IMSAI to SMPS

[rthorntn]

Hi All,

I purchased a USA IMSAI with the old 110v only transformer and so I would like to replace it with switch mode power supplies that handle 230v but how to get 18v, -18v and 8v?

For anyone with S100 knowledge, is there a rule of thumb about the percentages, for example I'm guessing a large percentage of boards use predominantly 5v, so if a board uses say 17w perhaps 5v (8v) uses like 3A and 12/-12v (18/-18v) uses 0.1A each, like 80-90% of the power needed is 5v, so the other voltages can be much lower amps, so maybe I would go AC to 9v and then boost the 9v to 18v (how to get -18v though), or would I just buy 3 seperate PSUs or maybe 2 (one PSU that could do both 18/-18v)?

I'm building up the IMSAI 8080, initially I will probably use a JAIR 8080 with the front panel before trying to get the CCS Z80 and other boards working.

Thanks for looking.

Richard

[Kean]

There are certainly plenty of triple output SMPS available, but possibly not one with your particular voltages.
Outputs of +5 & +/-12 or +5 & +/-15 are common enough.  e.g. at 200W there is the Meanwell MPT-200B and MPT-200C (medial versions, but cheaper ones would be available)

Not sure if there is one that is pre-configured for +8 & +/-18 or adjustable in that range.
Maybe start with a look around https://www.meanwell-power-supplies.com.au/multi-rail-enclosed/triple-rail for some potential candidates and check the datasheets for voltage options.

Otherwise, yes you could use three separate SMPS and just common the 0V rail as required, and those are likely to be easier and cheaper to source.

But if I was doing this I'd be wanting to keep as original as possible by keeping the old linear supplies and just sourcing a replacement transformer.  Jaycar/Altronics likely have something with suitable windings as similar voltage windings are often used in amplifiers.  Or there are several companies in Australia than can custom wind a transformer for you.

[MLXXXp]

Note that voltages on an S100 bus are assumed to be unregulated. Each board provides it's own regulation. If you're going to provide regulated voltages, they only have to be above the dropout voltage of the on board regulators, which would be about 2V for regulators of that era. To be safe, I'd add another half volt, so you need (minimum) 7.5V and +/- 14.5V. You can go higher but the regulators used tended to be linear, so the higher the input voltage, the more excess heat has to be dissipated by the regulators.

As far as current goes, you are correct that 5V used the majority of the current. +/-12V was normally only used for RS232 drivers and some other chips such as EPROMS or the 8080 (which needed +5V, +12V and -5V). However, some systems also had 12V fans, which would require more 12V current (I don't know about the IMSAI). Disk drives also tended to use a fair amount of 12V current but commonly had their own separate supply.

[rthorntn]

Jaycar/Altronics likely have something with suitable windings as similar voltage windings are often used in amplifiers.  Or there are several companies in Australia than can custom wind a transformer for you.

Thanks Kean, could you please name some companies, any ideas on cost, it's a pretty hefty transformer I think its 28amp and it weighs nearly 10kg?

[Kean]

Jaycar/Altronics likely have something with suitable windings as similar voltage windings are often used in amplifiers.  Or there are several companies in Australia than can custom wind a transformer for you.

Thanks Kean, could you please name some companies, any ideas on cost, it's a pretty hefty transformer I think its 28amp and it weighs nearly 10kg?

I'd try Peter at Harbuch.  They used to be just up the road from me, but moved out to Blacktown where rent is much cheaper.
Contact details on their website at http://harbuch.com.au/
No idea on costs, especially for such a large one.  He may have something in stock close to your needs, but also see http://harbuch.com.au/made_to_order_10.html

There is also Tortech https://www.tortech.com.au/ who sell step-down and isolation transformers (I have some).  I'm not sure if they wind custom ones.

[ebastler]

Do you think the old transformer is still OK? Could you test-drive the system via a step-down transformer, to get a better idea how much current your IMSAI actually draws? 28A sounds intimidating... 

[Kean]

28A sounds intimidating... 

At 8V though it is only 224W.  And that may be worst case with all slots full of RAM expansions.

[David Hess]

I purchased a USA IMSAI with the old 110v only transformer and so I would like to replace it with switch mode power supplies that handle 230v but how to get 18v, -18v and 8v?

How do you get down off of a elephant?  You don't; you get down off of a duck.

Bypass the linear regulators and use a switching power supply with +5, +12, and -12 volt outputs.  Or alternatively, bypass only the +5 volt linear regulator and use a switching power supply with +5, +15, and -15 volt outputs; 3 volts is plenty of headroom for the +12 and -12 volt regulators and if it is not, replace them with their modern low dropout alternatives.

If you are determined to find a triple output switching regulator with +8, +18, and -18 volt outputs, then it is likely that a +5, +12, -12 volt unit can be adjusted to +8, +19.2, -19.2 or close to it.

[james_s]

Given the historical significance and collector value of the machine, I would keep it completely original and run it off a stepdown transformer. I mean I assume this is not going to be a 'daily driver', but rather you'll fire it up to play with it now and then or show it off to guests.

If you insist on installing a modern SMPS, I'm betting you could get a 24V PSU and tweak it a bit to dial it down to 18V, or a 15V PSU and tweak it up to 18V.

[NiHaoMike]

How easy would it be to bypass the linear regulators such that the mod would be easily reversible with no sign that it has been modded at all?

[Kean]

How easy would it be to bypass the linear regulators such that the mod would be easily reversible with no sign that it has been modded at all?

Not hard to solder a wire jumper across a 7805 and keep it neat.
I'd be concerned about voltage drop across the S-100 bus running 5V instead of 8V, so keeping the point of load regulation may be wiser.  Depends a lot on the number and type of boards installed.
Also may be some side effects of using an SMPS on older hardware only having design considerations for use with linear PSU.

[james_s]

It could certainly be done, but why even mess around? These are rare collectible machines from the dawn of the personal computer era. A stepdown autotransformer is relatively cheap and readily available, I use autotransformers to run 240V gear I have.

[jmelson]

I ran an S100 system a long time ago.  After fooling around with oscillating regulators on memory boards, I bypassed all the 5V regulators and fed regulated +5 V from an SMPS to the motherboard.  Worked great!

Jon