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I am in the process of converting a uATX power supply into a lab workbench. I will add a 10W, 10ohm resistor as a load, but it works without a load( I don't know how long it would continue to work without a load). Anyway, it has been suggested from diy sites that I can get 3.3, 5, 12, and -12 Vdc coming off the rails. But if I wanted a 7 or 24 VDC source I could connect the positive to 12V and the negative to 5 volt. Or 12V postive -12V negative to get 24V. How is this possible and what are the current sourcing effects of doing this. In the power supply the rails at different voltages have different current sourcing abilities, so am I to assume I just take the lower of the two current ratings or would this combine them to the max the fuse/power circuitry allows. I'm very confused about non 0 V grounds, which means I don't understand OP amps either, or anything else that takes negative voltages. Can anybody help clarify with formulas(non calculus or set theory) or analogies how this is possible. I can probably assume this combining voltage mess only works with voltage sources that share a common ground? I loaded some power sources and a multimeter up in MultiSim and confimed what the diy sites were telling me, I just don't know if it is safe or reliable to do in real life, especially the current ratings.