You don't say why you need to do this, even lightly loading a supply is of no use as it won't show any hf ripple current's on what should be a DC supply line...
... as you start to draw current this ripple caused by the switch-mode action increases with the load..... as this increases the dc voltages begin to fall. This ripple can cause havoc with digital circuitry, if in doubt try by substitution. Failing capacitors is usually the cause of this or a underated power supply.
>how can i fire up an atx PS without using a motherboard? jumping 2 of
>the 20 pin contacts perhaps.
A motherboard is really about the cheapest way, if you just pick up a
used one. It acts both as connection for the power-pins and dummy load.
Go to any computer show or even computer repair place; and ask them for
an old model they're going to throw away. That gets *you* a free
motherboard to act as dummy-load; while *they* get out of paying recycle
fees. Sometimes you can get an entire computer for nothing, if you
promise never to bring it back.
A dummy test-load to see if the PSU runs at all, costs about $20.
bumfightsri@aol.com wrote:
> how can i fire up an atx PS without using a motherboard? jumping 2 of
> the 20 pin contacts perhaps.
>
If you want to provide a dummy load to the PSU, connect a couple
disk drives to the 1x4 Molex connectors. You don't need to connect
anything to the ribbon cable interface on the drive. (Use disposable
drives - I have a 4GB drive I can use for this. Don't use a good
drive!) The drive will still draw some +5V and some +12V. I cannot
say it is absolutely necessary to do it, but it will calm the people
who feel some load is necessary.
To fire up the supply, connect PS_ON# to an adjacent COM pin. PS_ON#
is a logic signal, and normally it floats high. Connecting to COM
pulls it to ground level, and makes the signal active. The power
supply will run, as long as PS_ON# is connect to COM. Without a
multimeter, you can use the fact that the fan spins, as a
sign it is not completely dead.
Note that, when PS_ON# is not connected to COM, the supply is
still working a bit. It is making +5VSB, as long as the switch
on the back is in the ON position, and the cord is plugged in.
So, one of your first tests, in fact, doesn't require firing up
the PSU. Plug it in, then turn on the switch on the back. Using
a multimeter, check to see if +5VSB and GND can deliver 5 volts.
If you don't find 5 volts on there, then there is no point in
wasting time connecting a paper clip to PS_ON# and COM. If
+5VSB has 5 volts on it, then it is at least worth trying
the connection from PS_ON# to COM, and measuring the other
outputs.
In other words, the ATX PSU consists of two parts. One part
makes +5VSB and some voltage to run the other part. Seeing
some +5VSB means there is a better chance the second part
of the supply can try to run.
For a sample ATX supply schematic, try this page. And remember
that there can be dangerous voltages in here (like on C5 and C6),
so use tube radio precautions if playing around.
atx power supply 
Linear Mode
