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Backlight Dummy Load???

I pick up free lcd monitors at garage sales and fix 'em. Biggest problem is getting them to run while you have 'em disassembled. Seems as if they TRY to make it hard to run 'em.

Would be much easier if I didn't have to hook up the backlights. But running the inverter with out a load is surely a BIG STRESS on the transformers and maybe the driver.

What's a good dummy load that I can put on the connections to keep the voltage spikes from arcing/shorting the transformer secondary? The lamp load is decidedly nonlinear. And there's several watts to dissipate.

Cutting the power supply trace to the inverter works, as long as the problem isn't in the inverter supply, but it's kinda brute force.

I have a bunch of laptop backlights, but I'm afraid to use them as load for bigger monitors. Big current difference.

Anybody successfully constructed a backlight proxy (dummy load)? Explanation?

Thanks, mike

Reply to
mike
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I've just put a cheapo, relatively large lamp on a scavenged connector (with lots of insulation, of course; there's kilovolts there). JKL made a bunch of inverters available standalone, and a few lamps, and there were only a few HV connector types we had to deal with, so it was possible to test lamps with known-good inverters, and inverters with known-good lamps.

Reply to
whit3rd

Some turn off if the backlight not running, too. Backlight CCFL is run on constant current (there's a little rectifier/smoother in the HV return lead), so perhaps a smallish value power resistor in series with a 400V or so TVS diode (bidirectional or two unis in inverse series) would be a nice load? Say 470R to 2k2 HV resistor + TVS?

Given that the HV is current regulated, maybe they run at similar currents? Longer tubes would have higher voltage? Tube diameter seems the same (tiny! 3mm?) on the few I've come across. I see two in parallel on larger screens rather than one CCFL with a larger diameter.

Not tried it (yet), but what I wrote above seems logical start point, try it on what spares you have now?

Depends on what the fault is, for how far I'm willing to dismantle an LCD, usually obviously stressed power supply caps, or blown CCFL inverter needing replacement tank cap (thanks to Internet goodness for that fix :) I've been able to connect CCFLs and all for testing before reassembly. Haven't felt the need yet for dummy CCFL.

My repair success rate is 5 1/2 of 7.

The half working one is usable but has odd shimmering pixel effect in one corner no one here could help with at the time. Here's the web page in case somebody has seen the effect, but missed my first post:

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I didn't dismantle further because the LCD is usable as it is, hasn't got worse over last few months. I'd have another try if I received some positive information, or get another screen so it matters less if I bust this one on further dismantling. I'm wary of breaking the film to glass connections by taking the metal frame off. My working space is a little cramped.

Grant.

Reply to
Grant

Why not borrow two tubes from a broken screen, mount them in a box, and use them as ballast?

Reply to
Sjouke Burry

Make a small lightbox for checking PCB prints, useful too ;)

Grant.

Reply to
Grant

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