More on Fav Designs - switchmode power supplies

[Paul Schreiber]

Well, I think this is not so great.

1)PC type supplies are designed for a heavy loading. If fact, most "flyback topology" switchers suffer from the fact they need a minimum load to not have the outputs over-voltage. In a PC, this can be as much as 500ma. As I said before, a typical syth doesn't need much CURRENT: it needs low noise, no ripple voltage. The terms "switcher" and "low noise" are seldom used in the same way (yes, there are ways to get this, but a LM723 is easier).

2) For fooling around, look in the Newark or Allied catalog. Here some suggestions:

Newark (800-298-3133 ext. 48)

87F2913   ELPAC model WM113-TT  +5 at 860ma +-12 at 300ma   about $60
44F3198   ELPAC model WM113 (same as above, wall wart style)  about $45

for European voltages

50F3431  ELPAC model WM220-1 +5 at 2A, +-12 at 500ma  about $98

These are very nice, fully enclosed supplies with nice, LONG, cords. I buy 50 at a time!!

Allied (800-433-5700)

921-9410  SOLA model SLT-12-20404-12 "open frame" style 5 at 2A, +-12 @400ma  about $60
(the exact same supply is also a 218-3015 Power-One model HTAA-16W-A, same price)

218-3016  Power-One model HBAA-40W-A "open frame" 5 at 3A, +-12 @1A  about $78

Both of these are also 110V/220V switchable.

Allied also has many more to choose from (about 15 pages in the catalog!)

Resist the $20-30 temptation to grab a PC supply. It's not going to work.

3) For serious lab/bench work, you need a triple output variable supply. I simply love my Leader Model LPS-152. It is a clone of a proven HP supply, but for 1/3rd the price. A lower wattage version is also available (LPS151). I paid around $300 for mine. I've had it 7 years, and I expect it to last 15-20 more. It's built like a tank! $20/year for a lab quality supply is worth it!

I have ask for permission to scan Walt Jung's "super linear" design (a discrete LM723). If you have access, it was in EDN Magazine Jan. 2.

Paul Schreiber
Synthesis Technology


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From: 	Doug Forbes[SMTP:av599 at lafn.org]
Sent: 	Saturday, April 19, 1997 3:21 PM
To: 	synth-diy at horus.sara.nl
Subject: 	More on Fav Designs - switchmode power supplies

>> [Dave Halliday]  I think that the question is more one of using
>> computer surplus supplies and these have been designed without analog
>> considerations.  With PC Clone supplies selling for $20 new, it would
>> be tempting to be able to re-work one of them to give +/- 15 volts at
>> a couple of amps - maybe a good design project...
>
>Is this feasible at all?  I'm philosophically very
>much in favor of gleaning what ever we can from the mass
>manufacturing machine; let it work to *our* advantage once
>in a while.  Since they're cranking out enough of these
>things to make them dirt cheap, it'd be excellent if they
>could be adapted to our purposes; even if that adaptation
>is a little convoluted (I think I have a couple of blown
>PC PSs in my basement right now).  Or would one still
>spend more in the process than on a from-scratch approach.
>
>steve - the scavenger!

All you have to do is find the +- 5 volt supply windings on the
transformer. Usually it's only a couple of turns. Rewind it for whatever
voltage you want and change the filter caps' for ones with the new working
voltage. The new windings can be snaked through the ferrite core without
much trouble. The supply remains regulated because of the feedback winding
being independant of the +- 5 and +- 12 volt windings.

Hope this helps some.

Doug