S-100 Power Supplies: thanks!

From: Richard Erlacher <edick_at_idcomm.com>
Date: Tue Aug 28 22:32:12 2001

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 _at_ 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_at_p850ug1.demon.co.uk>
To: <classiccmp_at_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
>
>
Received on Tue Aug 28 2001 - 22:32:12 BST

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