video screen inverter question

Discussion in 'Computer Information' started by pmaczik, Dec 4, 2012.

  1. pmaczik

    pmaczik Guest

    12-04-2012 05:51 AM

    I have a hp pavillion dv9700t CTO entertainment laptop running Windows
    7 with Nvidia GeForce 8600 video drivers.

    I might have a bad video inverter. I sort of have a black screen
    problem. I am connected to my LCD TV now so I assume my video adapter
    and drivers are working. The odd thing I discovered is, I can see my
    laptop video if I shine a flash light or a desk top condescend lamp on
    it. I can read this text that I am writting now. I can see color but
    it is like looking at my desk top through a dark filter lens. With
    out the lamp my laptop screen just looks black.

    Any clues as to what is going on.

    Thanks for your time,

    pmaczik, Dec 4, 2012
    1. Advertisements

  2. pmaczik

    Ken Guest

    Does your computer display properly BEFORE it enters Windows? See if
    you can boot from a CD and have the LCD display the characters properly.
    That will tell you if you have a problem unique to Windows or not.
    Ken, Dec 4, 2012
    1. Advertisements

  3. pmaczik

    Paul Guest

    Since you've done the "shine a flashlight" test
    and the LCD is actually loading the pixels
    with the right information, your problem
    is with the backlight (source of light
    that normally drives the display).

    As you surmise, on a CCFL lit monitor, the
    backlight subsystem consists of one or more
    cold cathode fluorescent tubes (thin, and as long
    as the monitor is wide). A power inverter,
    converts a low voltage DC, like 12VDC inside
    the monitor, into a 700-1000VAC voltage at
    a low current. Each CCFL tube draws around 3 watts
    of power at that high voltage.

    The inverter accepts low voltage DC as input, but
    also accepts a control signal that sets the intensity.
    The intensity control can be PWM or pulse width
    modulation. The PWM might be at 100 Hz, while the
    high voltage is at around 25KHz (above human hearing).
    The picture here, is intended to show a roughly 50%
    duty cycle driving signal for the CCFL (bursts of
    25KHz voltage). I can't draw an accurate waveform
    this way, without it being many pages wide.
    _ _ _ _ _ _
    ____| |_| |_| |________| |_| |_| |________

    A typical situation, is a user notices the
    light comes on for two seconds, then goes off and
    stays off. Power cycling the LCD monitor, might
    give another two seconds of operation.

    The startup of a CCFL has two phases. The higher voltage
    "ignite" phase is first. There is no mercury vapor at
    first when the tube is cold. The striking voltage is
    higher as a result. After two seconds, the plasma inside
    the tube, has more of the volatile mixture flying
    around, so the tube conducts a bit more. The inverter
    voltage drops. It is at this point, that a weak inverter
    just shuts down instead of continuing the "burn" phase.

    Reducing the brightness setting, sometimes gives relief
    from shutdown. For a few weeks. But eventually, the
    inverter will quit altogether.

    If the load is temporarily removed from an inverter
    (like the wires from the CCFL tube are loose), the
    voltage coming out of the inverter can shoot up,
    causing damage to the inverter. On the old
    piezoelectric inverters, they'd go up to 4000V
    and then the crystal would crack and it would be
    ruined. On the transformer based ones, I suppose
    they could arc between windings and be ruined.

    They're sensitive to stray capacitance, because
    the inverter connection to the CCFL is via
    capacitive coupling. Only pure AC is allowed to
    appear on the terminals of the CCFL. Any DC component
    (or apparently, even some harmonics), can cause
    degradation of the CCFL and a shortening of its
    life. If you drove the CCFL with a DC supply and
    no AC at all, it would probably be ruined by the
    end of the day. In normal usage, they last for
    something like 25000 hours. But various kinds of
    defects in the inverter, can shorten that life.

    Few people suffer from "worn out" CCFLs. If you
    had one of those, the tube light turns "brownish"
    as a hint. The vast majority of the time, the inverter
    is cutting out.

    Now, decent sized monitors, can use multiple CCFL
    tubes, and multiple inverters. It would have to be
    a relatively small monitor, for one inverter to die
    and wipe out all the light. Some of the big monitors
    I've seen at Best Buy, have 16 CCFLs and
    4 (quad) inverters, and you can get a
    "sun tan" in front of them :)

    Companies like this sell replacements. But, these
    are typically "generic" replacements, and we don't
    know how well they match the load. I'm only showing
    this page, to show a picture of the inverter. This
    one is long and narrow. Some (multi-channel) are
    perhaps a bit wider and shorter in length. The image
    on this page, has been altered by the website, to
    hide component details. The extensive usage of the
    word "US" in this advert, means you're dealing
    directly with China :)

    To learn more about the art of inverter design and
    the careful packaging of CCFLs, you can try this
    book. This guy analyzes failed efforts by other
    individuals, to build devices with CCFL backlights.
    So sometimes, your equipment is doomed, by the
    nature of the stupid design decisions made in it.
    When you see bits of tin foil inside the LCD housing,
    put them all back the way you found them. They're
    there for a reason (sometimes a good reason, and
    sometimes not).

    I also have this in my archive. Possibly the same
    content, and a bit easier to read. The author, Jim
    Williams, I think he appears occasionally over on
    sci.electronics. Note/an65f.pdf


    The other kind of monitor, uses LED (light emitting diode)
    lighting. The LEDs could be tri-colored or the
    light source could be white LEDs. A white LED,
    if driven hard, has a life of around 50,000 hours
    or so. The failure modes could be entirely
    different, than the ones you see on a CCFL
    monitor. There is no inverter. I don't think
    I've run into anyone yet, who has needed to take
    the back off one, so we don't know what's inside :)

    Paul, Dec 4, 2012
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.