Another question about shared grounds

[kae]

This question is similiar to one posted earlier.

I am using a chassis ground to earth for the negative side of my DC stepper supply. I am using a PC (ATX personal computer switching supply) power supply to power the BOB, case cooling fans and any aux equipment. When I powered up yesterday, I reached in (thinking I was safely away from any high voltage) to check something and got a bad shock across the PC supply heatsinks (used to sink the voltage regulators of the supply). I measured the voltage and it was 350 volts AC (not even sure how you can get that much voltage from a 115v outlet). I assumed that the PC supply had somehow failed (it was a used unit out of an old computer) so since I had another unit to try, I did so. These PC supplies are grounded at the corner PCB mounting holes to the chassis. So, everything in my box is grounded to earth through the chassis.

This is my question, I am using the correct grounding scheme? Is it a bad idea for the DC stepper supply to share the earth ground with another power supply? I would like to make sure that my ground method was not the cause of the PC supply failure due to some sort of ground loop or something that I don't understand. Something also to note is that my Geckos are mounted to the chassis (via heatsinks). If the back plate of the Gecko goes to ground, then they are also grounded to the same scheme.

I hope that this starts some good dicussion for novices like myself.

[Al_The_Man]

A DC P/S. is a switching supply and the switching is done on the High voltage/input side, so the switching device heatsinks are most likely well above ground, the voltage you see is due to switching pulses. There is isolation between these and the DC output via the toroidal transformers etc.
Your PC power supply low voltage DC output is usually connected to the service ground via the chassis screws, at least on a tower or Desktop model.
I usually make a connection from the actual PC common (Black) to the star point ground just to be on the safe side and ensure a direct path from ground back to PC DC common.
Al.

[kae]

Al,

I just spoke to a friend who said the same thing concerning the PC supply heatsinks. Since you didn't comment on the sharing of the ground I assume that you feel it is okay that they do so. I am using a star ground as you suggested to me earlier and I will send a ground wire from the PC supply over there. Thanks.

[Al_The_Man]

JEEZ, I spent 20 minutes typing a reply and when I posted it , the site collapsed again
I am not retyping it, but I use common grounds, and I know by previous experience here that some have had problems with noise etc, and ensuring common earth ground has eliminated the problem.
But the bottom line is whatever method is used it should be done with care.
Al.

[Mariss Freimanis]

Ground (supply return) and supply distribution seems complicated but it really isn't. Follow a few simple rules:

1) If it runs from a power supply, give it two wires (gnd and +supply) that run to from the power supply to the device. All wires, +supply and -supply, return to a single point, usually the "+" and "-" terminals of your power supply capacitor. Don't have one? Add one then! This is called "star" distribution because each device's supply wires emanate from the supply like sun or star beams.

2) Ground the chassis with a wire from your -supply terminal. Connect the earth ground from your AC mains to this same point. Never use the chassis to distribute -supply anywhere.

3) Keep your supply wire lengths short. Put the power supply and the motor drives physically near each other. Wire lengths are really good if they are 6" long. Wire lengths greater than 12" long can probably be improved on. Just because you have a big control cabinet doesn't you should spread things out.

4) Try to keep your PC interface grounded stuff (USB, DB-25, breakout board, whatever) isolated from chassis ground if you are using optocoupled STP/DIR drives. If that's impractical, run one single wire from the PC grounded stuff to the chassis grounding point. That's the same chassis point that gets your motor power supply "-" wire and the AC earth ground wire.

Mariss

[Al_The_Man]

As well as grounding, bonding is important, this is ensuring that all metallic parts of the machine, enclosure, motor frames etc have a ground conductor back to the central earth ground point.
Equipotential Bonding
http://www.divshare.com/download/5002681-38f
Al.