S-100 Power Supplies: thanks!

From: Richard Erlacher <edick_at_idcomm.com>
Date: Wed Aug 29 19:31:59 2001

comments inline:

Dick

----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, August 29, 2001 4:54 PM
Subject: Re: S-100 Power Supplies: thanks!


> > > > 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
> > +
> > >
> > > You've made this sort of comment before, and I still can't understand
it...
> > >
> > Well, take it in context. Go back and read the first post from Jeff
Hellige.
>
> Sure.
>
What I meant here (and it wasn't Jeff, but I said that already), was that the
original post contained distinct references to both a moderate aversion to
building what he could buy, and to spending too much money. That's what started
me down this road.
>
> It's reasonable for the original poster (whoever it was) to say that he
> doesn't want to spend the time winding a transformer and would rather buy
> a PSU.
>
> But that's not the same thing as never wanting to spend the time winding
> a transformer because you somehow put a financial value on the time spent
> on hobbies. It is that last part that I have problems accepting.
>
No, it's not, and I didn't mean that doing such a thing is not worth the effort
or investment. I simply meant that I didn't think it met the original poster's
model.
>
> [...]
>
> > > For many of us on this list, classic computers are a hobby. We work with
> > > them because we enjoy them. Not because we make money with them (at least
> > > not all the time).
> > >
> > Classic Computers as a hobby includes a vast array of things aside from
> > hand-building components. If one's hobby is fabricating components, which
may
>
> Of course...
>
> FWIW, I don't particularly enjoy winding transformers. I have done it. If
> I can buy a suitable transformer off the shelf, you can bet I'd do that.
>
> But in just about every hobby there are going to be aspects of it that
> you don't particularly enjoy, but which you have to do anyway. The
> enjoyment you get when the machine finally works, and the enjoyment you
> get from using it outweigh the bits you don't enjoy doing.
>
> [...]
>
> > sense "classic." I'd never use a switcher to power S-100 box, simply
because
> > they weren't part of that culture, but that's me. OTOH, I've machined
>
> Actually, that wouldn't bother me too much. Well, unless I was restoring
> a name-brand S100 system (whatever that really means) when I would want
> to keep it original. If I was making a new system now that happened to
> use some old S100 cards from the junk box, then I would have no problem
> using an SMPSU.
>
> The practical reason for not doing so is that 8V SMPSUs are not at all
> easy to find, as we've all heard by now.
>
> One thing I certainly wouldn't do is bypass the regulators on the cards
> and supply 5V (and +/-12V) to the bus. The reason is that if those cards
> ever ended up in a normal S100 backplane, you'd get a lot of magic smoke
> leaking out.
>
> > enclosures, heatsinks, etc from solid blocks of metal, just to have them
"just
> > so."
>
> As have I on occasions. I've certainly made a lot of
> spacers/screwposts/terminal pins, etc from metal rod. Because the exact
> part I want is not listed in any catalogue.
>
> >
> > That doesn't make it unreasonable to wind a transformer, but, I certainly
didn't
> > get the impression from Jeff's original post that he was wanting to do
anything
>
> OK. I was simply suggesting that _a_ way to get a transformer was to wind
> it yourself, and that if you use the kits it's not too hard. The primary
> windings are pre-wound, so you don't have any HV insulation problems to
> worry about.
>
> > like that. It's not an issue of cost, though that's probably a factor, but
I'd
> > submit that one who isn't interested in building his own switcher because of
a
>
> I would not recomend that anyone who can't design and build linear PSUs
> attempts to build a switcher from scratch. It's a lot harder.
>
> > > And IMHO it is wrong to value time spend on a hobby at anything other
> > > than $0 per hour. You spend that time because you enjoy it. Not to make
> > > money or save money. If you weren't working on classic computers, what
> > > else would you be doing?
> > >
> > It's like fishing ... it won't save money on dinner.
>
> Exactly!
>
> [...]
>
> > > Hnag on...
> > >
> > > Are the S-100 PSU voltages 8V DC and +/-16V DC, or are they what you get
> > > if you rectify 8V AC and 16V AC? I assumed the former, but perhaps I am
> > > wrong.
> > >
> > They are what they say, so your guess was correct. The 8-volt raw supply is
> > regulated on each of the boards to produce the logic supply and, where
needed,
>
> Yes, that's what all the S100 schematics I've seen show...
>
> [...]
>
> > were doctored. There's a 3-terminal replacement for the LM340T-5 that is
> > actually an encapsulated switcher and it would surprise me to learn that it
> > wouldn't save considerably on heat if the input voltage were too high as
> > compared to a linear. That's another option.
>
> Again, if I was making an S100 system, I'd want to be able to use any
> S100 cards without modification. Which would mean having the +8V line
> high enough that any 3 terminal linear regualtor would work. I'd not want
> to have to replace regulators on every card I wanted to try.
>
I agree. However, sometimes compromises are necessary. The SCHOTTKY rectifier
trip would probably meet the 8V spec if the full load voltage of the transformer
is 10% down from the no-load voltage, however, since the 6.3 * SQRT(2) - 0.5 =
8.3 or so at full load and full load is unlikely to occur. A lot depends on
what the numbers stamped on the transformer really mean.

There are, BTW, some pretty interesting things one could do to a PC PSU to
increase the output on the 5 volt supply. It's risky, of course, because the
capacitors are designed for a 5-volt supply, but, perhaps with an extra turn on
the secondary of the main transformer, and a tweak of the output voltage, one
could squeeze 8 volts out of the thing without running up to the limits. The
consequence would be a proportional increase on the complementary 12Volt
outputs. That's where you might wonder about the capacitor voltages, since it
will be as much as 20 volts. I've got some commercial linear S-100 supplies
that provide 21 volts on what should be +/-16 volts. That suggests it's
survivable at the S-100 end.
>
Received on Wed Aug 29 2001 - 19:31:59 BST

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