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Testing CCFL tubes and invertors

Any good tips for testing LCD inverters and the CCFL tubes they hook to?

I am working on more of these and think i need to rig up an external inverter to test the tubes in a LCD unit in some manner. When these tubes age and degrade, do they just get weaker like home units, or do they take more start voltage or current? How do you really gauge when the bulbs are really bad?

What i am trying to trouble shoot in some cases of units starting up for a few seconds and shutting down whether its the inverter, or because the lamps are bad the inverter is shutting down but otherwise ok.

I have seen some information that its hard to test the inverters because a slight load on the outputs affects there operation. Should most scopes have a high enough input z on 10x to do this?

Anyone that has tried some of this please let me know what your results are.

The gateway i posted about (and got no responses) try's to start up but shuts down. Caps were OK, and I soldered up what looked bad. So, short of ordering new bulbs for it i need to know which part i can eliminate as the problem.

What i thought about is making a external jig or such to power each bulb up and seeing if they light. Is there a way to sub a dummy load on the Inverters for the bulbs to test? It would have to have a high voltage zenar with high z series resistor or such on it to simulate the CCFL plasma voltage of conduction?

Bob

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Reply to
bob urz
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In my limited experience, most of the time the inverter (or a fuse on it) is bad. However, on my laptop, when the CCFL would light up for

15-300 seconds and then go out, the problem turned out to be the tube itself. Once the tube was replaced, it stayed lit.
Reply to
Caesar Valenti

I usually find bad caps or solder connections. But i don't have enough spare parts laying around to swap stuff in and out. It seems many of these inverters have multiple protection systems to shut down the inverter for over current or over voltage conditions. Short of having known good replacement lamps, i will have to get the spec sheets on the controller chip (if it can be ID'd) and see if i can hot wire the protection to see if the lamps stay lite.

There is going to be a lot more of these units floating around broke, so i guess need to bone up on how to trouble shoot them. It will be a little difficult to make a generic test supply, since the lamps draw different depending on how long they are and such. You almost need a storage scope or a data logger to see the lamp ramp up voltage, lamp strike voltage, then the operating voltage. Then how to monitor the voltage without loading down the power supply.

Maybe sam can add a bit to his site on these issues once researched...

bob

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Reply to
bob urz

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I did find a few links to the inverter controller that were somewhat helpful(oz960) Now,to figure out how to use that information to trouble shoot.

It may be possible to hot wire some of the protection circuits to do some testing. or monitor the controller pins to figure out what shutdown mode it is in.

bob

Reply to
bob urz

On Sun, 27 Jul 2008 13:55:16 -0500, bob urz put finger to keyboard and composed:

FWIW, this manufacturer tests their inverters with a 120K resistive dummy load:

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It simulates a 6mA, 720V lamp.

This one uses a 110K dummy load (for 6.7mA lamp current):

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This one specifies a 100K dummy load for 6-7mA:

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I'm not sure that such a steady state test would be conclusive, as it may not test the inverter's ability to generate the much higher striking voltage.

AFAICT, the current rating of an unknown lamp can be inferred from the current sense resistor (say 470 ohms) in the lamp circuit. It seems that many lamp controller ICs expect to see a 3V feedback voltage across this resistor.

- Franc Zabkar

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Reply to
Franc Zabkar

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