the ground loop thing

[r123]

Recently, having read the page about Ground Loops when there is a USB connection to a computer came away quite concerned, because have done this plenty. Talked it through with a friend who explained that the vast majority of the power supplies used for 3dp are "floating" - their outputs not associated with ac lines or earth in any way. Checked for my Meanwell and that's certainly the case, so that's a bit of a relief. Unless...

Any comments?

https://duet3d.dozuki.com/Wiki/USB_ground_loops

[fcwilt]

@r123

I have built a number of "kits" and I normally toss the power supply provided and get a Meanwell.

The power supplies I use do not have any connection between the DC outputs and the mains ground.

The only connection I make from the mains ground is to the metal case or metal frame of the printer.

I have never had a problem with a USB connection under these circumstances.

Frederick

[r123]

@fcwilt Thanks, your experience is quite reassuring. Agree, there's no point in economising on power supplies.

Incidentally, is there a view on whether or not it's OK to have a USB connection to the Duet when the power is off? Meaning the USB seems to satisfactorily power the 5v side of things allowing one to play around.

[fcwilt]

@r123 said in the ground loop thing:

@fcwilt Thanks, your experience is quite reassuring. Agree, there's no point in economising on power supplies.

Incidentally, is there a view on whether or not it's OK to have a USB connection to the Duet when the power is off? Meaning the USB seems to satisfactorily power the 5v side of things allowing one to play around.

There are some things you cannot do when powered only from the USB connection.

I don't have a list of them.

I've been using Duets for awhile and I have a standard procedure to go from "out of the box" to being able to complete the configuration from the DWC so I'm not sure what can/cannot be done.

Frederick

[Dr_Ju_Ju]

Personally and what I WOULD have had to do in a previous working environment, is to ensure that ALL exposed metalwork is earth bonded, i.e. linking wires connecting back to the incoming, mains earth point.

Also to ensure that there is no possibility of any form of interference between disparately powered devices, where a supposed common 0v could have the potential to rise above a true 0v, e.g. earth point, I would bond each power supplies 0v to earth...

[fcwilt]

@dr_ju_ju said in the ground loop thing:

Personally and what I WOULD have had to do in a previous working environment, is to ensure that ALL exposed metalwork is earth bonded, i.e. linking wires connecting back to the incoming, mains earth point.

Good.

Also to ensure that there is no possibility of any form of interference between disparately powered devices, where a supposed common 0v could have the potential to rise above a true 0v, e.g. earth point, I would bond each power supplies 0v to earth...

Why? That just creates the possibility of a ground loop via the USB connection that we want to avoid.

Doesn't everything in your printer on the DC side already have the "0v points" connected together?

[Dr_Ju_Ju]

@fcwilt
In your printer the USB 0v may well be close or even at, earth potential, but connecting to another device e.g. computer / laptop etc. which will have a different power supply, whose 0v may or may not be at the same potential as the printer 0v / earth potential.

If they are different, connecting the two devices via the USB ports, would initiate a current flow from the higher to lower potential, i.e. a loop.... It may not be much, and have no discernible impact on performance etc. but I have experienced issues with data transfers being slowed down as the retry count increases.

If these were audio systems you may well perceive the current flow as a hum at the frequency of the mains supply, which in the UK would be 50 Hz.

[fcwilt]

@dr_ju_ju

If the printer power supply is fully isolated from the mains, a typical design, then there will not be any current flow as the printer will reference it's 0v point to the 0v point of the computer USB port.

Frederick

[Falkia]

@fcwilt
Without having the 0V connected to ground/earth and the frame of the printer connected to earth/ground, then for example the fuse for the bed heater may not trip if you get a short to the frame in the heater.
I always have the 0V connected to ground/earth and if there should be any problem with ground loop I use the laptop running on battery only. Just my opinion.
/Fredrik

[Phaedrux]

@falkia said in the ground loop thing:

@fcwilt
Without having the 0V connected to ground/earth and the frame of the printer connected to earth/ground, then for example the fuse for the bed heater may not trip if you get a short to the frame in the heater.
I always have the 0V connected to ground/earth and if there should be any problem with ground loop I use the laptop running on battery only. Just my opinion.
/Fredrik

That is the reason we recommend bonding DC- to earth. Though as with anything complex, there are valid reasons you may not wish to do that.

[fcwilt]

@falkia said in the ground loop thing:

@fcwilt
Without having the 0V connected to ground/earth and the frame of the printer connected to earth/ground, then for example the fuse for the bed heater may not trip if you get a short to the frame in the heater.

I use AC powered heaters and the frame of the printer and the associated parts so a short from heater to frame would blow the fuse.

If I used DC powered heaters a short from the heater to the frame would not be dangerous as the DC outputs of the power supply are floating but there would also be no sign of that condition without a measurement being taken.

Yes, I understand that many folks recommend bonding the "ground" output (whatever that means) of the DC power supply to the "earth" ground.

Over the years (and there are many behind me) I never have done that and it has never caused any problems for me.

Thanks for the feedback.

Frederick