Since I don't have enough money at the moment to purchase a good bench power supply, I want to 'mod' a PSU. In the first instance not by opening the PSU, but just by making an adapter that also puts a load on it's 5V rail.

But after taking a closer look (with a multimeter) I found an issue that could be a problem. In contrast to bench supplies (from my University) I'm familliar with a PSU is not floating. It's ground outputs are connected directly to ground.

This could be a very big problem if I mistakenly connect the ground hook of a probe to a higher potential point, thus creating a short which probably will evaporate the probe lead. I know myself, this will happen someday.

As for the AC side I think without opening it should look like this:


simulate this circuit – Schematic created using CircuitLab

Where the Live, Neutral and Earth wire are labeled, My personal safety simply modeled as a resistor, and a failure that should trigger the RCD/Circuit-breaker as a switch.
RLOAD being of course an very basic simplification of the rest of the circuit.

Now for my question: Are there any issues that could occur by making the modification:

  • Lifting the Earth from the neutral line @RLOAD
  • Keeping the Earth connected to the metal chassis of the supply.


simulate this circuit

Are there also concerns with quality/stability. As in +5 or +12V rails being way out of spec when measured?

I've been thinking about this for some hours and I think there wouldn't be any major issues, but since I do not have a background or vast knowledge in power supply design I'd like to hear your take on this.

  • \$\begingroup\$ What is the "Potential disaster" switch intended to represent? There is nothing like that in any normal PC PSU. \$\endgroup\$ – brhans Apr 29 '15 at 16:26
  • \$\begingroup\$ I probably could have left it out, but it's just as an representation to me: An unknown event where the Earth/Chassis could get, due to mechanical/electrical failure, shorted to the mains. It is drawn in these two schematics as a method to see if my hand has a problem if this event happens. I think there is no really big problem, in contrast to when I would lift the ground 'earlier' than at the neutral node at RLOAD, but since I don't have a lot knowledge about electrical safety I want to ask. \$\endgroup\$ – Rick van Loo Apr 29 '15 at 16:59
  • \$\begingroup\$ @brhans: your average terribly cheap china smps has this switch, it is otherwise known as "cut off tth component leads stuck in flux residues instead of being cleaned up" \$\endgroup\$ – PlasmaHH Apr 30 '15 at 9:43

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