B+ supply for tube equipment? While also
possibly tiddling one or more of the 5 volt DC outputs closer to 6.3
volts for tube heaters?

Rectified 10 or 20 kilohertz wouldn't need much filtering compared to
60 or 120 DC ripple of a conventiaonl power supply? Recall building a
number of conventional 50 and 60 hertz power supplies many years ago
with heavy chokes and large capacitors. But now have a number of
slightly older ex computer power supplies of various wattages around.

Or is the idea completly off base?

The "higher voltage" is usually on the line side of the isolation
transformer. You would create a safety hazard by accessing that voltage.

A thought might be to use the non-rectified low voltage side to drive a
small high frequency step up transformer or use a diode voltage
multiplier circuit or a combination of transformer and voltage multiplier.

There is a company that builds high voltage switchers for amateur radio
amplifiers, and there have been construction articles to build these.
(Check QEX magazine).

Building from a surplus PC power supply would have a cost advantage.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

"RFI-EMI-GUY" wrote in message
g.com...
terry wrote:
Understand that inside a typical computer switching power supply there
is a higher multi kilohertz voltage (350v RMS????) produced by the
switching action of the rectified 115 or 230 volt AC input?

This high freq AC through various usually toroidal step down
transforner windings is then rectified to provided the plus and minus
5, 12 volt outputs etc. Correct?

We recently modified such a power supply to get a single 12 volt DC
output at about 20 amps for a particular, amateur radio application.
In order to do so we had to load one of the 5 volt ouputs with a
couple of amps in order to get the unit to work; but that's normal.

Question: Is it possible to get at that higher AC voltage inside and
directly rectify it as a B+ supply for tube equipment? While also
possibly tiddling one or more of the 5 volt DC outputs closer to 6.3
volts for tube heaters?

Rectified 10 or 20 kilohertz wouldn't need much filtering compared to
60 or 120 DC ripple of a conventiaonl power supply? Recall building a
number of conventional 50 and 60 hertz power supplies many years ago
with heavy chokes and large capacitors. But now have a number of
slightly older ex computer power supplies of various wattages around.

Or is the idea completly off base?

The "higher voltage" is usually on the line side of the isolation transformer.
You would create a safety hazard by accessing that voltage.

A thought might be to use the non-rectified low voltage side to drive a small
high frequency step up transformer or use a diode voltage multiplier circuit
or a combination of transformer and voltage multiplier.

There is a company that builds high voltage switchers for amateur radio
amplifiers, and there have been construction articles to build these. (Check
QEX magazine).

Building from a surplus PC power supply would have a cost advantage.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©




There's a schematic of a typical (although a bit dated) ATX power supply that
shows the concept of a PC power supply design at
http://www.pavouk.org/hw/en_atxps.html. Current models might use different
components but the basic design will be the same.
Here, you can easily see the building blocks of the supply and how the various
voltages are generated.
There's also a link on that page (http://www.webx.dk/oz2cpu/radios/psu-pc1.htm)
that demonstrates a way to modify a power supply for high current +13.8V output.

Modification for HV output would obviously involve rewinding the switching
transformer to give it a HV winding.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.

RFI-EMI-GUY wrote:

There is a company that builds high voltage switchers for amateur radio
amplifiers, and there have been construction articles to build these.
(Check QEX magazine).

Regrettably the author of the key article, and owner of the company
"Watts Unlimited", died a few years ago.

The commercial unit followed some years after the QEX article. It
contained a number of important improvements but schematics were never
published. The big unanswered question is why that design had no voltage
feedback, and therefore had quite poor output regulation? (We can only
speculate that a feedback loop must surely have been tried, but proved
unworkable... and then wonder why that should be?)

An example application for the commercial unit:
http://www.gare.co.uk/k5and/8877.htm

Building from a surplus PC power supply would have a cost advantage.

These days, tube RF amplifiers only begin to show a clear advantage over
solid-state at 500-1000W RF output, ie 1-2kW DC input. That is way
beyond the capabilities of PC power supplies.

A small-scale project based on a PC supply would be a good way to begin
to understand the problems, at relatively little cost, but it would be
another big step to build (or modify) a much larger SMPS for a serious
tube amplifier.


--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
RFI-EMI-GUY wrote:

There is a company that builds high voltage switchers for amateur
radio amplifiers, and there have been construction articles to build
these. (Check QEX magazine).

Regrettably the author of the key article, and owner of the company
"Watts Unlimited", died a few years ago.

The commercial unit followed some years after the QEX article. It
contained a number of important improvements but schematics were never
published. The big unanswered question is why that design had no voltage
feedback, and therefore had quite poor output regulation? (We can only
speculate that a feedback loop must surely have been tried, but proved
unworkable... and then wonder why that should be?)
(snip)

I worked on a 13.8 Volt SMPS made by TODD for Motorola as an OEM without
any documentation. There was a large stud mounted 13.8 Volt zener on the
output to clamp the voltage and some op amp circuitry to sense if the
output voltage fell below 13.8V it would start increasing the PWM to
charge the filter caps. This PS apparently never worked right because
the transformer leads were never soldered and then were plastered in
conformal coating.


--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

The "higher voltage" is usually on the line side of the isolation
transformer. You would create a safety hazard by accessing that voltage.

Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
,

Pshaw

I used to make a living fixing TV sets that ran off of the mains wit
no isolation. Needless to say there were millions of these TV's back
in the day, The trick was to use half wave rectification.
You can do the same thing with a PC supply. Instead of using an AC
input you would half wave rectify the AC and and a suitible filter
capacitor to make up for the frequency change. Also you has to use the
neutral as ground, and use a polarized plug in the USA, Now it's true
if the wall outlet got wired the wrong way you could be in for a treat
of epic proportions. So a GFI would be in there somewheres to protect
you. Then your HV DC ground will be at neutral. At which point you
could use a Voltage multiplier to get close to any whole number
multiple of the mains voltage.

Now it's true you you may not get away with this trick on 3 phase or
split phase 220 mains, because then where is the neutral?


73 OM

n8zu