Power hungry lcd CCFL tube

[jss]

Hi all,
My laptop lcd was becoming pink at power-on during a couple of minutes.

So I looked in ebay and bought a similar tube, 2mm thick by 305 mm
long. Replaced the tube to find that the inverter fuse blew. Replaced
it with a calibrated wire from a bigger fuse and everything seemed fine
apart a bit more heat which is tolerable by me once that this is an old
laptop close to its life limit.

Then I tried to operate the laptop on batteries just to find that the
inverter shuts off after a little while. After a thought I guessed that
by reducing the input voltage to the inverter, which happens when it is
operated on batterires, the input current rises once that the output
power is constant. So the conclusion is that it must be close to the
power handling limit. Not difficult to admit once that I had to raise
the fuse caliper.

As working only on AC is not acceptable, I decide to invest a bit more
time and measured the current of both the old and new lamps to be 6 and
13 mA. All the specifications that I could find for this inverter
states 6 mA at maximum brilliance, which is correct for the old bulb.
Tried to measure the voltage under load (bulb lit) but this is a bit
tricky although I have a good Fluke multimeter although not equiped
with High Voltage probe. Anyway, the measured voltage is about the same
with both bulbs, about 450 Vrms.

So, after all this, it's apparent that the new bulb consumes about
twice the power for the same light output, any comments on this and how
to overcome it in order to have my laptop working on battery also?

jss

 

[Jim Land]

Hi all,
My laptop lcd was becoming pink at power-on during a couple of minutes.

So I looked in ebay and bought a similar tube, 2mm thick by 305 mm
long. Replaced the tube to find that the inverter fuse blew. Replaced
it with a calibrated wire from a bigger fuse and everything seemed fine
apart a bit more heat which is tolerable by me once that this is an old
laptop close to its life limit.

Then I tried to operate the laptop on batteries just to find that the
inverter shuts off after a little while. After a thought I guessed that
by reducing the input voltage to the inverter, which happens when it is
operated on batterires, the input current rises once that the output
power is constant. So the conclusion is that it must be close to the
power handling limit. Not difficult to admit once that I had to raise
the fuse caliper.

As working only on AC is not acceptable, I decide to invest a bit more
time and measured the current of both the old and new lamps to be 6 and
13 mA. All the specifications that I could find for this inverter
states 6 mA at maximum brilliance, which is correct for the old bulb.
Tried to measure the voltage under load (bulb lit) but this is a bit
tricky although I have a good Fluke multimeter although not equiped
with High Voltage probe. Anyway, the measured voltage is about the same
with both bulbs, about 450 Vrms.

So, after all this, it's apparent that the new bulb consumes about
twice the power for the same light output, any comments on this and how
to overcome it in order to have my laptop working on battery also?

Eh, you just bought the wrong tube. Easiest to throw it away and buy the
right one. Look at a site such as

http://www.lcdpart.com/doc/ccfl.html

 

[jss]

Eh, you just bought the wrong tube. Easiest to throw it away and buy the

right one. Look at a site such as

http://www.lcdpart.com/doc/ccfl.html

Thank you Jim Land for your reply.

I considered that but after searching the net for many tubes of 2 x 305
mm, including the site you indicate, and I didn't find one that
consumes more than 7 mA, on the stated characteristics I mean.

So I assumed that there are no tubes consuming 13 mA and mine is
probably manufacture defective or faulty in some way; am I wrong?
I think the inverter is ok because the current is normal with the old
tube.

One other thing I didn't refer is that I can feel the tube warmer than
I would expect, more to the hot side, from the outside of the LCD.

Do you care to comment, or anyone else?

Regards
Joao

 

[Jim Land]

Thank you Jim Land for your reply.

I considered that but after searching the net for many tubes of 2 x
305 mm, including the site you indicate, and I didn't find one that
consumes more than 7 mA, on the stated characteristics I mean.

So I assumed that there are no tubes consuming 13 mA and mine is
probably manufacture defective or faulty in some way; am I wrong?
I think the inverter is ok because the current is normal with the old
tube.

One other thing I didn't refer is that I can feel the tube warmer than
I would expect, more to the hot side, from the outside of the LCD.

OK, your new tube is drawing too much current. Maybe it's designed that
way, or maybe it's defective, but either way, it doesn't matter, because
it doesn't work with your lcd. Throw it away. Buy a new tube (from a
different source!) and install it.

The point, after all, is to get your lcd working, correct?

 

[jss]

The point, after all, is to get your lcd working, correct?

Jim Land,

Many times I try to learn from facts of life that cross my path, I
guess we all do; this is called evolution.

But you're right, this is not worth it; the point is that I was just
being too cautious to avoid buying a new tube just to discover that the
problem remains; after all, a new tube shall travel a long way from a
shop in the US to my country in Europe.

Thank you for your guidance, I'll do as you said.

Best regards
jss

 

[James Sweet]

@m58g2000cwm.googlegroups.com:




Eh, you just bought the wrong tube. Easiest to throw it away and buy the
right one. Look at a site such as

http://www.lcdpart.com/doc/ccfl.html

Throw it away? Those things are cool, if you can't use it for the laptop
get a cheap surplus inverter and build a desk lamp, under cabinet light,
accent light, nightlight, etc.

 

[James Sweet]

I considered that but after searching the net for many tubes of 2 x 305
mm, including the site you indicate, and I didn't find one that
consumes more than 7 mA, on the stated characteristics I mean.

So I assumed that there are no tubes consuming 13 mA and mine is
probably manufacture defective or faulty in some way; am I wrong?
I think the inverter is ok because the current is normal with the old
tube.

One other thing I didn't refer is that I can feel the tube warmer than
I would expect, more to the hot side, from the outside of the LCD.

Well I haven't encountered differences like that myself yet, but it's
possible the tube for whatever reason was designed for a lower voltage,
which will often cause the current delivered by the inverter to be higher.

 

[Jim Land]

Jim Land,

Many times I try to learn from facts of life that cross my path, I
guess we all do; this is called evolution.

But you're right, this is not worth it; the point is that I was just
being too cautious to avoid buying a new tube just to discover that the
problem remains; after all, a new tube shall travel a long way from a
shop in the US to my country in Europe.

My apologies. I was not being sufficiently sensitive to the cultural
differences between the US and other countries. I was advocating the
contemporary US attitude of "throw it away and buy a new one." This is,
of course, wasteful. Many times, parts can be repaired, circuits can be
modified, or compromises can be made. If you can make your new tube work
satisfactorily, you will save money and have a good learning experience.

If you wish to experiment with the tube you have, you might try a
resistor in series with it, to reduce the voltage across it and the
current through it.

If the tube is too dim, or does not light, the resistance is too large.
If it is too bright, the resistance is too small. If the tube is the
same brightness as the original tube, the resistance is just right. If
the resistor gets too hot, replace it with a resistor with the same
resistance but higher wattage.

Best wishes,
Jim

 

[Ancient_Hacker]

Don't try measuring the tube voltage or current right at the HV
leads!!-- the circuit is very unfriendly, with wild voltage and current
peaks. It's likely to overheat your meter's voltage dividers and.or
the inverter.

You might have too high a current if the HV wires to the tube or the
tube itself are too close to any nearby metal. Try running the wires
EXACTLY as the used to run. Be sure the tube is EXACTLY where it used
to be, no closer to the LCD panel or the reflector tape.

you might also check the 22pf or so capacitor or capacitors in line
with the HV leads. If they short out, which is quite common, the
tube will run at really high current.