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Our hall has a projector hooked up to a PC via a VGA cable but we're seeing horizontal bars on the projector image when we turn on the lights in the hall. I'm think it's a ground loop problem. I'm trying to eliminate ground loop between PC and projector via a VGA cable. I've tried hooking them to the same power socket which didn't eliminate the horizontal bars on the projector image. I disconnected the safety ground inside the projector plug and the horizontal bars on the image went away, but I believe this is not a long term safe option. but does seem to indicate that the ground loop is the cause of the horizontal flicker/bars. Is it possible to disconnect the ground signals inside the VGA cable to eliminate the ground loop without making it unsafe/losing the picture entirely? (pins 5-8, pin 10?) What would be the consequences of doing so? I don't want to degrade my image quality, or risk damage. I'm also not super keen to "experiment"if I might damage something, and also re-soldering the vga cables is fiddly work.

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  • \$\begingroup\$ Don't disconnect the ground wires in the VGA cable! The signals are not differential, they're single ended, in coaxial cables. They need the ground, otherwise what is their voltage referenced against? Disconnecting the ground isn't an option. With no ground reference, the signals will certainly have more 50 Hz buzz on them. You also run the risk of having the half-supply-voltage appear on the pins, maybe 60 V, which will destroy the projector, computer or both. \$\endgroup\$ – tomnexus Sep 17 '15 at 3:58
  • \$\begingroup\$ @tomnexus That was what I thought might be the case, but also I'm not comfortable with disconnecting the earth on the 3 prong plug which is the only thing I've tried that works so far. the common power cord extension didn't work, I think because the ground loop is caused by EM interference from the lighting system (old CFLs). The VGA cable and power cable run up the wall and across the ceiling to the ceiling mounted projector. Websearching found a VGA isolator cable but I can't find a local one to try out (I'm in South Africa) \$\endgroup\$ – Gen Sep 17 '15 at 18:46
  • \$\begingroup\$ I realise this is kinda old, but to anyone else going here (like I did today), if you also have an audio cable plugging into the computer that may create bit of a ground loop also, I fixed my flicker/bars issue with just adding a Ground Loop Isolator to the audio cable. \$\endgroup\$ – Mint Feb 14 at 8:51
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That approach is common in audio and industrial sensor signals for the reasons you've identified. The screen is usually left connected at the transmit end and disconnected at the receive end so that may be worth a try. Get a short male-female cable to hack and install it at the projector end.

We're short a few details on your setup. What kind of lights? Fluorescent, incandescent, dimmers, etc.

Ground loops cause problems because of either a) a difference in ground voltages between the signal source and destination or b) because of induced currents in the loop due to EM interference. Something that may be worth a try is to power the PC and projector from the same socket. This should eliminate any ground potential differences. A temporary extension lead should be good enough to give an indication. If you can run the mains cable close to the video cable it will reduce the cross-sectional area of the loop and that may help too. (I know this is usually not considered good practice and you may get some switched mode power supply interference.)

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  • \$\begingroup\$ This is a terrible idea! VGA signals are not differential, they're single ended! Disconnecting the ground isn't an option. With no ground reference, the signals will certainly have more 50 Hz buzz on them, but you also run the risk of having the half-supply-voltage appear on the pins, which will destroy the projector, computer or both. What you're thinking of is disconnecting the screen from a differential cable, which is a valid way of stopping a ground loop. VGA cables are coaxial, the outer is not a screen, it's the other half of the signal (and there's an overall screen). \$\endgroup\$ – tomnexus Sep 17 '15 at 3:55
  • \$\begingroup\$ You're right about the power extension cord idea, that should solve the ground loop problem. \$\endgroup\$ – tomnexus Sep 17 '15 at 4:04
  • \$\begingroup\$ I tried the power extension cable idea before I disconnected the safety earth. That didn't work. The lights causing the problem are CFLs \$\endgroup\$ – Gen Sep 17 '15 at 5:10
  • \$\begingroup\$ @tomnexus: Good point about VGA not being differential but where would half-supply voltage come from? Which supply - mains or DC? \$\endgroup\$ – Transistor Sep 17 '15 at 6:14
  • \$\begingroup\$ Mains. If one end were to be badly earthed, which can happen on a desktop and is often the case on a laptop, then the chassis might float at about half the AC voltage. Very little current available, so it is grounded as soon as you plug in a printer or a screen and there's no problem, just a bit of audio buzz maybe. But if there's no signal ground, it's bad. There's a question about this voltage each week on ee.se. \$\endgroup\$ – tomnexus Sep 17 '15 at 17:39
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What you really want is a video 'longitudinal stop coil' sometimes known as a 'video Humbucker', basically a big common mode choke on the video lines. It acts as a current balun, and push comes to shove you could build one out of a VGA cable and suitably large power transformer core by winding as many turns as possible of the vga cable onto the core.

One other thing that may help is to run a heavy gauge earth conductor in parallel to the video cable (Tape it to the side of the cable so the loop formed has minimum area), The objective is to bond the projector and the source so that the flowing current (Not a problem in itself) develops little voltage between the two because the parallel earth conductor is of such low impedance.

In really nightmare cases, both can be used, but the need seldom occurs.

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