Switching PS for S-100 Computer?

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Aug 23 22:12:05 2001

If you want a switchmode PSU you have to know what the load will be. That
includes the MINIMAL load, which must be within specified limits, else the unit
won't regulate. The problem with S-100 is that most of them have a PSU that's
capable of 100x the power needed. IF you design a switcher for 100x the power
needed, it won't regulate with the 1x load.

I have half a dozen S-100 systems, none of which need more than seven or eight
slots populated, and those only when I'm using the ultra fast SRAM boards. In
most cases, the load on the +8 is about 4 amps and the load on the bipolar 16 is
less than an amp. If you use a 22 slot backplane with the idea in mind that
you're going to use the 16 4K SRAM boards that justify such an arrangement, you
might need a BIG power supply. If you're going to use a 65K DRAM board, as most
S-100 systems use, you won't need a big supply. It's all in making up you mind.

If you want to use a switcher that you can afford, then you use a PC PSU at the
low end, and something intended for a UNIX server at the upper. It has the
necessary power and voltages, but requires that you modify each S-100 board by
bypassing its regulators. Is that what you want to do? Keep in mind that these
boards were designed to work with the on-board regulation.

Whereas a HUGE transformer from a distributor may cost $200 a switcher at 8
volts with enough current to cover the most heavily loaded S-100 will cost more
than that. You might get by with a heavy duty bipolar 15-volt switcher if it
has an output voltage adjustment. A transformer capable of doing the job will
cost $25 from a surplus vendor, though it may cost $30 to ship. Isn't that an
easier solution?

Check with www.mpja.com, www.mouser.com, and the like and see if you don't find
a suitable transformer. If you really can't find one, let me know and I'll send
you an S-100 PSU for $5 + freight. I'm sure I've got one or two to support
between 15 and 21 slots, though they've been on the shelf since the '70's.

Dick

----- Original Message -----
From: "Rich Beaudry" <r_beaudry_at_hotmail.com>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, August 23, 2001 1:34 PM
Subject: Re: Switching PS for S-100 Computer?


> >Date: Wed, 22 Aug 2001 19:44:49 -0400
> >From: "ajp166" <ajp166_at_bellatlantic.net>
> >Subject: Re: Switching PS for S-100 Computer?
>
> >From: Rich Beaudry <r_beaudry_at_hotmail.com>
> >>I would like to ask if any of you know of an existing, available,
> commercial
> >>switching power supply that could be used to power an S-100 computer?
> >
> >WHY???? The bus voltages for S100 are UNREGULATED.
>
> OK ... so how does this change things? If I have an unregulated 8 Volt
> power supply, and the regulators on the cards regulate that down to 5 V, how
> is that different from having a regulated SMPS at 8 volts, and still having
> the regulators drop it down to 5 Volts. Perhaps stupid, but is it
> technically possible, and will it work without harming the boards?
>
> I thought it would be easier to get a modern supply, so I wouldn't have to
> try to build one... Of course, now that I look at the prices for SMPS....
> YIKES!
>
> >> realize I could get a +5V and +/- 12V supply
> >> and just remove the regulation circuitry on the cards
>
> >Only if its an old LINEAR supply as SMPS designs mostly will not dot it.
>
> I'm not sure what you mean by that....
>
> >The S100 power supply is terminally simple. Three transformers
> >{8,16, 16 V AC}, three bridge rectifiers and three caps... thats all
> >folks.
>
> Yup, I know ... Try finding a new, "available from a distributor"
> transformer these days that will handle these voltages and currents, and I
> will quietly slink off... That's the reason I'm looking for alternatives. I
> can't find a transformer that will give me the voltages and currents I need.
> Links to these distributors are welcomed...
>
> Rich B.
>
>
Received on Thu Aug 23 2001 - 22:12:05 BST