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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Hi all.
Among my other goodies I now have 12 20" CCFL tubes, harvested from a WII'd to death Panasonic IDTV.
Am trying to determine if they can be driven from the original (bulky) drive circuit or something smaller, but it appears that the drive is bipolar (+/- HV around 6KV to strike then dropping to around 600V) with all tubes in parallel.
Would I be better off using individual drives as they are to be fitted inside clear plastic pipe onto an outdoor umbrella ?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I'd say you were unlikely to design and build anything smaller and cheaper than what Panasonic have optimised for the applicaiton. I'd just go with whatever drive board is there, or copy their design if you want more!
Are you sure the tubes are in parallel? I thought gas discharge tubes had a negative slope to their resistance, so wouldn't naturally share current?!?!
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Unless the tubes are EEFLs, which draw the correct amount of current without external ballasting as long as the drive is right, there has to be some form of ballast in series with each tube, usually in the form of a capacitor. Richie is correct, CCFLs don't share current on their own if directly paralleled, after the first one strikes, it will pull down the voltage, preventing the others from striking, and even if you could get all of them to strike, they wouldn't share current, because of the negative resistance characteristic Richie mentions.
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
Bear in mind that you will have problems if you try to extend the wiring from the inverter to the tubes due to stray capacitance, so if you are spreading them out you may hav no choice but to use seperate inverters close to each tube. While we're on the subject, I've recently been involved with an installation containing 1520 little CCFL tubes
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Not bad! But CCFLs take several seconds to a minute to come up to full brightness, they need to warm up and get the mercury vapour pressure up. Surely that would mess up your greyscale. How do you cope with that?
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
There is a noticeable delay in reaching full brightness when started from completely cold, but once going, the brightness is pretty consistent, and it seems that the cool-down time is somewhat longer than the startup time - of the order of a few tens of seconds. This unit will mostly be displaying scrolling messages, so this shouldn't be a problem.. I would like to try some video at some point, just to see what it looks like! As the tubes are viewed directly, any differences in brightness are to some extent masked as the light source is small - you see similar effects with LED arrays with & without a diffuser - with a diffuser, variations in colour and brightness are much more noticeable, but without, the small intense sources tend to saturate the eye.
Control is by PWMing the DC supply to the inverters- if warmup time turns out to be an issue we can 'simmer' the "off' ones at a few percent to keep them warm. More pics here
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Linear 20" CCFLs driver?
How fast can they be addressed?