You have to fake out (short??) at least one signal on the motherboard connector for the PSU to continue delivering power.
Whether draw on only one rail causes a problem will depend on the internal design. You could hook up a 9V-battery-powered low-voltage alarm to let you know the PSU died if that's a major concern. I'm assuming you aren't going to run out of spare AT PSUs anytime soon ;-).
-Adam

On February 15, 2016 3:39:02 PM CST, Trevor Cordes <trevor@tecnopolis.ca> wrote:
Is it possible to use a standard AT (old style) computer power supply just 
for its +5v out?  I have a project and I just need 5v with a beefy amount 
of amps, and I have lots of AT PS's around that would fit the bill, but 
I'm wondering if running a computer PS just for its 5v line might damage 
it somehow?  There would be no load on the 12v (and other) lines.  
Anything I should worry about?

Thanks!

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