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!

see below, plz. Dick ----- Original Message ----- From: "Tony Duell" <ard(a)p850ug1.demon.co.uk> To: <classiccmp(a)classiccmp.org> Sent: Wednesday, August 29, 2001 12:04 PM Subject: Re: S-100 Power Supplies: thanks! + 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. 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. It's like fishing ... it won't save money on dinner. he to build There are lots of other options, yet, Tony, short of scratch-building a transformer, or even building a kit. www.mouser.com, was transformers, certianly 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 ... gee

Yes, ... it's hell getting old ... Unfortunately, I figured that out AFTER sending the post with the erroneous reference. Dick ----- Original Message ----- From: "Jeff Hellige" <jhellige(a)earthlink.net> To: <classiccmp(a)classiccmp.org> Sent: Wednesday, August 29, 2001 3:17 PM Subject: Re: S-100 Power Supplies: thanks!

comments inline: Dick ----- Original Message ----- From: "Tony Duell" <ard(a)p850ug1.demon.co.ukTo: <classiccmp(a)classiccmp.orgSent: Wednesday, August 29, 2001 4:54 PM Subject: Re: S-100 Power Supplies: thanks! at $5 it... Hellige. 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. 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. may because "just didn't anything I'd a needed, 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.