Good day,

I found this question asked before (not exactly this) but the answers were wrong (at least at some level).

So i have two 300W PSU's (different manufacturer so different schematics too). I don't really have any purpose for them but i could use a 24V high amperage (up to 10A) power supply.

So i took one apart, disconnected ground from few screws to chassis (just so that output ground would not be connected to socket/chassis ground). I left ground for all other uses (chassis, grid and some other grounds are all connected). Now i had output ground separated from main ground. I then supplied 12V from other PSU to first PSU's ground. Voltage between PSU1 ground and PSU2 12v then became 24v. 5v became 17, 3.3 became 15.3...

So my question is this: Since all i could find was that this is impossible/very bad to do, it kinda contradicts my findings. The 24V output did hold over 7A load and it seemed to not matter in which pattern i turned them on (i turned "free" PSU first, then the "non-grounded" one second). Is it really that bad to use those PSU's this way? Is it "theoretically bad" or like "100% break down pretty soon"?

  • \$\begingroup\$ 12+5, 12+3.3. What is the main question? \$\endgroup\$ – User323693 Jul 17 '15 at 9:07
  • \$\begingroup\$ The main question is: "Is it really that bad to use those PSU's this way? Is it "theoretically bad" or like "100% break down pretty soon"?" its the last sentence. \$\endgroup\$ – avuthless Jul 17 '15 at 9:08
  • \$\begingroup\$ Randomly disconnecting grounds from chassis is a very bad idea, and may lead to unexpected electrocution. \$\endgroup\$ – Nick Johnson Jul 17 '15 at 9:32
  • \$\begingroup\$ Thanks for warning. Though since my chassis is still connected to ground on grid i think the only risk remains in wires, which do not give out more than 24V (for now lol). \$\endgroup\$ – avuthless Jul 17 '15 at 9:41

It would depend on the type of PSU used, linear PSU's normally use a regulator of some description at the output stage and can sink current (but could get damaged over time) as PSU1 would have to do this. Most switch mode PSU's have a capacitor and diode. These types don't really care as the diode would prevent the current from PSU2 from going where it should not.

I have done this a lot with LAB grade PSU's and had no problem as well as some high current switch mode power supplies (30A) and had no problems, but our power here comes with separate earth as we have Live, Neutral and Earth. As long as your mains side is earthed to your chassis and the output is isolated from the input (most PSU's have a transformer), I can't see any problems other than stated above.

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  • \$\begingroup\$ Thanks, other than this you don't see any problems? Since you pointed out one possible issue i am not certain there are no others left. \$\endgroup\$ – avuthless Jul 17 '15 at 9:42
  • \$\begingroup\$ Yea i do have separate earth in my grid too. This separate earth is then used throughout the chassis and several components inside. Thanks for info! \$\endgroup\$ – avuthless Jul 17 '15 at 14:35

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