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VGA LCD flicker

Discussion in 'Hobby Electronics' started by TT, Apr 3, 2004.

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  1. TT

    TT Guest

    I am having a problem with a VGA LCD display device when powered from a
    different AC power circuit to the video signal source (PC).

    If both the LCD monitor and PC are plugged into the same power point,
    everything works fine. The problem arises when I power the LCD from a
    different power point on the other side of a warehouse. When powered from
    different circuits, the LCD flickers or waves/pulses to what looks like a
    50Hz type flicker.

    I am using a VGA splitter to boost the video signal with a 30M VGA cable to
    get across the warehouse floor. I am confident that this isn't what is
    causing the problem because I have tried connecting the LCD directly to the
    PC through a short 2M VGA cable and run an extension cord across to the
    power point on the other side of the room to try and eliminate the cause. In
    this case, the flicker is still apparent.

    I have also tried a CRT monitor which exhibits the same problems. Also, if I
    power PC and LCD together at either side using the same power point, there
    is no problem, so I don't think it's a power problem at one particular

    Has anyone solved a similar problem before or have any ideas what might be
    causing the problem? Thanks.
  2. Phil Allison

    Phil Allison Guest

    ** The power points are possibly on different AC phases.

    To check this use an extension lead and connect the probes of a DMM
    between the two actives, if they are the same phase the reading will be
    near zero volts - if different phases the reading will be about 415 volts.

    .............. Phil
  3. C3

    C3 Guest

    This isn't directly related to your question, but I have also noticed
    flicker on a number of monitors in apparently good order.

    Could this simply be caused by the flickering of the fluorescent backlight?


  4. Newsy

    Newsy Guest

    Probably the easiest way to fix it is to run a power cable along with the
    VGA cable and power the screen from the same power point as the PC. You may
    have to ensure the power and VGA cables are separated by 30cm or more.

  5. Mike

    Mike Guest

    Beat freq, sync source,

    up your refresh rate or lower it as LCD does it by-itself

  6. If the LCD monitor is earthed, you may have an earth loop - you can have
    substantial current flowing between the earths of two power points.

    If the LCD monitor is powered by a "brick" with only a 2 core mains lead,
    then an earth loop isn't the problem.

    If the LCD monitor does have an earth, then you can disconnect it via a
    short extension lead with disconnected earth just to see if that cures the
    problem. If it does cure the problem and you need the earth for safety,
    then you can use an isolation transformer to give the the same safety as an

    Someone else suggested running a mains lead along the same path as the
    video - this might work too.

    Generally it is not possible to pipe unbalanced video or audio all around a
    building and plug it into earthed equipment along the way. Ultimately you
    need isolation via transformer or opto coupling.

    Other possiblilities are -

    - don't use a monitor, instead put a PC at the other end connected by a LAN.
    Of course this changes your software setup and may not be possible. LANs
    can handle earth loops because they are transformer / opto isolated.

    - use a video transmitter. May be OK if your PC has a TV output which can
    plug into the transmitter. You may have to use low resolution in order to
    read text.

    - Look at . Give them a ring and tell them
    about your problem. They have the sorts of gadgets you may need.

  7. TT
    Could be 50 Hz hum = earth loop, very common in audio, it can also affect
    video and computer gear
    Try an isolating transformer
  8. BOB URZ
    Real video isolators use high frequency transformers, for VGA it would
    need to isolate all three colours and maybe a bit more as well?
  9. Roger Lascelles
    - Get another power point put in on the same earth circuit.
  10. TT

    TT Guest

    Thanks for all the replies. I think the problem is an earth loop as when I
    open circuit the earth on one end of the system (either end), it resolves
    the flicker. I measure about 200mVac between earths when open. Turning up
    the monitor refresh rate does help, but I cannot completely eliminate it as
    anything higher than 75Hz refresh is unfortunatly 'out of range' for the

    A few people have mentioned an isolation transformer, I would like to give
    this a try. I thought an isolation transformer only isolated the active and
    neutral though, doesn't earth just pass straight through on one of these
  11. Phil Allison

    Phil Allison Guest

    ** Often it does - though it is considered OK to lose the earth if an
    ISO is in use.

    .......... Phil
  12. TT

    TT Guest

    Yes, you would have to isolate all. The extron units are 5 channel
    I found this 'Ground Loop Inhibitor' device at Extron. It has a high density
    d-sub 15 pin (VGA) connector for input and output, exactly what I want.
    Sounds like it would solve my problem. The box doesn't require any power, so
    how does it work? I assume it uses some kind of transformers for isolating

    Does anyone know who the local distributer for Extron Electronics is in
    Australia? I would like to get a price on the GLI 2000 15HD (PN 60-602-01).
  13. Newsy

    Newsy Guest


    The local distributor is RGB Integration in Adelaide. However, they will
    tell you to go to one of their distributors. I am a distributor, and the
    price is $1,850 + GST. It would be easier to run an additional power
    circuit off the same circuit as the computer.

  14. TT

    TT Guest

    Geesh, I was right when I thought the box looked kind of expensive. Alot of
    money for something that 'does not require an external power supply as they
    are non-powered' ;)
  15. TT

    TT Guest

    Unfortunatly the display device is mounted in a metal cabinet that is
    earthed for other reasons in my case. (ie. the LCD chassis bolts directly
    onto the earthed metal cabinet). Would it be electrically legal and more
    importantly dangerous to float the computer end with an isolation
    transformer? The computer has a metal case.
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