J. P. Gilliver (John) wrote:
In message , Paul
writes:
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The front button does not have "direct effect".
It's a [partially] software mediated button.
The switch on the back of the PSU, is a
direct effect switch that cuts the mains.

If it's there; not all power supplies _have_ the back panel rocker
switch. (Or have regulations changed and they have to nowadays?)
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the schematic. There's a 4.7K resistor to +5VSB
and there would be 1 milliamp sunk to ground on
the motherboard end of PS_ON# pin 14.
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Or slightly over 1 mA, of course. Isn't it funny that resistors (and
capacitors, and many inductors) are still made to the E12 series, which
dates from when they were sorted into bins as they came out of the
machine that made them, whereas they're usually precision-made to 2% or
even 1% these days!

It's a pullup resistor. Just an order of magnitude of
current flow is required in a discussion of such. You
could use a 4.7K 5% or a 5.1K 5% and nobody would know
the difference.

I'm feeling nostalgic now, for my TI Bible...

It probably had the equations spelled out for working
out what pullup to use :-) [It would involve a discussion
about wired-OR logic using open collector drive, N outputs
wired to M inputs, and what pullup resistor is required.]
In this case N=1 and M = approximately 1.

https://cdn.eeweb.com/articles/artic...web-dsl-01.jpg

And switching a 1mA current flow with a relay, may not
provide enough current to keep the relay contacts clean.
Relays have a maximum current, but they also have a
minimum current flow they can handle if you expect
the contacts to remain clean and trouble-free. Even
with a precious metal coating there will still be
a minimum current flow spec. I might go looking for
a reed relay to do it perhaps. Rather than the
contactor off an air conditioner. Contact bounce ?
Not a problem driving the front panel Power button.
Might need to think about the consequences of
driving pin 14 with such. This is why semiconductor
drive is "nicer". And in this case, you could use
an optoisolator to close the contacts if you wanted.

Paul