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Should we earth V- (GND) on the DC side of the PSU

Should we earth V- (GND) on the DC side of the PSU
January 12, 2024 11:00AM
Spent a few hours searching about this question...

I'm currently finalizing a Voron 2.4. Everything is earthed, and I get 0.3 to 1.5 ohm from any point of the frame to the wall wart end of the power cable (measured from screws as the Misumi extrusions are powder coated) ; after zeroing the ohmeter ; Not great, but most likely good enough ; I know that it should be measured with specialized equipment, but I don't have such devices...

The assembly manual does not talk about earthing V- ; Google didn't bring interesting information. Even read that V- should never be grounded earthed because it is dangerous (!!!)

I found 2 examples only where V- is explicitely not floating :

- the UMMD : [drmrehorst.blogspot.com]
- Duet instructions on Reprap.org : [reprap.org]

I'm not comfortable with floating voltages. On PC's, GND is earthed at the PSU level, and motherboards mounting holes are grounded. When I made my machine tools electrical enclosures, I earthed GND on the low voltage side (24V DC) including VFDs. Built a couple audio amps, earthed GND.

But on printers, no such things. No grounded mounting holes, motherboard and Raspi. On the Raspi, the RJ45 socket has galvanic isolation (schematics show the transformer) ; not the USB sockets.

Just the blue Y capacitor between GND and earth, that can be seen through the grill, between HV and LV sides.

The electrical compartment (in a earthed drawer) is built so there's a clear separation between mains and low voltage. Wires are in drag chains, and the only point where mains are crossing two of the four Z motors, the motor cables are in a 1000V rated duct.

The machine has a CANBUS toolhead (U2C + EBB36, didn't use the Octopus RJ11). Seems to be working properly. It's differential, but common mode is something I don't fully understand. Can't even look at the signals, the benchtop scope being earthed, no handheld one, and no isolated diff probe. (unearthing the scope it is not an option)

I see one potential problem only : if the printer is connected over USB to a desktop computer, creating a ground loop. Isolated USB cords are a thing...

The main question is : when should we earth GND, and when shouldn't we ? And a subsidiary one : what about CANBUS, earthing GND and common mode ?

Edited 1 time(s). Last edit at 01/12/2024 12:51PM by yet-another-average-joe.
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