Need 28VDC 4Amp PS...

Ethan Dicks ethan.dicks at
Wed Jan 23 16:08:08 CST 2008

On Wed, Jan 23, 2008 at 04:52:23PM -0500, Bill Sudbrink wrote:
> I'm fixing a PowerMac G4 for my Dad's wife.  The original PS is dead.
> It was a pretty standard ATX supply, the only difference is that it
> supplies 28VDC at 4 Amps on two extra pins.  Rather than spending a
> lot of money on a new supply (more than $250 from the few sources that
> will guarantee the PS) or a not insignificant amount on a used PS that
> could go at any time, I want to put in a new standard ATX supply and
> "tuck in" a small 28VDC PS to drive the two extra pins.  The supply
> is always on, so I can power it from a pass through on the ATX supply.
> Does anyone have (or know where to get) a small supply?


I don't know where you'd find a 28VDC supply without building one, but
since AFAIK, it's just for externally-powered FireWire devices, you might
be able to get along without it.  Alternately, if you find an old laptop
FireWire interface, I know I've seen them with external PSUs (I have one)
since there's no way to pull the required current through the PCMCIA
socket.  You could probably rig one up as an external supply to the G4
and run it off the same power strip as the CPU.

Have you tried running the box with a standard ATX supply?  One thing
to check, if you haven't, is the state of the -5V pin from the PSU.
On my B&W G3, it's grounded on the motherboard.  Given that there are
already plenty of grounds on the cable, I just poked the offending pin
out with a soldering iron.  I _think_ Apple may have come to their
senses after the G3, but it's something to be aware of.

Good luck,


Ethan Dicks, A-333-S     Current South Pole Weather at 23-Jan-2008 at 22:00 Z
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