Another Capacitor Value Question

Firstly, thanks to all you GURUS that help us out in this newsgroup from time to time.

I have another Cap question for ya,

This guy is marked 10J 3KV

I know that it is rated at 3000 volts of course, but the 10J has me stumped.

This is used in the final stages of a CCFL circuit in a LCD monitor. I suspect that these are leaky and causing the HV to shut down after a couple of seconds.

Thanks In Advance,

- Tim -

Reply to
Tim
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Tim wrote in news: snipped-for-privacy@allnews.nbnet.nb.ca:

10J means 10 picofarads at 5% tolerance.

I am NEVER a guru, but I wanted an answer too so I looked and found this:

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Reply to
Lostgallifreyan

With my background, I would have taken the 10J to mean 10 Joules. My background is in lasers and 10J at 3kV seems about right to pump flashtubes for many small pulsed lasers. A helpful clue would be an indication of how big the capacitor is.

I do not understand why an LCD would need a 3kV capacitor. But I do not know that much about the nitty gritty of LCD displays. But I do not know what CCFL means either.

Enlighten me.

Bill

-- Fermez le Bush

Reply to
Salmon Egg

Tim,

Some pics of a typical CCFL inverter on this page

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The bottom 3 pics indicate a simple 2 transistor oscillator and step-up transformer arrangement. A small blue high voltage cap is next to the output socket. Is your unknown cap similar? If so then the answer of 10pF 2kV proposed by Lostgallifreyan sounds right.

Reply to
Ross Herbert

Salmon Egg wrote in news:C1814FB3.4B2F9% snipped-for-privacy@sbcglobal.net:

Me too. :) Not with that level of experience, but it meant the same to me too. And most of Google's results. I think that's maybe why the info wasn't easy to find. I saw two occurences of 'picofarad' that helped me narrow it down to find a decent page about it.

Cold Cathode Fluorescent Lamp. Those thin bright tubes used to modify computer cases, car hi-fi's, backlight LCD's and give good spread of light in tight spaces generally. They use a high voltage high frequency AC to drive them, I think, closely related to the suppies used in electroluminescent displays.

Reply to
Lostgallifreyan

As I have a CCFL in an existing design...

Typical CCFL drive is at between 20kHz and 100kHz (it varies according to load amongst other things) and the AC output ranges from perhaps

400Vrms to 750Vrms depending on model. [You have to choose the appropriate parts for each different one or have quite spectacular results ;) ]

Note that during startup (strike, as it's known) the terminal potential can be as high as 1.5kVrms.

Quoting the datasheet for my CCFL for which I designed the inverter: "The Non-load output voltage (VS) of the inverter should be designed to have some margin, because VS may increase due to the leak current which may be caused by wiring of CFL cables. (Reference value : 2015Vrms Min.)"

The spec table states that the unit will strike at 1550Vrms worst case,

0C.

As most (all?) controllers have an internal sequencer with operate and short detection, a flaky output cap will cause the unit to shut down almost immediately (the controller will sense a short). Usually the strike is tried 3 or 4 times before autoshutdown (assumes no current drawn), but heavy loading usually operates an electronic circuit breaker.

Hope that helps

Cheers

PeteS

Reply to
PeteS

That helps. I still hate cryptic acronyms.

Bill

-- Fermez le Bush

Reply to
Salmon Egg

PCMCIA -> People Can't Memorise Computer Industry Acronyms

;)

Cheers

PeteS

Reply to
PeteS

Salmon Egg wrote in news:C181F98A.4B444% snipped-for-privacy@sbcglobal.net:

There was a time when the same thing could be said of 'LASER'. :) Not that it would be true. New words can either come from Latin, Greek, Old High German even, whatever is a good root with long standing, or, they can be useful condensations. There is ONE reason the term laser became a word, and that's because someone had the sense to know you had to be able to say it. Similar things that formed true new words are Radar, Sonar. Some idiot tried with Lidar, but that's just a cheap derivative, it feels like one, no-one could actually drop that word in conversation without looking like a dork. Even TLA works better, as do some of the better TLA's like MRI, VGA, CRT, LCD... They can be said, they make sounds like words. Can anyone say CCFL as a word? Even the Welsh with practise at syllables with no vowels as most of us know them would have trouble with it, and would know, more than most of us, there is NO poetry to the term. In short, it sucks. If people really want their technically creative legacy to last, they must think this stuff through. It costs nothing, and being subject to copyright, it might be better and longer lasting than a patent.

/end rant. I like rants. Without them I am useless, and that one is mine.

Reply to
Lostgallifreyan

Yup, that's like the ones on my board. There are 4 of them, one for each output I guess, as they are located at each connector. I'll just replace them all just to be sure. The 3kv kinda threw me for a bit, but someone explained about that HV spike at turn-on now that makes sense too.

Thanks,

- Tim -

Reply to
Tim

Thanks for the info and the link. I bookmarked it for future reference.

- Tim -

Reply to
Tim

Hey now I know why the cap has that 3kv rating.

Thanks for your input.

- Tim -

Reply to
Tim

I think AIDS is the stupidest acronym ever.

Health aids? Home aides?

Reply to
Homer J Simpson

"Homer J Simpson" wrote in news:Q6u7h.15159$C94.2313@edtnps82:

Yep, that one IS hard to beat, is it not?

Reply to
Lostgallifreyan

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