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I'm constructing a custom(cheap) power supply, with digital current meters and such. I'm using 2 ATX power supplies to build it. I'm using 1 for the standard -12, -5, +3.3, +5, +12 outputs. I've modded the other to output ~0 - ~24V(off of the 12V line). I've tested both of them separately, and they work great.

My concern is if I have 2 separate GNDs. If I change the voltage on the one power supply, would it affect the other power supply?

added explanation

I had the 1 atx power supply stay to same because it is easy to use the standard ATX powers supplied.

I wanted something that I could change as well. So I did a little research on the circuit inside the ATX power supply. There is a voltage divider that controls the output voltage on the power supply. It is on pin 1 of the TL494 chip (guessing on actual chip). You also have to change the Over voltage protection on (I think) pin 4. I placed a small circuit turn pot on the Pin 1 with a resistor. I tested it, and I was able to get 0 - 24V on the 12V line. I estimate that I would be able to get 5amps out of this variable control.

I also added some Amp meters and 1 voltage meter. Here is an image my ideal design. enter image description here

Thanks

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I realize I'm just reading a sketchup, but you might consider using orange for 3.3v, red for 5v, and yellow for 12v. These are the "standard" colors in the ATX harnesses, which will make at-a-glance identification easier. –  insta Jan 18 '13 at 21:00
    
I have to remake the design to fix different dimensions. I'll make them match. –  Ashitakalax Jan 19 '13 at 4:10
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1 Answer

up vote 2 down vote accepted

ATX power supplies have floating outputs. If you connect the output common returns (grounds) together, the outputs will remain floating. They will float together. Whether you connect the grounds or not, changing the voltage on one unit should not affect the other one. If you connect two power supplies in parallel, however, you will have load sharing problems.

At the same time, I don't know exactly you are wiring them together, and what modifications have you made to the 2nd unit in order to get 0-24V output.

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I added explanation above. I also am wondering about the Load sharing problems. The outputs would have separate loads. I wouldn't combine the outputs together. Would there still be an issue with combining the grounds? –  Ashitakalax Jan 18 '13 at 20:40
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