ATX Power supplies

[Bob Drzyzgula]

> You might notice that our subsequent clusters are space-wasting boxes
on
> shelves. The power supplies were never the problem, it was the
physical
> mounting that wasn't robust or easy.

I often wish I'd paid more attention in high school metal shop...

> I don't think it requires -5V, but it does add 3.3V to the list. The
other
> voltages are required for ISA bus compatibility, although -12V is used
only
> for serial ports.

The ATX spec does call for -5V, on pin 18. I can certainly imagine that,
as a practical matter, one would find that motherboards never use it.

> The standby current isn't well specified. That's why it's a
unpredictable
> if wake-on-LAN will work on a randomly configured system.

The manual for the Intel board I mentioned had a spec of a little over
700mA. But I can imagine that this would be highly dependant on the
board.

> You can jumper the standby voltage to the on switch line. Two of our
> motherboard types had undocumented headers to do just that. I found
the
> first header while looking for the easiest place to solder a jumper
wire.
> Without the soft-on bypass these motherboards would leave the machine
> powered down after a power failures.

The original Sun UltraAX didn't support the ATX soft power on/off, so
one had to buy an ATX power supply with an actual power switch on it.
Sparkle was the recommended vendor, and I have a couple of cases of
those. Perhaps feeding the standby off the real +5V on power-up.

> In theory you can share a ATX single power this way, but I haven't
tried it
> myself. Mostly because of the lack of connectors. Wiring up our first
> cluster was painful and expensive, since contacts and connector shells
that
> cost pennies in Pacific rim countries were expensive and had long lead

> times here.

Molex Part # 39-01-2200, AMP part # 2-106527-0. The Molex part is about
$1 each in moderate quantities from Allied/Avnet; Digi-key and Newark
also carry Molex and AMP. Female contacts, Molex part # 39-00-0039 in
brass for 24-18AWG wire, run about $20 per 100. Molex Crimp tool for
24-18AWG, part # 11-01-0197, goes for about $335 (acutally low for
ratcheted crimp tools...).

Jameco sells an unbranded equivalent part for around $0.35 each; I don't
know if the Molex tool will work with their contacts, however.

Painful I will agree with... crimping==cramping, after a while. This is
one reason I was interested in SBCs... generally they have all the
DC-to-DC converters they need onboard, and can simply run off of +5V.
For the +/-12V on the RS-232, they generally just use tranceivers with
charge pumps.

--Bob