see below, plz.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, August 29, 2001 12:04 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.
See how far we've drifted from his problem, which is finding an inexpensive
(that was why he wanted to use a switcher) PSU for an S-100 system. Consider
that he freely admits to knowing relatively little about his requirement.
Consider also, that he freely admits knowing little about how to build such a
thing. Consider, also, that those who WANT to learn about how to build things
have done so by the time they're adults, though the process does go on. My take
is that he wants to USE the PSU for an S-100 system, not BUILD one. The fact
that he's not ruled it out is simply a product of the fact that he may not have
that option.
>
> 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
be closer to my own principal area of interest than Jeff Hellige's, then cost of
a transformer kit might not be too much. However, I know some of the list
readers are only casually interested in whether or not their computers work,
while others wouldn't even want one that doesn't work. I personally have a
position somewhere in between, yet still don't endeavor to fabricate components
that are (a) not exact replacements for the desired ones, and (b) not in any
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
enclosures, heatsinks, etc from solid blocks of metal, just to have them "just
so."
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
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
shortage of technical know-how as well as interest, (he's interested in the
computer, not the components) yet isn't interested in buying a switcher that
meets his requirements because of cost, probably isn't interested enough, or
probably willing, to design and build a transformer around available wire,
cores, etc even if they're presented in kit form.
>
> 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.
>
> > 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.
>
> Sure, if you can find a suitable one surplus...
>
There are lots of other options, yet, Tony, short of scratch-building a
transformer, or even building a kit.
>
> > 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
>
> 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,
and it's quite seldom that it's needed, the +/-16 are used for analog or, more
commonly, RS-232 supplies, normally bipolar 12. The 8 volt number is there
because a 5-volt regulator with a pass transistor needs that much to operate.
Using more volts dissipates more watts, however, and benefits nothing. However,
if one can get a transformer that has enough 13.6, center-tapped, it could very
well work out if one uses the newer low-dropout regulators, e.g. LM330T, even
with a pass member, if it's well filtered. There are lots of transformers out
there that nearly do the job, some of them would do quite well if the regulators
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.
Something like that would probably work out better for Jeff than having to wind
his own transformer, even if the transformer did the job correctly the first
time. Jeff hasn't considered all the available options yet, or at least hadn't
when the discussion began. From where I sit, however, building a transformer,
even from a kit, is a mite farther than Jeff's original post suggests he's
willing to go. One thing that would concern me is that he's looking at using a
very long backplane, suggesting he's going to populate it with 4K SRAM boards.
That will require lots of power. If, OTOH, he uses a 64K DRAM board, instead of
the 16 4K boards, he'll save 90% of the power. Of course, it's likely 10-12 of
the slots will be empty ...
>
> > than I am, but winding a transformer in order to build an S-100 supply ...
gee
> > ... I don't know ...
>
> I can assure you I wouldn't do it if I could find a suitable transformer
> or transformers in the catalogues. But I would do it if it was the only
> way to keep a machine running.
>
> -tony
>
>
Received on Wed Aug 29 2001 - 15:17:32 BST