28 Aug
2001
28 Aug
'01
10:32 p.m.
Winding a transformer is not a way to save money if you value your time at
50-cents per hour. I offered to ship him a complete S-100 mainframe for $5 +
freight, and that didn't interest him, even as a parts supply. I do believe he
has specific needs that he's trying to meet, but I really doubt he's likely to
roll-his-own transformer, since, though they're expensive, the parts to build
them still cost more than a ready-built transformer from a surplus house.
Marlin P. Jones & associates, www.mpja.com, Mouser Electronics, www.mouser.com,
among others, certainly HAVE the things. Finding one that's "just right"
was
never easy. Nevertheless, for $50 or so a guy can have a pair of transformers,
e.g. 8 VAC @ 10 A and 34 V center-tapped, at, say, 2 amps tops, would certianly
be adequate for systems that don't try to use the old 4K or 8K SRAM boards. A
transformer capable of that, under load, which, BTW, I got from MITS when they
were liquidated, is about the size of a small grapefruit and weighs about 6#.
My last one is destined to be used as a replacement for one just like it in a
box I built in '82 or so.
With today's low-dropout regulators, e.g. LM330, it's perfectly reasonable to
use a 6.3 volt transformer, e.g. the filament heater for a 4-1000 (?)
transmitter tube (I think that's what my first one was) and there are plenty of
those around, still.
Now, I know you're a purist, and that you're much more oriented to such things
than I am, but winding a transformer in order to build an S-100 supply ... gee
... I don't know ...
Dick
----- Original Message -----
From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Tuesday, August 28, 2001 2:53 PM
Subject: Re: S-100 Power Supplies: thanks!
The main
problem in all of this was not ever the design. As Allison pointed
out, an unregulated supply is dead simple to design. Even looking at some
True, and there are plenty of S100 PSU diagrams available to give you ideas.
commercially designed/built S-100 supplies, you
can see it's not rocket
science. The problem nowadays is finding one part -- the transformer. Most
Have you considered winding your own transformer.
In the UK we can buy 'transformer kits'. Certainly ones up to 100VA are
easy to find, and I suspect larger ones exist as well. The kit contains a
bobbin with the primary windings pre-wound and tested (2 115V primary
windings, connect them in series for UK 230V mains, in parallel for USA
115V mains as usual), and a pile of laminations, clamps, mounting
brackets, etc.
You have to provide the wire to wind the secondaries, wind them, and put
the laminations together.
It works out more expensive than a ready-made transformer, but of course
you can wind it for any voltage you like.
100VA is a bit small for an S100 PSU (although maybe it would do for the
+8V only,and use a second transformer for the +/-16V lines), but as I
said you might be able to get larger kits.
According to my calculator, 6.3V * sqrt(2) = 8.9V or thereabouts. Since
the 8V line is unregulated anyway, I would think you could start with a
6.3V transformer. Can you really no longer buy valve heater transformers???
-tony