From: A.R. Duell <>
Date: Mon Jun 23 20:50:20 1997

> Tony,
> > > Center tapped ransformer is used. Both 17v ac at 2amp each on each
> > > outmost pins, center pin is the center tapped wire for transformer.

> I vaguely remembered had to run up the power card with dc
> transformer so I could puzzle out last one or two voltages on it.
> Had to do several steps to find all the voltages.
> From there, wired up a adapter which worked. Note, it is not 24vac
> on each line! it is 12v on each wires if you buy center tapped
> transformers you will see like that added up by outside two wires
> measured. It goes like that marked on the transformer and the
> transformer has three wires, of two wires same color and one is
> different color, then it's a center tapped.

I'm not sure I understand that at all. I assume you're saying that it's a
12-0-12 trasformer (or actually 17-0-17 - I am _sure_ 17V is mentioned
somewhere in the IBM manual).

Do you agree with the circuits I've just sent out? If not, could you
please tell me where I've gone wrong?
> Hello? Knock, knock...:) Anyway I did clearly remember that 17v ac

Yes, I'm here...

> on each wire (of two) and the center tap on one wire.
> The power card rectify and regulates all the voltages for the

Yep, I'll agree to that.

> computer. That sort of thing I did build several power supplies
> based on center tapped transformers, using center tap as common.
> Consider this: Take two aa battries and wire up in series, total
> voltage is 3v assuming they're fresh. But put a wire as common
> between two batteries and measure two wires in turn without swapping
> those probes! This will give you one positive 1.5v and negative
> 1.5v which that how center tapped transformer power supplies in this

Yes, that sounds 100% reasonable. I may be from Cambridge, but I do
understand _some_ electrical engineering :-) (Seriously, I had to explain
jsut that to a 3rd year student in electrical engineering the other
week..... There is something wrong with the world.)

After all 'There is no such thing as ground' What you take as a 0V
reference is entirely up to you.

> configuration worked. But part of the circuity requires components
> to rectify and smooth out the ripples caused by ac to dc
> rectification process then final voltage regulation to complete the
> inefficient ac to dc conversion process.

The inefficiency in a linear PSU is in the regulator (which wastes energy
as heat) and not (in general) in the rectifier or smoothing circuit. Of
course a high-frequency SMPS can get away with smaller smoothing
capacitors and a smaller (ferrite-cored) transfoemer.

> Efficient power supplies are switching type, little heat
> and low losses in conversion process plus very light weight.
> But they use same design in outputs (secondaries but for some who
> knows electronics, these uses high frequency or fast recovery diodes)

See my comment on that crazy Zenith PSU that I mentioned in an earlier

The gates in my computer are AND,OR and NOT, not Bill
Received on Mon Jun 23 1997 - 20:50:20 BST

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