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Grounding Bench Power Supplies (connecting GND to mains PE)
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neo2001:
Since I have four different power supplies by now, I was thinking of how to handle the "GND situation" of the different devices.

I have one old ATX power supply, which I modified to be a bench power supply a few years ago. Since it delivers all the commonly used voltages for digital devices, without the need of adjustment, I kept it. Like all(?) ATX power supplies, the protective earth wire of the mains connection is tied to the GND pole of the power supply's DC output.

The next power supply is a "classic" 30 Volt, 5 Ampere linear power supply with a big transformer an power transistors. On the front there is a small piece of metal, which connects the terminals of PE/earth/case and DC GND. Since it came this way, I kept it in place.

Additionally I have two DPS power supplies from Ruideng. Both of these are "floating". Both use a grounded power supply (36 and 60 Volt respectively). The PE wire is connected to the metal case of the power supply, but not to the GND pole of the DC output.

I was asking myself, if it would make sense, to tie all GND poles to the PE/earth wire, especially if I use two of the power supplies on the same device, in which case they would need a common ground connection anyway.

Does this make sense? What would or could be the downside of "grounding" the latter two devices by connecting the DC GND pole to PE of the mains supply? I also thought of using a switch to make the connection optional.

Somehow having all devices on the same ground potential, seems favourable to me. Especially with USB devices or digital stuff connected to a PC. But I wouldn't ask, if I already knew the answer. :-)

janoc:

--- Quote from: neo2001 on March 01, 2019, 06:14:03 pm ---Somehow having all devices on the same ground potential, seems favourable to me. Especially with USB devices or digital stuff connected to a PC. But I wouldn't ask, if I already knew the answer. :-)

--- End quote ---

Actually, you most certainly don't want that. Try to think what will happen if you make a mistake and connect a ground lead of an USB oscilloscope/logic analyzer/whatever to a place of your device under test that is not ground.

Yep, the magic smoke escapes and depending on how you had it connected and what the voltage (and amount of current available) was, you will most likely blow out the USB port of your PC if not the entire motherboard.

It is exactly the same problem like with earthed oscilloscopes (watch the Dave's video on how to not blow up your oscilloscope). Having the device you are working on earthed severely limits your options and greatly increases the risk of causing a short when connecting two devices together.

Another reason why you don't want this is for connecting multiple of them together (e.g. to get a dual supply for testing amplifiers, opamps, etc.). Once you earth them you can't do that anymore or you will short something out.

For development work it is much better to use a lab supply with floating outputs - that's the reason your linear supply has the grounding strap there instead of being hardwired (I would take it out, IMO) and why the Ruideng supplies are floating. It is the configuration you want in 99% of the cases. The exceptions are few and far between.
Benta:
What a mess!
PE is exactly what it says: protective earth. It's there to prevent dangerous voltages on something you can touch: enclosure, screws etc. and has nothing at all to do with your test setups.
All lab power supply outputs should be floating. The ATX supply's output should be disconnected from PE, the bridge on your second supply should be removed as well.
If at some point you need a PE connection in a test setup, fine, do it consciously. Not through the power supply, that's looking for trouble.

magic:
Is it a good idea to disconnect GND from PE in a PSU which was designed that way?
I sometimes wondered if it's possible that some PSUs may rely on noise being shunted to Earth for EMC? Could things be so bad that it wouldn't work at all without Earth?

And one remark regarding the original topic: if you happen to use multiple PSUs to provide different voltage rails to one circuit, either none or only one of them should be grounded. Otherwise some current flowing through PE installation in the building could find its way through one PSU, grounding cables, the circuit's ground traces, another PSU and back to PE, injecting noise into the circuit's ground that really isn't needed at all.
Benta:

--- Quote from: magic on March 01, 2019, 09:02:18 pm ---Is it a good idea to disconnect GND from PE in a PSU which was designed that way?

--- End quote ---

A very good point and one that I didn't mention. ATX supplies have a precisely defined function.
Using a modified ATX supply in a lab environment is questionable at least, but defensible when you know what you're doing.
My feeling is that it's not the case here.

In general: confusing/mixing PE with ground is not a good idea.
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