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PostPosted: 06.10.2008 11:02 
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Junior Modder
Junior Modder

Joined: 22.04.2003 12:45
Posts: 12
Location: Finland
Need to make my car-pc LCD to dim automatically at nights and first thought was of course to add resistor to the backlight inverter power supply.
Question is: Is it harmful for the CCFL to do this? I've read many people has done this and with too big voltage drop the light wont lit fully or will flicker and such, so will it shorten the life time of the CCFL and is there a better way to control the backlight brightness dynamically?

Thanks in advance!

Ja varoiksi sama lyhyesti suomeksi:
Kärsiikö LCD:n taustavalo (kylmäkatodi) jos himmennän sitä alentamalla invertterille syötettävää jännitettä? Onko parempaa tapaa kontrolloida taustavalon kirkkautta?


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PostPosted: 08.10.2008 00:27 
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Site Admin
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Joined: 24.01.2002 10:09
Posts: 4683
Location: Finland
It is quite common to dimm the CC lights with a resistor on the low voltage side... Can't say that I have heard people having problems with them though. It should be quite safe and on the other hand, these kind of lights are not that expensive to replace. Sorry that I don't have any numbers to give you on this. :(


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PostPosted: 09.10.2008 18:24 
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Junior Modder
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Joined: 22.04.2003 12:45
Posts: 12
Location: Finland
Thanks Japala!

Tho as I made some tests with potentiometer, I noticed that this specific inverter didn't seem to work well with lower voltages. I was only able to get a very gentle dim before it started flickering badly / shut down. Maybe I'll give another shot with a more accurate trimmer.
If someone could tell how with low voltages you have been able to feed it without severe flicker, please post.

Replacing the backlight with leds would of course be the best solution but I think it would need quite powerful leds for adequate brightness in daylight, and that would need quite a lot more power. I measured ~50mA current draw (12V) to the inverter. Panel is 8".


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PostPosted: 09.10.2008 22:00 
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Regular Modder
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Location: Pori *** CPU: Intel E8400 RAM: 4 Gigabytes HDD: 596 Gigabytes GFX: 8600 GTS
I'm no expert on this but it just came to my min that maybe you should try to limit the current that flows after the inverter.
If you adjust the voltage before invertor, it dramatically changes the output voltage.... so adjust just that output. : )

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PostPosted: 09.10.2008 22:05 
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Junior Modder
Junior Modder

Joined: 22.04.2003 12:45
Posts: 12
Location: Finland
Might be, just that I dont have a single clue what voltage it outputs. None of my multimeters could show anything else than overflow when trying to measure the output (also gave nice sparks). I think it's around ~150V AC but maybe it's so high frequency that my multimeters go nuts.

Anyhow, I'm not sure if it's a good idea to choke things from output side? Japala?


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