S-100 Power Supply Design Question

[DeltaDon]

Pardon me for asking; I know little about the specifics of the S-100 standard but out of curiosity I have begun building a system using Monahan-design boards. I have the backplane finished except for an op-amp (which the USPS is sheltering in an undisclosed location) and have acquired a nice big case to house everything. Soon it will be time to provide power and that brings me to my question:

On the advice of the Google S100 group I will try to use switching power supplies instead of a linear supply, as was the custom back in the day. One Meanwell 7.5VDC 10A will provide 8 Volts (they assure me it can be "tweaked" up to 8V) and two Meanwell 15.5VDCs to provide plus and minus 16VDC. If I understand correctly they are yoked - + : - + to provide the 32V differential. (It's always possible I don't understand correctly, however.)

Is this a good idea? Are there any caveats about this plan? Is there a better solution?

Thanks for your suggestions.

-CH-

John Monahan often spec's using the Ezsbc or Pololu regulators which greatly reduce the heat generated on each board compared to the old 7805 and 7812 regulators.

[Hugo Holden]

Given that these things are designed with no-load core saturation, I've played with the notion of adding a control winding and a capacitor and turning one into a ferroresonant transformer.

But--so many projects, so little time.

There are some other types of transformers with another form of magnetic shunting. These are used in small neon signs with long thin tubes to provide 2 to 3kV. The transformer core wraps around to partially isolate the magnetic field of the primary from the secondary, shunting the lines of flux away from the secondary. The output waveform on the secondary becomes somewhat peaked. At turn on the voltage on the secondary presented to the tube is high enough to strike it, then when its impedance drops, the load on the secondary is better accommodated because, in essence, the regulation is degraded, so the assembly becomes similar in its function to a normal transformer with a series inductor or ballast on its output.

I have used these transformers for other applications to run CRT's in TV projects, by filing the small magnetic gap open to at least a 4 to 5mm gap so they behave like a normal transformer. Also I rewind the primary. The secondary on these are great because they are resin potted and there is a low chance of insulation failure

[Chuck(G)]

Most of the neon-sign transformers that I've run into are in the 10KV+ range for large signs.

Rewinding the primary? There must be easier ways to get a couple of KV!

[Hugo Holden]

Most of the neon-sign transformers that I've run into are in the 10KV+ range for large signs.

Rewinding the primary? There must be easier ways to get a couple of KV!

Yes, there are much easier ways to generate a couple of kV, but I needed a 230V primary (the original was 115v) and a winding for a heater on a tube rectifier, for this home built vintage TV project:

http://worldphaco.com/uploads/ARGUS.pdf

[Dwight Elvey]

Hugo, your post has made me smile. Knowledge of these various side things related to electronics has been a great boost to this MB. Rewinding a primary is not the same as rewinding a low voltage secondary.
Stay safe
Dwight

[RichCini]

[QUOTE=Dawsoca;646668]Hello Rich, yes I have a print for the open frame S-100 project (enclosed). It is based on the
older version of the S-100.com 8-slot motherboard. It should work for the 9-slot one with
adjustments. The card guides were purchased from Mouser (Bivar VG2-5). You don't need the
power-on delay relay setup but I like it as it prevents me from removing cards with power on and
it allows the supply to stabalize during turn-on (mainly for linear supplies-not so much for the
switchers).

>>> Charles


I got around to building this setup (converting from my CompuPro cage-on-plywood) and it's an excellent design that was pretty easy to build. My bottom frame was about 1/2" shorter so I had to adjust the offsets on each end to make up for it.

Overall, the diagram was very thorough and was easy to build from. Here are a few observations and suggestions:

* Multi-decimal measurements are very hard without a digital caliper since I didn't have a tenths-ruler. There was enough wiggle-room in the holes that any inaccuracy did not have much of an impact.

* Having the drill hole dimensions was helpful, but the intended screw size (4-40 or 5-M3) wasn't indicated.

* The mounting screw sizes on the bottom of the S-150 supply are 5-M3 and the sides are 3-M3. Since I didn't have metric screws, 4-40 worked, but when all you have is a hammer, everything looks like a nail as they say.

* My plastic board was shorter by about 1/2", so I mounted the power supply stack so that the sheet metal lined-up with the outside edge and I shifted the entire board stack to the left a little. I also moved the power supply stack further back to accommodate a small switch/LED panel I used on my other setup. If the plastic sheet has a paper protective layer, drawing this out is easy. Mine didn't have that (it's a satin black base). So, blue 3M painter's tape, a center punch, and a Sharpie worked well for this purpose.

* I would recommend mounting the middle power supply 1/2" forward of the current dimension so that you can wire-up the supplies when mounted to the metal (I built it as a sub-assembly). If they're mounted in-line, you can't get to the screws.

* I did not use fuses, although I drilled the holes for the holders. These supplies have short circuit protection which of course doesn't necessarily protect the traces. I've never had an issue, but I can add them later. I was looking into self-resetting fuses for it...the new revisions of the backplane have that, but the older ones have an error in which the PTCs protect the LEDs and not the slots. Those LEDs are expensive you know

* I'm a little concerned about the accumulation of static on the plastic; not sure how to deal with that, but I guess the cage of the power supply "connects" to the base through the mounting screws. I punched some card stock and put it under the backplane for good measure. I did not use copper tape or anything.

* For power input, I have an "input block" which has a switch, IEC connector, fuse and line filter in a single package. I mounted it in a piece of wood which I then mounted on the board behind and inset to the power supplies so that the 0.25" terminals clear the S-150 supply. I did not build the relay part of the setup.

All I'm waiting for at this point is a new spool of black 16AWG wire I ordered, and then I can finish wiring the supplies to the backplane.

Thanks again for a great design and sharing it with the list!

Rich

[Dawsoca]

Hello Rich, glad the S100 Open Frame drawing was useful. The multi-decimal measurements are a
product of AutoCAD as I don't think the version I use has fractional dimensions. Mostly using a
1:10 scale with 1 decimal accuracy is sufficient for assembly. The staggering of the power
supplies is a good idea as you are correct, it would make wiring easier. Fuses are a force of
habit, the supplies have auto short circuit protection so are not necessary. I haven't noticed
any static buildup on my setup as the plastic is grounded by the supply as you indicated. In any
event, thank you for the feedback it always helps to get another viewpoint to improve a design.

>>> Charles

[RichCini]

I really appreciate you putting the drawing together. It was very easy to work from, and I agree, if I had a 1:10 engineering ruler, it would have made it a tad easier. Really what's important is the measurements around the backplane. It was the design that I was looking for. If you saw the nonsense I did with pieces of wood with routed slits, you'd chuckle.

Have a great weekend Charles.

Rich

[RichCini]

S-100 Power Supply Design Question

Here are a few pictures from my setup (which, essentially, is a CompuPro ZPU setup). Works great.

Just curious if you considered cooling at all. In some of my previous plywood-based hacks, I had two 80mm fans on the left side of the cage (they're left over from old DEC MC200 serial concentrators). I'm a little leery about relying solely on convective cooling for basically 40-year-old boards (other than the mini front panel).

Thoughts?

Rich

[Dawsoca]

Hello Rich, I like the way your system turned out. The power supply stack worked out very well. As to heat concerns, I use my open
frame unit for de-bugging boards and times when I just want to tinker so it is not my main S100 system. If it was the main or only
S100 unit I had, I would consider cooling fans. As it is, most of the S100 cards I own have good heat regulator design S/T you can
hold your fingers on the regulator during operation without getting burned. For the few very high current video boards I have, I
started using CUI (V7805-2000R) switchers which generate no heat at all, so unless enclosed, heat build up from regulators has not
been a problem for me. In any case, cooler is always better so a fan would not be a bad idea at all.
Also, good job on the mini-IMSAI front panel card.

>>> Charles

[RichCini]

Thanks very much. It was a great project to do.

I have several S100 crates, but they all have something in them -- an 8800b, an IMSAI, a TEI 22-slot with a Seattle Gazelle and a TEI 12-slot with a "new" system based on cards from S100Computers. This one is, like yours, a "random testing" bed...except right now it's basically a fully functioning CompuPro ZPU (Cromemco CPU, CompuPro RAM17, CompuPro Disk 1, front panel, and CompuPro System Support Board) that runs CP/M 2.2 on 8" drives.

I have not actively replaced any of the regulators with switching equivalent. On "new" boards, I use 3A regulators from Pololu. I'll see if I can incorporate fans without making it look ugly.

Rich

[clh333]

Once again, thank you all for your input. I took on this project for its educational value; I have learned a lot already*. The backplane is constructed, the power supply passes initial tests, the chassis is being modified to accommodate the various elements and now I have come across another question I would like to ask:

The backplane and cards will be consumers of the output of the three Meanwell switching supplies, nominally +/- 16VDC and +8VDC. The chassis will also have a need for +12VDC for fans and for any (future) drives and +5VDC for drive logic. Would it be better (in the sense of advisability) to add a fourth power supply with these 5/12 voltages as standard output or to derive these voltages from the existing supplies? Either approach is feasible; I am just wondering which is more sensible.

Thank you for your responses.

-CH-

*
- You can substitute a Hakko heating element in your Zeny soldering station iron but the Hakko puts out significantly more heat. Optimum soldering temp is about 70 C. lower according to the display.
The Zeny's display will need recalibration but for now I just dial down the heat.
- The "power entry module" - Schaffner FN9260-10-06 - had, in addition to an L-C filter circuit, two 5x20mm fuses. Who knew? Not me, until I looked up the datasheet on line, but it simplified the design of the AC power control.
- The Meanwell RSP-75 lit right up when power was applied but the two LRS-150s did not, until I saw the note and found the internal switch and set them from 230 AC to 115.

[RichCini]

Good question. I think it would be too hard, and probably not too efficient, to further regulate these voltages down for use with a floppy drive. I happen to have a 45W switcher that was scrapped from some PC-like thing and it has the right voltages (and connector) for a floppy. I would consider finding a small dual-voltage switcher on the surplus market and use that. Of course you can use any XT/AT/ATX supply that you may have, but those are overkill for a single floppy and may not regulate properly if the load isn't great enough.

Rich

[clh333]

... I think it would be too hard, and probably not too efficient, to further regulate these voltages down for use with a floppy drive.
Rich

Thanks, Rich; makes sense to me. There is plenty of room in this chassis so one more switcher is pretty easily accomplished. And thanks, too, for maintaining classiccmp.org which has been very useful to me as well as many others, I'm sure.

-CH-

[RichCini]

Thanks, Rich; makes sense to me. There is plenty of room in this chassis so one more switcher is pretty easily accomplished. And thanks, too, for maintaining classiccmp.org which has been very useful to me as well as many others, I'm sure.

-CH-

Glad to help. I used to hunt for surplus switchers at BG Micro, MPJA, or All Electronics.