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#204838 - 01/15/12 11:14 AM VGA ground loop isolation?
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
Is there anyone here who has dealt with ground loops in computer display systems? I recently added a connection between the VGA input of my Sony LCD TV and my PC, and it works, but there is a 60 Hz buzz in the audio and a hum bar rolling through the picture.

Temporarily lifting the AC power ground at the PC clears it up, but obviously that isn't a safe permanent fix. The PC and the TV are already on the same branch circuit. Everything works fine when testing with a laptop (on battery), but plugging the laptop into AC brings the hum back, as well.

The connection between the TV and PC consists of 2 cables, a 15 pin VGA cable and a 1/8" stereo audio cable. An isolation transformer for the audio link is easily available for a few bucks, and has already been ordered. A suitable VGA isolator seems a LOT harder to find. I only seem to find ONE manufacturer of such a device, at a cost of ~$140:

http://www.markertek.com/Video-Equipment...AT-VGA11S.xhtml

Before I drop the money for one of these, I would like to be SURE that it is going to clear things up. Has anyone used one of these gadgets successfully, or found a cheaper solution to this problem?

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#204839 - 01/15/12 01:40 PM Re: VGA ground loop isolation? [Re: NJwirenut]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Verify the VGA cable you are using is a true one to one cable. If the cable tied multiple pins together internally, that might be the source of the problem.

pin 1 red video
pin 2 green video
pin 3 blue video
pin 4 monitor ID bit 2
Pin 5 system ground
pin 6 red video ground
pin 7 green video ground
pin 8 blue video ground
pin 9 N/C or missing (Key)
pin 10 Horiz & Vert sync ground
pin 11 if tied to ground, the display is color or Monitor ID bit 0
pin 12 if tied to ground the monitor is monochrome or Monitor ID bit 1
pin 13 Horiz sync or Composite video sync
pin 14 Vert sync
pin 15 Monitor ID bit 3

Can you open the cable shield on the VGA cable? That is probably what is connected between the two chassis.

Edit: Additional questions.

1) Is the computer also tied to shielded network cables?

2) Do the hum bars go away when the audio is disconnected.

3) Does the audio buzz go away when the video cable is disconnected?


Edited by LarryC (01/15/12 01:47 PM)

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#204841 - 01/15/12 04:46 PM Re: VGA ground loop isolation? [Re: NJwirenut]
KJay Offline
Member

Registered: 11/27/07
Posts: 763
Loc: MA, USA
Do you possibly have an available HDMI connector on the TV? If so, Iím wondering if using a VGA to HDMI converter or a combo VGA/audio to HDMI converter would help. Iíve seen these for around $30.00 or $40.00.

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#204842 - 01/15/12 04:52 PM Re: VGA ground loop isolation? [Re: LarryC]
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
All pins are wired straight through. Tried breaking the shield at one end to no effect. The outer cable shield was tied to the connector shell at either end, but not tied to any of the grounded pins (5-8 or 10).

Network cables not shielded, but computer frame is also grounded via a couple USB cables (to UPS and printer).

Hum bars present with only VGA connected. Audio buzz present with only audio cable connected. Connecting both gives bars and buzz.

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#204843 - 01/15/12 05:11 PM Re: VGA ground loop isolation? [Re: KJay]
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
HDMI port available, but trying to keep it open for a bluray player to be added soon.

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#204844 - 01/15/12 05:50 PM Re: VGA ground loop isolation? [Re: NJwirenut]
KJay Offline
Member

Registered: 11/27/07
Posts: 763
Loc: MA, USA
If things donít work out and you want to try it, you can find a decent 3 or 5 port HDMI auto switching hub for around $25.00 these days. This will allow you to connect multiple devices to the one HDMI port on the TV.

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#204847 - 01/15/12 08:32 PM Re: VGA ground loop isolation? [Re: NJwirenut]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Did you try disconnecting the USB cables to determine if the groundloop is being generated by a USB device?

Back to basics. You stated the receptacles feeding the TV and the monitor are on the same circuit. Can you verify that the outlets are not miswired?

In my AV experience, hum bars and ground buzzes are most often caused by a slight voltage difference between two "ground" potentials. Possible sources for minor differences include multiple connections between the neutral and ground, leakage to ground from defective equipment and surge suppressors / EMI filters.

1) Is your house an older house were someone before you swapped black and white wires on daisy chained outlets?

2) Could have someone cheated by tieing the ground screw to the neutral to make a 3 prong outlet out of a two wire circuit?

3) Do you have any two wire circuits in the house where perhaps a ground leakage current is trying to get back to the neutral via the UPS / printer / PC / TV / sound system?

4) Did someone add a "safety" ground or bond between a device and the water piping?

5) Do you have cable or satelite hooked up to the TV? DSL for the computer? Are the earth connections for the surge suppressor blocks for the telephone, TV, or satellite securely bonded to the grounding network for the house?


A quick test to determine if there is a voltage difference between the outlet grounds, is to run a bonding wire between the TV outlet ground and the computer outlet ground and see if the hum bars and ground buzz disappear.

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#204848 - 01/15/12 09:34 PM Re: VGA ground loop isolation? [Re: NJwirenut]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
You could try Isolated Grounding type Receptacles, with all the Grounding Conductors bonded to a single Star Point.

This is one application, where IG Circuits actually are of some benefit!

Otherwise, go with some type of Modulated A/V signals.

-- Scott
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#204852 - 01/15/12 11:15 PM Re: VGA ground loop isolation? [Re: NJwirenut]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
I had an old Daewoo TV that would trip a GFCI if it was hooked to a PC. My bet is they have tied the neutral/hot to the DC ground some way in the TV, maybe in some crude attempt to provide surge protection. (I have seen delta bridges across all three with something that might have been a MOV type device)

In the PC, DC ground and the EGC are rigidly connected together. (via the mounting pads on the system board to the base plate, bolted to the power supply case).
In my wooden machines, this was not the case. (no pun intended). It showed up for me when I was installing them in cars.

Is there a ground terminal on the TV for intersystem connections? Try bonding that to the PC case. If not, try to bond the DC ground in back of the TV to the PC in some other way. (RCA connector shell) Use something like a #12 stranded.
Since you are incidentally bonding these anyway, give it a shunted path with a lower resistance than your signal cables. That is what fixed the car/PC problems. (a 12ga from the PC case to the chassis of the car).
_________________________
Greg Fretwell

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#204863 - 01/16/12 10:37 PM Re: VGA ground loop isolation? [Re: NJwirenut]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
Well, I'm not particularly worried about being electrocuted by my computer chassis. I wouldn't put up with what I imagine is really a 120Hz hum in the signal which you will have a hard time getting rid of with that ground connected. When you had your ground lifted, you should have measured the AC voltage between the prong and the outlet. Let's say that you see 3 VAC between ground and the prong that should be grounded. That's a deal killer for your video & audio signal but probably not an NJwirenut killer. So what if you clamp your ground somewhere between a signal killer and a wirenut killer??? Consider inserting a bidirectional transorb like a 1.5KE6.8CA or 2, series inverse unidirectional 1.5KE6.8A between chassis ground prong and Earth. That way, you can avoid the nuisance low voltage ground loops while still retaining 1500W of clamping ability to ground should the chassis try to elevate to a dangerous voltage. Just my 6 volts worth.
Joe
PS: The cost of a Littelfuse 1.5KE6.8CA at Digi-Key is $0.77, assuming you bought enough other goodies to satisfy their $25 minimum. I never seem to have a problem doing that.

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