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Thread: Xenarc CCFL brightness reduced after modification - please help me debug

  1. #1
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    Xenarc CCFL brightness reduced after modification - please help me debug

    recently I extended the high voltage wires from the inverter to the CCFL on my Xenarc 700TSV. I'm still alive after the experience and so is my Xenarc. I used hi-flex PVC insulated test lead cable. The extended cable run is about 8 inches long. There's no crackling, or arcing that I can see going on but my fear is that somehow the voltage is being lost along the run.

    My problem is the CCFL brightness now seems to be low. I like to be as precise as possible about things but I have no way of measuring how low it actually is. My Xenarc has been in bits for best part of 3 months so I can't remember how bright it should be. I know these screens are not exceptional in this respect (hence the upgrade project), but this just seems too dim.

    Using the OSD menu I have checked that the backlight setting is on 99. Indeed reducing it reduces the backlight intensity even further so the control mechanism seems to be working.

    I've read about people saying that the Xenarc benefits from being supplied from cleaner power supply than may be available from a car, so I'm supplying it from the output of a 12V linear regulator. It says in the manual that the screen's minimum DC input voltage is 11V. I've just measured the input on my screen and it falls to 10.2V when everything's on. I think this is to do with the regulator's internal loss as my bench power supply is 12V. Technically this is out of spec so maybe that's my problem. This is something I'm going to investigate and I'll probably try bypassing the regulator on the bench to see if that extra 1.8V makes any difference to the brightness. Anyone have any anecdotal evidence that backlight brightness on a Xenarc is a function of input voltage in the range 10-13V?

    I suppose I really need to be able to measure the output voltage from the inverter, but I suspect that's not easy with basic test equipment. Maybe there's some kind of resistor network I could apply to it to scale the voltage range down so that it's measurable with a standard digital meter? That then begs the question:- what voltage am I expecting to see out of the inverter?
    If that method works then I guess I could then tap off the voltage at the CCFL end of the extension and see if there's been a voltage drop alon the wires. BTW are CCFLs supplied with AC or DC? and if it's AC, what sort of wave is it?

    The other thing I wondered was whether there is any way I can probe the (low voltage) 4-pin input connector to the inverter board to test for expected voltages. I don't know the pinout of this - maybe someone could assist. I assume it's got to be a supply rail, a ground, a brightness analogue/PWM input and something else? I just wondered if it was this side of the inverter where the problem lies.

    Any help greatly appreciated.
    James

  2. #2
    Variable Bitrate
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    the wires from inverter to buld may not be lengthened on a ccfl tube
    time for the sig:

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  3. #3
    Variable Bitrate NRGZ28's Avatar
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    Agreed. Those wires need to be as short as possbile! They're very high resistance wires...
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  4. #4
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    okay, I think I can help.

    1.) 10.2v will result in reduced brightness. 11v is the minumum, but even that is low. you need to have 13-14v for ideal brightness.

    2.) lengthening the wires is fine, as long as you use good enough wire. you seem to have used the proper wire, as yours is rated at 1500v. I do ask, though, why didn't you lengthen the low voltage side?

    @NRG: the wires pass a few milliamps. Ohm's law:

    V=I*r

    even if the wires were 10 ohms, which would be terrible:

    10*.005=.05v

    Hardly an impact when you're talking 600-800 VAC

    3.) If you have a fluke DMM or similar, you can test. It should be around 1000v starting, and 700v running. You can apply a resistor network if you're careful, but chances are you won't maintain 1000v of dielectric insulation.

    4.) 12v, ground, ground, 3-5v PWM for brightness.

    good luck

  5. #5
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    greenman100:-

    thanks so much for your input on this, I've definitely got things to go at now.

    Unfortunately I can't lengthen the low voltage side because of space constraints around the LCD module. I physically need to get the inverter away from the module. It's for a homebrew Xenarc to motorised in-dash conversion so there's just no room on the sliding plate to put the inverter.

    the pic makes it clearer...

  6. #6
    Fusion Brain Creator
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    I have personally extended wires about 8", but I also replaced the inverter while I was at it.

    Let me know how it goes for you. If need be, I could swap inverters with you, and give you my cabling

  7. #7
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    Well I went out and bought a decent bench supply with variable voltage to see what effect varying the Xenarc input voltage has on the backlight brightness.

    I found that:-
    • The Xenarc doesn't regulate the supply voltage to the inverter, it just passes it the supply voltage directly.
    • CCFL brightness only started falling-off below 11V (this agrees with spec) and was essentially constant up to 15V
    • Rapid changes in input voltage (say from 11-13V) would cause momentary increase in CCFL brightness. I conclude from this that the inverter has a slightly noticable feedback loop that corrects for input voltage fluctuations. This highlights the point made in the 'powering your LCD monitor' sticky that you may see brightness 'blips' as your engine revs (& hence supply voltage) change.




    For the second graph I monitored the brightness voltage pin whilst adjusting the Xenarc's On Screen Display control for backlight (an integer from 0 - 99). The actual curve isn't as linear as that but you get the point.

    Remember I did this to debug my dim screen so I'm sort of hoping someone might notice an anomaly with my results. I'd particularly like to validate the transfer characteristic of the brightness control voltage.

    Unfortunately for me though I didn't solve my reduced brightness problem so I'm going to rip the whole thing apart and re-locate the inverter close to the LCD to see if it's my wiring

    James

  8. #8
    MySQL Error Scouse Monkey's Avatar
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    hey good work with the plotting the inverter stuff.

    The control voltage range will be 0V for full and 5V for dimmest as you have seen.

    I would not recommend testing the inverter with anything rated for less than 1500V just in case. The Vstart should not be over 1000V but you never know. You could always try and test the votlage once the tube is running as then it should be about 600V but as we dont know what it shoudl be this is pretty pointless.

    the tube brightness is controlled by a PWM output so you would need to check this anyway.

    I would say it's probably the way you have joined the wires. You should get some proper high voltage inverter wire and properly insulate any joins. Also be careful how you route the cables as metal can cause losses. up to 6-8" should be fine to be honest. I know this may seem long but i was amazed when i was told it's fine.

    It is interesting to see the xenarc doesnt regualte the input voltage. The inverter should be run at 12v. Any more and it is shortening the life of the CCFL.

  9. #9
    MySQL Error Scouse Monkey's Avatar
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    I just noticed you are in the UK from the RS link. you could see if you can get some proper CCFL cable or CCFL extensions with proper connectors on each end from visualux in cornwall.

  10. #10
    FLAC coucouillu's Avatar
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    Quote Originally Posted by jwexqm
    greenman100:-

    thanks so much for your input on this, I've definitely got things to go at now.

    Unfortunately I can't lengthen the low voltage side because of space constraints around the LCD module. I physically need to get the inverter away from the module. It's for a homebrew Xenarc to motorised in-dash conversion so there's just no room on the sliding plate to put the inverter.

    the pic makes it clearer...
    I have had the same problem when I did my indash screen.
    You have a private message.

    It seems that you have a motorized mechanism.
    From wich unit have you take it?
    personnaly I have taken it from this unit:



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