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'[EE]: help with CCFL backlight supply circuit'
2001\02\18@213736 by JB

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face
Hello,

I'd like to get one of these tubes up and running from junkbox parts. I have
no transformer (designed for the purpose), no SCRs, and no good circuits for
generating the HV. On the other hand, I do have a huge spool of some tiny
magnet wire and some forms, plus some small-signal transistors, so I could
put together something ugly for now. From the looks of those inverter
boards, they don't consist of much more than that. I need a reasonably
simple circuit, so some guidance would be most helpful. A search of the 'net
was not too revealing.

The tube itself is white, about 5" long, and 1/16" diameter. Single lead on
each end, no other markings. Unit pulled from working equipment, which I
don't have. The entire backlight is about 4" x 5". See link below.

Somewhere, I saw a tube like this that required a start voltage of 1200V,
running at 700V at perhaps 500Hz? That could be way off. I can afford to
waste a few, so I'd like to err on the high side and cut it down from there.
These are freebies from a friend, thirty in total. Will share some for the
first junkbox-grade supply circuit that works.

One thing I do know - holding one end in my hand, and the other to a
television screen will cause brief flashes.

       http://www.rof.net/yp/alphaone/ccfl.jpg

TIA,

JB

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2001\02\19@101047 by Olin Lathrop

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> I'd like to get one of these tubes up and running from junkbox parts. I
have
> no transformer (designed for the purpose), no SCRs, and no good circuits
for
> generating the HV. On the other hand, I do have a huge spool of some tiny
> magnet wire and some forms, plus some small-signal transistors, so I could
> put together something ugly for now. From the looks of those inverter
> boards, they don't consist of much more than that. I need a reasonably
> simple circuit, so some guidance would be most helpful. A search of the
'net
> was not too revealing.

Fluorescent tubes are really current devices.  They need a high voltage to
get going initially.  Once the mercury vapor is exited, the resitance drops
dramatically.  You need to find specs for the starting voltage and the
operating current.  If you don't have specs for your tubes, look up the
specs for tubes of similar length and diameter as starting points.  The
parameters are mostly a function of the tube geometry because most tubes use
pretty much the same mercury vapor inside.

Note that small two terminal tubes are different from the large 4 terminal
tubes.  These larger tubes have heated cathodes.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\02\19@110908 by JB

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So - similar to a HeNe laser tube, except AC instead of DC?

Like I noted in my original message, the only specs I have seen were for a
tube of similar size, and they suggested 1200V starting, and 700 running at
5mA. I'm guessing that in a pinch, one could use a large value, high-wattage
resistor to limit the current on the HV side.

I did eventually find some inverter circuits here that may work:

       http://www.repairfaq.org/REPAIR/F_samschem.html#SAMSCHEM_008

JB

> {Original Message removed}

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