Problems with 12V rail on ATX Power supply conversion

I am trying to convert an ATX power supply to a lab bench supply following this Instructable, but with one change: Since I only had one red plug on hand, I connected the +5V and +12V rails to a SPDT switch with the common lead then connected to the load resistor and the (1.5A fused) output:

The problem I am getting is that when the 12V rail is selected, the PSU turns off and does not supply power, i.e., neither the 5V or the 12V rails are "hot" and the fan spins down. When 5V is selected, it works fine (and both the 5V and 12V rails are "hot.").

Why is this happening?

• Ideally, both 5v and 12v would have individual load resistors. – Passerby May 25 '14 at 15:12

• I agree but I'd go with a bigger resistor. That's 2.5W of pure electronic heat on that poor $R_1$ – Vladimir Cravero May 25 '14 at 17:32