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Backlight Dummy Load???

Discussion in 'Electronic Repair' started by mike, Aug 22, 2010.

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  1. mike

    mike Guest

    I pick up free lcd monitors at garage sales and fix 'em.
    Biggest problem is getting them to run while you have 'em
    disassembled.
    Seems as if they TRY to make it hard to run 'em.

    Would be much easier if I didn't have to hook up the backlights.
    But running the inverter with out a load is surely a BIG STRESS
    on the transformers and maybe the driver.

    What's a good dummy load that I can put on the connections to keep
    the voltage spikes from arcing/shorting the transformer secondary?
    The lamp load is decidedly nonlinear. And there's several watts to
    dissipate.

    Cutting the power supply trace to the inverter works, as long as the
    problem isn't in the inverter supply, but it's
    kinda brute force.

    I have a bunch of laptop backlights, but I'm afraid to use them
    as load for bigger monitors. Big current difference.

    Anybody successfully constructed a backlight proxy (dummy load)?
    Explanation?

    Thanks, mike
     
  2. Grant

    Grant Guest

    Some turn off if the backlight not running, too. Backlight CCFL is run
    on constant current (there's a little rectifier/smoother in the HV return
    lead), so perhaps a smallish value power resistor in series with a 400V
    or so TVS diode (bidirectional or two unis in inverse series) would be
    a nice load? Say 470R to 2k2 HV resistor + TVS?
    Given that the HV is current regulated, maybe they run at similar
    currents? Longer tubes would have higher voltage? Tube diameter
    seems the same (tiny! 3mm?) on the few I've come across. I see
    two in parallel on larger screens rather than one CCFL with a
    larger diameter.
    Not tried it (yet), but what I wrote above seems logical start point,
    try it on what spares you have now?

    Depends on what the fault is, for how far I'm willing to dismantle
    an LCD, usually obviously stressed power supply caps, or blown CCFL
    inverter needing replacement tank cap (thanks to Internet goodness
    for that fix :) I've been able to connect CCFLs and all for testing
    before reassembly. Haven't felt the need yet for dummy CCFL.

    My repair success rate is 5 1/2 of 7.

    The half working one is usable but has odd shimmering pixel effect
    in one corner no one here could help with at the time. Here's the
    web page in case somebody has seen the effect, but missed my first
    post: http://grrr.id.au/odd-lcd/ I didn't dismantle further because
    the LCD is usable as it is, hasn't got worse over last few months.
    I'd have another try if I received some positive information, or get
    another screen so it matters less if I bust this one on further
    dismantling. I'm wary of breaking the film to glass connections by
    taking the metal frame off. My working space is a little cramped.

    Grant.
     
  3. Sjouke Burry

    Sjouke Burry Guest

    Why not borrow two tubes from a broken screen, mount them in a box,
    and use them as ballast?
     
  4. Grant

    Grant Guest

    Make a small lightbox for checking PCB prints, useful too ;)

    Grant.
     
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