ATX PS turns off when blue(-12V) and red(+5) get loaded

Hi there,
I am using an extra ATX power supply to power some surveillance cameras that need about 17V to run.

My problem is that the power supply turns off when I try to power the cameras.

I created two rails to power the cameras, one connected to the ATX blue wire (-12V) and the other to red (+5V). A multimeter shows 16V between the two rails -- good.

I also tied the green wire to ground so that the power supply will turn on.

As soon as I connect even one camera, though, the fan stops on the power supply, and I get no more voltage.

I tried connecting a smaller "wall-wart" 17VDC power supply to my rails, and that works fine.

So why is my ATX power supply turning off??

-Andrew