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S-100 Power Supply Design Question

clh333

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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-
 

Dawsoca

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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

Image1.jpg

Image2.jpg
 

glitch

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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.
 

Chuck(G)

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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 I²R 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.
 

WSM

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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.

S100_Power.png
 

Chuck(G)

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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).
 

Al Kossow

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RichCini

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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

View attachment 65060

View attachment 65061

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.

Rich
 

Hugo Holden

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Leave it at 7.5
The voltage difference is being dissipated as heat in the linear regulators so they will just run cooler.

https://electronics.stackexchange.c...an-a-voltage-regulator-work-below-minimum-vin

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.
 

Dawsoca

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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

Pic1.jpg

View attachment S100_Draw.zip

Pic2.jpg

View attachment Relay_Delay_On.zip
 

clh333

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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 S100computers.com 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.

-CH-
 

Chuck(G)

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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.
 

Dwight Elvey

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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.
Dwight
 

Chuck(G)

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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.
 

DeltaDon

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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

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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
 

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Chuck(G)

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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

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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

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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
 
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