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Old 07-27-2010, 08:48 PM  
PlainBill
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Default Re: LG Flatron 1950s-sn 2 sec to black

Quote:
Originally Posted by sceva
I plugged in two sets of working ccfls. They are the same length or smaller than the originals. When turning on the originals I don't think they come on full bright, but they are on so short it is hard to tell for sure.

I have repaired several monitors, but still a lot I don't know - can you elaborate on the "sense circuit"?
A common description of a CCFL inverter is 'A constant-current high voltage AC source'. A constant current source will tend to destroy itself if it doesn't have some provision for limiting the output voltage. Also, some provision must be made for the rise in current when the CCFL reaches the end of it's life, especially if there are multiple CCFLs driven by one inverter.

I've attached the schematic of a typical inverter. Note the circuit running from pin 2 of CN2, through CR1, R16, R15, R13, and to pin 13 of U2. The voltage on that line is proportional to the current through the CCFL. As the voltage on pin 13 rises above the set point, the drive to the transformer decreases, reducing the output voltage. Thus, constant current.

Next, look at the circuit from pin 1 of CN2, through C8 and C13, CR2, R14 and C16, and to pin 2 of U2. As the voltage out of T1 rises, the voltage at pin 2 of U2 rises. If it rises above a set point, the inverter shuts down and 'latches' - stays off until the control input turns it off, then on again. That protects against a voltage that goes too high.

Other designs sense the current through the CCFLs and shut down if there isn't enough current through a CCFL. Different design, same reason.

Many inverters have another circuit in parallel with the current control circuit. If the current through any CCFL rises too high, the inverter shuts down and latches. Sometimes they trigger

'Two Seconds to black' occurs when the sense circuitry detects a problem - output voltage too high, current through a single CCFL too high. Sometimes this occurs because of a broken CCFL, a broken wire to a CCFL, a CCFL that is dying, etc. And sometimes it occurs because one of the components is defective. For example, if C8 was leaky, the inverter would shut down shortly after the CCFL lit.

If you look at the back of your inverter, you may be able to see the equivalent of these components. I didn't work with that because it is very difficult to trace green circuit paths on a green circuit board.

Quote:
Originally Posted by sceva
(My father in law had the same eye surgery. He didn't need glasses anymore, but had worn them for so long he felt naked without them!)
I know how your Dad feels. I've worn glasses since I was (roughly) 8 years old. The first thing I did when I woke up was grab my glasses. Now they aren't there!!!

PlainBill
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File Type: jpg Inverter 1.JPG (47.0 KB, 467 views)
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Last edited by PlainBill; 07-27-2010 at 08:52 PM..
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