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Hardware Thread, VGA Ground Loop in Technical; Hi All, We have an issue with laptops using external screens as second monitor, this causes a ground loop and ...
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    british_government's Avatar
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    VGA Ground Loop

    Hi All,

    We have an issue with laptops using external screens as second monitor, this causes a ground loop and interference on the second display.

    I have done a fair amount of research and found 2 solutions, the first is to disconnect the ground on the mains lead for the second screen, obviously fine until PAt test comes around. The second is a "VGA Ground Loop Isolator" and I cannot find any re-sellers of these.

    Has anybody had the same problem? What solution do you use? Do you have a source for ground loop isolator's or a different solution?

    Thanks in advance

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    mdench's Avatar
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    You able to get a picture of the interference?

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    british_government (6th November 2012)

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    "Disconnect" the earth on the laptop power supply. You can quite often find power bricks which are double insolated.

    Notice the difference on the power input.

    dell-laptop-ac-power-supply-adapter-d500-1-big.jpg

    ADEPA12.jpg
    Last edited by j17sparky; 6th November 2012 at 10:36 AM.

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    you can often get round this by making sure the socket for the projector and the socket for the laptop are on the same circuit - it's sometimes less grief to get the sparky to fling a bit of T&E rather than throw technology at it.

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    british_government (6th November 2012)

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    Quote Originally Posted by british_government View Post
    the first is to disconnect the ground on the mains lead for the second screen,
    DO NOT DO THIS - EVER!

    Double insulated supplies are designed to be double insulated, it is NOT the same as lifting the earth wire. A different PSU, Class 1 or 2 may well fix this, or look at filtering the VGA signal. I'm looking into this in a sort of "maybe next week" way.

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    british_government (6th November 2012)

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    Thanks for all your replies.

    Unfortunately it isn't that easy to get a picture, did I forgot to mention it is our head and the most important problem in the world?
    @j17sparky sorry im not sure what you mean, are you suggesting a new power supply with 2 pin mains lead instead of 3?
    @Oaktech both on the same curcuit, plugged in the same double plug in a new build school so supply should be clean.
    @Andrew_C never ever! Basically a better ac adapter for the laptop could sort it? Obviously a significant cost for a "maybe" fix thought but I will look into it.

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    Quote Originally Posted by british_government View Post
    @j17sparky sorry im not sure what you mean, are you suggesting a new power supply with 2 pin mains lead instead of 3?
    Yes.

    @Oaktech both on the same curcuit, plugged in the same double plug in a new build school so supply should be clean.
    Unsure what the problem is then as the cure points to that being the problem. Maybe you've got a dodgy power brick/projector psu...?

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    It will be an artefact of the power supply architecture (almost certainly).

    However, if the two devices were on significantly different mains circuits, you can get "earth loops". This is where there is a small difference in the potential in the earth at the socket. This can lead to currents flowing in the screens of signal leads, and this leads to interference. Trying to clear earth loops in a big sound system is really difficult!

    "Clean supplies" are sometimes needed to get sensitive equipment away from big motors in factories, or buildings with lifts. Here, having a lot of cable between any common points in circuits reduces the impact of back emf, and switching spikes.
    Last edited by Andrew_C; 6th November 2012 at 10:51 AM.

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    I found a bit of info that points to analog signal being converted to digital with vga into a tft and suggests using hdmi out from the laptop and converting it to dvi.

    Im going to try that but if not will have to try a power supply with only 2 pins on the lead, that makes sense as a solution as it obviously isn't earthed.

    Thanks all for suggestions I will post when I have an update

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    Just going for a 2 pin supplied pack may not clear the problem. You need to find the right one!

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    Quote Originally Posted by Andrew_C View Post
    Just going for a 2 pin supplied pack may not clear the problem. You need to find the right one!
    Thanks, luckily enough I have a universal one on my desk which only uses 2 pins, so that is probably a good place to start!

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