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# The dilemma of the dimming LCDs

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re: The dilemma of the dimming LCDs
1/25/2011 5:29:00 PM
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SUNSPOT is the name of my company - I do custom product design, typically by word of mouth. No website yet, no catalog of products (soon!). Right now I'm finishing the interior of our cabin addition. 'SUNSPOT' from Bob Seegar's 'Sunspot Baby' song.

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re: The dilemma of the dimming LCDs
1/17/2011 5:28:42 PM
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the coupling cap eliminates a DC component that would plate out the mercury in the CCFL that aids ignition. The LC of the AC secondary, the cap and the CCFL load is resonant and swamping this circuit with a BFC would defeat this operation. There are other styles of HVAC supplies that brute-force the CCFL; this wasn't one of them.

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re: The dilemma of the dimming LCDs
1/15/2011 10:54:45 PM
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Something I learned many years ago from a great teacher. To electricity, everything has some R, some C, and some L, even when it is not supposed to. It is just a matter of how much relative to each other. At high voltage, insulation starts to conduct. At high frequency, C between 2 conductors allows current to flow, etc. Just EE 101 basics. Do they teach that anymore? Or is everyone just a code jockey?

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re: The dilemma of the dimming LCDs
1/15/2011 9:56:47 PM
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The challenge is always that somebody in charge wants it to be cheaper. The result is usually that cheaper equates to less suitable for the application, less margin for variation, and usually a shorter time until failure. One way to control tube current is to use resonance, and have an inverter that changes frequency with load. This means that the lamp starts with a high voltage, but as it conducts and loads the inverter the frequency changes away from the resonant peak. It may not work in every instance, but it is worth considering.

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re: The dilemma of the dimming LCDs
1/10/2011 5:40:46 PM
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Good story, also use low capacitance cable(thick insulation and far apart)and keep those HV lines as short as practical.

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re: The dilemma of the dimming LCDs
1/10/2011 8:48:59 AM
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I had a similar thing recently - an inverter that ran an 8W fluorescent tube off a 4.8V battery. A bit more beefy than an LCD backlight but very similar principles. It used a 680 pF cap in series (no starter) and when I used a long length of twisted wire between the inverter and the lamp I had a noticeable dimming. You want a high voltage to ensure the tube/CCFL starts, but that means a low cap (Hi-Z) to limit the current to a value that won't overload the inverter or burn out the tube. The frequency is usually in the tens or hundreds of KHz. I guess its our old friend the trade-off....

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re: The dilemma of the dimming LCDs
1/9/2011 11:33:17 AM
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An AC power supply that is susceptible to a few pf of load sounds suspicious. Why not increase the ballast (blocking) cap instead?

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re: The dilemma of the dimming LCDs
1/8/2011 12:59:55 AM
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Like most successful diagnostics, there is a good deal of value in understanding just exactly how something is supposed to be working. The better one understands how it works, the more valuable all of the diagnostic information becomes in the hands of one who understands how it works.