Carputer grounding through VGA cable

**ohioG35** · 04-30-2007, 12:18 PM

I have a carputer powered by a m2-atx and a Lili-629 display is connected through the on-board VGA. The display has always had a little noise on it but not enough to bother me but recently I had some free time and tried to fix the problem. In trouble shooting I realized that the carputer is grounding though the VGA cable via the ground on the monitor if my preferred ground (wired to the frame) is removed.

I tested it by removing the ground from the m2-atx and to my suprise the computer was still running. I then removed the power from the display ( though the 12v cig lighter) and the computer shut off. It will run on either the desired ground or the display if the other is removed and presumably either or when both are connected.

Does anyone know how to keep the computer from grounding through the VGA?
I have a asrock K7S41GX MB if it makes a diffrence.
Thanks!

DIGItonium · 04-30-2007, 02:00 PM

The "shell" of the VGA connector is grounded, but I don't think that will help you with your noise issues.

How long is the VGA cable, and does it have ferrite cores on both ends?

**ohioG35** · 04-30-2007, 02:12 PM

6' and it does have Ferrites. The noise isn't so much of a concern as the fact the power supply decided that it can ground through the VGA (when the true ground for it is disconnected). That bad isn't it?

**Arrow** · 04-30-2007, 02:22 PM

The VGA cable is usually grounded at both ends. This helps the shielding keep interference out. Thing is, the only way to ground is a direct connection to the ground wire on each end. So, disconnecting the ground will cause the shielding to be the conductor and and use the ground from the other end of the cable. It is this way by design and therefor not a bad thing. Actually using the cable as a ground could be bad, so keep you're grounds hooked up at both ends. Don't worry, the ground is not causing your noise... Unless there is a difference in potential at both grounds, that might do it... Hmm.. You could try putting a ground loop isolator on the monitor side. You might be able to temporarily disconnect the ground on the monitor to see if interference goes away. If so, the ground loop isolator should help.

**v8 scimitar** · 04-30-2007, 02:23 PM

that's normal, ground is ground and electricity will always find it for good or for bad if there is a hint of a path, in your case bad as your stressing components that shouldn't be. When you disconnect power you need to disconnect all power.

**ohioG35** · 04-30-2007, 02:43 PM

Originally Posted by Arrow

The VGA cable is usually grounded at both ends. This helps the shielding keep interference out. Thing is, the only way to ground is a direct connection to the ground wire on each end. So, disconnecting the ground will cause the shielding to be the conductor and and use the ground from the other end of the cable. It is this way by design and therefor not a bad thing. Actually using the cable as a ground could be bad, so keep you're grounds hooked up at both ends. Don't worry, the ground is not causing your noise... Unless there is a difference in potential at both grounds, that might do it... Hmm.. You could try putting a ground loop isolator on the monitor side. You might be able to temporarily disconnect the ground on the monitor to see if interference goes away. If so, the ground loop isolator should help.

As for noise, I've decided that I'm running too many other items too close to the VGA cable so i think re-routing is my best option.

As far as grounding the VGA, I don't know if I am reading this right but... When I remove the ground from my PC's power supply the computer still runs is because the display is properly grounded and all the power is redirected there. This seems like a bad thing since the whole unit is pulling ~120 watts and it all get funneled into my wimpy VGA cable. The only way to stop this would be to disconnect the shielding from either end of the VGA cable. I just find it hard to swallow that the whole PC could ground through the VGA cable but I guess it only make sense.

I assume that once I plug my power supply's ground in it takes over the ground for the system because it most likely has a lower resistance.