Peter Able wrote:
On 13/05/2021 22:39, T i m wrote:
On Thu, 13 May 2021 16:53:30 -0400, Paul
wrote:
Do you know what they mean by 'single rail'?

https://www.scan.co.uk/products/600w...-psu-black-psu



One +12v rail offering a lot of current as against two. That's how they
started out until someone decided that so much demand on +12v meant that
two +12v would be a better idea. Lower currents per rail. OK, but
loading the two rails correctly didn't work out for some systems. So
some have returned to a single, many ampere, rail.

I think that this really kicked off when all the PCIe connectors were
loaded onto the same +12v rail of a two-rail PSU.

PA


On ATX, they dabbled with a few ideas.

You could get some idea, from the length of some of the
ATX PSUs. Two dimensions are standard, the third dimension
(length) is variable.

A few supplies, really did have independent outputs. One
PSU was really long (would have bumped into the DVD drive),
and it had four transformers for 12V. There would be ATX12V1
and 12V2, and a couple rails for PCI Express connectors. The
hardest part of using those, was finding the wiring diagram,
so you could figure out how to load the thing (not overload
any one transformer.

But the idea of doing that, quickly died, and they don't tend
to do that any more.

Now, there's one huge transformer with 12V on it. What should
happen, is sections of the +12V loom, are protected by
current limiters. The main transformer might be 60A, but
there will be three current limiters installed, each connected
to the main output. The output wires then feed some part
of the system (like, just the motherboard). The limiter
might not be 20A, it might be a bit higher. It's there,
to ensure a 20A rated wire, doesn't have 60+ amps flowing
through it :-)

It's also possible they're selling some "let 'er fry" ones,
where there is the 60A transformer, but only the 60A limit
is checked, and maybe it can make some 12V wiring glow
with the heat. Generally, the documentation is negligent,
in that it does not give sufficient detail about the
sections of the loom and what limits are in place. The
marketing people want the message "it's got 60 amps, kids",
to get through, and any other level of detail would be
a distraction. Telling people there was a current limiter
for sound reasons, would only elicit a customer response
of "but I paid for 60 amps!" :-)

Since all the 12V current comes from the same transformer
winding, joining some 1x4 Molex to some other pins, should
not cause loop currents to flow. Loop currents would be
a consideration on the supply with the four transformers.

Paul