traumajohn said:
Hi,
I think this is an appropriate forum for this question. I have a Dell
running XP. LCD display worked fine until today. The laptop top cover had
some loose mounting screws so I dismantled and tightened while checking wire
condition and connections. Everything looked good. I see on boot a very light
image as it starts to boot then can no longer see any images in the
background. I connected to an external display and all is well. Is there a
way to find out if the problem is the backlight or the inverter? I never saw
discolorations or flickers. One day fine the next day very dark display.
Thanks,
John
The light source, consists of an inverter and a backlight.
(Very few laptops have LED backlights yet.)
The inverter converts 12V to somewhere between 600 and 1000VAC.
The power level is only about 3 watts, so the current is low.
The output voltage of the inverter varies a bit, as the load is
dynamic. When the tube is cold, a higher voltage is applied to
the tube. After two seconds, the voltage level may drop. And that is
when some inverters decide to shut off. It means they
cannot maintain the "burn" voltage, and are overloading or
weak. The "strike" phase works, but after two seconds,
they shut off while delivering the lower "burn" voltage.
In addition to the basic operation of the inverter, there is
a signal to control the output. With an analog signal (like
an adjustable knob on an older laptop), the voltage is
varied over a small range. The user may be able to reduce the
backlight by 30% with a scheme like that. The sine wave from
the inverter, is varied in amplitude by a small amount. If
adjusted too far, the tube will extinguish. The sine wave in
that case, operates continuously.
More modern inverters, use PWM (pulse width module) as a control
method. The high frequency AC output of the inverter, is
modulated. The output frequency might be sine waves at 25KHz
(some inaudible frequency), but the modulation might be applied
in the hundred of hertz. It would look like this. PWM allows the
screen to be turned down more, before the "burn" isn't enough to
keep the tube lit. The parts of the waveform with the sine
waves, are full amplitude. The duty cycle gives the appearance
of lower average intensity.
<---sine---> <---sine--->
waves waves
_ _ _ _ _ _
_| |_| |_| |______________| |_| |_| |______ 50% duty cycle pulsed operation
<---light---><--- dark--->
If the screen is dim and discolored, that could be a bad lamp.
But based on the report of symptoms here, the inverter is much
more likely to die, than a lamp.
An LCD panel can be set up to emulate the symptoms. For example,
if all the digital values used to paint pixels, where shifted, or
scaled or whatever, then you could make a dim image. But then,
you might see "light leakage" around the edges of the
screen, which would tell you the backlight was still doing
its job, and the problem was elsewhere. If you don't see
the normal level of leakage around the edge of the panel,
then it could be the inverter/lamp doing it.
When playing with the lamp, all the components found in that
area have a purpose. For example, you may find some foil
in the area of the lamp. That may affect how the thing
works, so should not be discarded. And if you short the
high voltage to ground, then obviously the lamp is
going to stay dark.
Some inverters have another failure mode. If the connector
falls off the lamp, and the inverter is operated with no load,
the voltage can shoot up. When the voltage hits about 4KV,
the piezoelectric crystal can crack. Some inverters use
coils and others use crystals to make the voltage. Depending
on how crudely the inverter is controlled, it can have
a failure mode like that. To give another example of a
piezoelectric crystal that generates a high voltage,
consider the starter for a gas barbecue. it uses a
piezoelectric crystal. But it isn't operating in a
continuous mode like in the inverter application.
http://www.pittjug.org/b2b/pics/Kitchen_Barbecue_Lighter.jpg
This is an example of the piezoelectric transformer in some
inverters. You can see the warning here, about operating the
thing without a load.
http://www.tamura-ss.co.jp/en/electronics/piezoelectric_ceramics/pdf/solid_former.pdf
"The unit may be damaged if a load is open when it starts driving."
This is an example of an inverter that uses a transformer,
so there is no piezoelectric crystal on this one.
http://www.mp3car.com/vbulletin/att...83d1145329210-backlight-inverter-inverter.jpg
It is possible, that "fine one day, dark the next", is an
inverter symptom.
HTH,
Paul
