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?

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

1 Answer

The PSU's output feedback is based on the +5V rail. You need a load connected always to that rail. I suggest you leave the +12V rail without load (if it works).

simulate this circuit – Schematic created using CircuitLab

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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
@VladimirCravero I would have said the same, but I've seen a TL494 based PSU whose maximum power (current) on the 12 V rail was directly dependent on the load of the 5 V rail. –  Cornelius May 25 '14 at 17:35
@VladimirCravero: If you don't have a resistor that can handle that much power, use a few larger ones in parallel, so that the total current remains unchanged. –  Ben Voigt May 25 '14 at 18:47
@BenVoigt well thanks, but I am not the OP. And I am lucky, I've got my own bench PSU ;) –  Vladimir Cravero May 25 '14 at 18:48
It works without a resistor on the 12V rail, just on the 5V one, and it doesn't get hot, so I feel like it's okay! Thank you all! –  Asa Graf May 26 '14 at 18:06