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I've an ATX PSU that I've modified to run without a motherboard, to power the many (6 or so) low amperage 12v and 5v devices at my workspace, to reduce # of power adapters plugged in and to provide dedicated toggle switches for some devices.

I've also got a 9v device, so I was wondering if it's safe to create a 9v line for that device by using a 12v line as the positive, and a 3.3v line as the negative.

My guess is that the answer is "no" because the PSU is not designed to have electrons flowing out of 3.3v into 12v, but perhaps I am wrong!

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  • \$\begingroup\$ I wouldn't do it on any equipment that I considered important, but it wouldn't hurt to try. I suspect that the only guys who would really be able to answer adequately are the designers of your PSU. Depending on your power requirements, it might just be easier to grab a $10 DC/DC buck from amazon and dial it back to 9V. \$\endgroup\$
    – slightlynybbled
    Commented Apr 22, 2016 at 13:57

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It depends on your 9V device and the load on the 3.3V line. You're essentially using the +3.3V to sink current from the 9V device. SMPS supplies can in fact sink current but only if they're designed to do that and plugging something into the output and hoping for the best will usually end up in tears.

It may be OK if you have significant load on the +3.3V line exceeding the 9V device current. In this case the +3.3V regulator will simply supply less current and all is good. If this is not the case, +3.3V voltage will start to rise and trouble ensues.

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