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S-100 Power Supply Design Question

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    S-100 Power Supply Design Question

    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.


    Yes, your plan should work. I have this arrangement for my open frame S-100 system. In the (low res pictures) the switching power supply on the top is (+16V), the middle one is (-16V) and the bottom one is (+8V). For the +/- 16V supplies the green GND wire is on the -V for the top supply and on the +V for the middle supply. This seems to work fine for me.

    >>> Charles




      If you don't need a ton of +/-16V (and most systems don't) you can use a single Mean-Well RD-3513 and turn the voltage up to +/-15V (it's nominally a +/-13.5V supply). That's what I typically use, saves a little space and cost.
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        You should be fine--note that the S100 spec of +8V is only approximate and is done to provide sufficient line supply headroom for the pretty-much-universal linear on-board 5V regulators. That +8 is assumed to be nominal and unregulated.

        If the line (as in line-versus-load) supply is too high, the on-board regulators will get very hot, as they just dissipate the excess voltage as heat as pure IČR losses. If the line voltage is too low, you sacrifice regulation and have to confront the possibility that the load side will sag below 5V.

        For nostalgia's sake, I like the old linear "boat anchor" supplies. You may want to consider building your own, as for S100, they're rarely more than a big transformer, rectifier and capacitor setup. Integrand used to advertise with their S100 boxes that they wound their own transformers.

        Old 6.3V filament transformers should still be pretty plentiful. With a Schottky bridge rectifier and generous capacitors, the voltage should be right around 8V.
        Reach me: vcfblackhole _at_ protonmail dot com.


          If you're interested in building a boat anchor linear S100 supply, it can be done with a single 12.6V CT transformer which is pretty standard. Back in the 70's, I built one from the following circuit and it was still working up to a year or so ago when I finally sold the system.

          Note that this is for older S100 systems and 15+ Amps at +8V plus 3A at +/-16V. If you're using just a few of the newer cards with linear regulators on them, the component sizes (and costs) could be reduced.



            You could also go the "really DIY" route and take an old microwave oven transformer and rewind the secondary. Beefy.

            YT video

            Reminds me of my old ham days when you'd rewind old TV power transformers (and they could get pretty darned big back then).
            Reach me: vcfblackhole _at_ protonmail dot com.


              Originally posted by clh333 View Post
              One Meanwell 7.5VDC 10A will provide 8 Volts
              Is this a good idea? Are there any caveats about this plan? Is there a better solution?
              Leave it at 7.5
              The voltage difference is being dissipated as heat in the linear regulators so they will just run cooler.


              so 5 +1.8 = 6.8 minimum


                Originally posted by Dawsoca View Post
                Yes, your plan should work. I have this arrangement for my open frame S-100 system. In the (low res pictures) the switching power supply on the top is (+16V), the middle one is (-16V) and the bottom one is (+8V). For the +/- 16V supplies the green GND wire is on the -V for the top supply and on the +V for the middle supply. This seems to work fine for me.

                >>> Charles


                I use the same setup, and the supplies are slightly tweaked to get closer to the expected voltages.

                @Dawsoca -- I really like the cage you built for your setup. Is there a CAD drawing of it anywhere? I'm not using the same motherboard (which looks like the S100 Computers 8-slot), but I do have one of those handy. How about the card guides? 3D printed? It's a very awesome look. Mine is just plywood (kind of hacked together) so I wouldn't mind re-doing it.

                Web site:
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                  Originally posted by Al Kossow View Post
                  Leave it at 7.5
                  The voltage difference is being dissipated as heat in the linear regulators so they will just run cooler.


                  so 5 +1.8 = 6.8 minimum
                  Yes, I agree with this. With the original analog supply (transformer, rectifier,filter capacitor) the S-100 on board regulators (like 7805) the input voltage needs to be a little higher than 7.5V (average) because of the supply ripple and not wanting the troughs to drop below 7.5V. But with a ripple free output from the SMPS, you can run it right on 7.5V which satisfies regulators like the 7805 and it keeps the heat dissipation in those regulators as low as possible.Generally for the vintage 7805, the minimum input voltage is 7.5V, but I have found many keep regulating a little below this value.


                    Hello Rich, yes I have a print for the open frame S-100 project (enclosed). It is based on the
                    older version of the 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

                    >>> Charles






                      Thank you all for your responses and suggestions; as you may surmise I've spent my life a long way from the bare metal. I undertook the project for the sake of learning and it has already met that objective.

                      I first considered the linear supply - I have already built another smaller one for testing purposes - but with the amperages I thought were required I wondered if the weight of the components would be prohibitive. I may have to move the thing some day.

                      The backplane has three regulators, each a TO-220. There are holes in the motherboard for mounting the tabs but I was wondering if it would be smarter to stand them up and attach a heat sink instead. There isn't a lot of real estate available, and they might interfere with the card edges, but heating the motherboard didn't seem like a great option, either. I will definitely follow Al Kossow's suggestion and leave the supply voltages lower.

                      After consulting with glitch and Herb Johnson I decided to build my own card cage; wood frame with aluminum channels inserted. But I'll check out the Mouser guides first, thanks to Dawsoca for pointing them out.

                      The op amp finally arrived from China and the Meanwell supplies are finally off Jameco backorder so things are moving again. Thanks again for the guidance; I'm sure I will have more questions as I go forward.



                        I'm curious, what is the magnetic shunt for in the original transformer?


                          Are you talking about the shunts in the MOT? There's a simple answer to that--and it's economics.

                          MOT's are designed to use the least iron and copper possible, so they always operate under full load, which limits the amount of flux in the core. The shunt is there to protect the magnetron from drawing too much current, by providing an alternate path for excess flux to dissipate. Consider the size of an ordinary transmitter plate transformer designed for 2KW applications by comparison.

                          Under no- or light-load conditions, an MOT runs very hot, as the core will tend to saturate. If you repurpose an MOT for low-or variable-load, you either have to reduce the voltage in the primary or add more turns to it.

                          That's one of the reasons that you see MOTs either used for high-current applications, like spot-welding or you'll see them used in series pairs, with each transformer taking only half the line voltage.

                          Very old MOTs didn't do this and were designed along traditional lines. I've got one from the early 70s and it's quite a bit larger than later ones.

                          I hope that's what you were talking about---if not, Emily Litella.
                          Reach me: vcfblackhole _at_ protonmail dot com.


                            Thanks Chuck
                            That makes some sense. I suspect it increased the inductive phase shift a lot though. I like the idea of re-purposing a microwave oven transformer. It is about the right size ( maybe a little larger ) for a S100 system. It is not really hard to make any typical voltage needed.
                            One does need to determine the average voltage needed, based on the current load, capacitance, rectifiers and peak voltage. Or, just make a guess and a little experimenting.


                              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.
                              Reach me: vcfblackhole _at_ protonmail dot com.