How can I get 17V from old PC PSU?
I have +12V and +5V output, can I get them to one +17V output?
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Many PC power supplies have both 12 V and -12 V outputs. It is also true that most power supplies will share a common ground, and if yours is such a PSU then it is not possible to add together two positive outputs (+5 V & +15V). It should be possible to add together a positive and a negative output however, in fact they are already connected via the ground.
To find out if this works for you try measuring the voltage between the -12 V wire and the +5 V wire. With luck it will measure approx. 17 V. In fact you can use your meter to measure the voltage between any negative output and any positive output to find one that works for you.
The +12V and +5V outputs are from the same power supply, so no. Both voltages were generated referencing the same ground.
With two power supplies, if the outputs are galvanically isolated from the inputs, it can be possible to connect them in series to stack a 12V from one and a 5V from a second.
WARNING! This is very important! Do not do this if you cannot guarantee that the DC side is isolated from the AC side! This is because the two supplies still share the same ground, which is from the AC input. That means the current can flow into one of the inputs, possibly damaging the supply and things around it! @russ_hensel has some experience with this problem (https://electronics.stackexchange.com/a/7659/6331). Could you elaborate russ_hensel?
This would be done by connecting the ground of one supply to the voltage of the other supply, i.e. connecting the supplies series. One example is connecting the ground of the second to the 5V of the first. Then voltages using the ground of the first and the 12V of the second would produce 17V. 5V+12V=17V.