back lit LCD questions

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This is the first time I really started to mess around with one of these, but I
have a display that is not
working correctly and have some questions as to how this is 'suppose' to work.

I am measuring 4.15v on all lines for the display, even on the pin that the
segment is 'clear'.
What kind of signal is used to activate the segment as it does not seem to be DC
controlled.

The driver is ok, I swapped in another display and it work fine. Is it possible
that the display is not good (no physical damage, however),
or maybe there is something just 'goofy' with the display? Is it possible that
the connections inside the
display have been servered on the non-working segments?

Not sure if back lit LCD's work the same as standard one (have no experience
with either), but I kind of
assumed that you applied a DC voltage to each segment to activate it (like a
LED), but this does not
seem to be the case in this situation.

I hope that is clear...

Re: back lit LCD questions



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have a display that is not
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segment is 'clear'.
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DC controlled.
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that the display is not good (no physical damage, however),
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the connections inside the
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with either), but I kind of
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LED), but this does not
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It was explained to me about 10 years ago, but you certainly dont
drive them anything like an LED or Vacuum fluorescent device.

As I recall there is a backplane voltage, and the segment/digit
drives are somehow driven in some relationship to that, and in a
certain phase relationship too - to make things worse :)



A backlit LCD is the same device electrically but has simply got a
light behind it, (whether it be a small light bulb, LED, or
electroluminescent material) where a non-backlit just relies on
ambient light, and would have a reflective backing on it.

Re: back lit LCD questions


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I have a display that is not
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segment is 'clear'.
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DC controlled.
Quoted text here. Click to load it
possible that the display is not good (no physical damage, however),
Quoted text here. Click to load it
the connections inside the
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with either), but I kind of
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LED), but this does not
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What kind of LCD is it? character ?
some types need a negative contrast
voltage to function drive them.

There used to be a LCD faq once that
was good for info.

Re: back lit LCD questions


It is a simple display similar to a digital clock. I know the pcb outputs the
correct
signal as another display works fine, yet not all the segments will 'light' up on
the defective display.

Is it possible that the signal used to drive the display could be weak and not
able to drive the 'defective' display?

I would like to know what signals are need to drive the display, that way I can
maybe bench test each segment.

thanks,
Tim
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Re: back lit LCD questions


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LCD's aren't driven by a DC voltage, as someone else said. They get fed with
a clocked waveform which has to have a zero DC voltage bias on it. You can
take a look at something like
http://www.pacificdisplay.com/ics_app%20notes/hitachi/HD44100.pdf
if you're interested in seeing how a driver works.

For your problem it would appear more likely that you have a connector fault
or a display fault. Check that the pins are mating properly and that there's
no cracks in tracks. The LCD module itself will sit in a connector by way of
a strip of conductive (I think) rubber - it's all pretty hairy IMO and you
need to make sure that it's mating properly. It may have been jolted and not
connecting properly. What make/model is the display?

Cheers.

Ken



Re: back lit LCD questions


It's a custom display and it does not use the rubber strips for connections, it
has physical
pins coming out of the display the push into through connectors on the driver
PCB. I have
connected another display to the PCB and it works ok, so that bascially
eliminates everything
to the display itself. The display in question has no physical damage to it
(that is visable), it is
just missing some segments. Actually in the area that is the problem, on the
segments that do light
up, the ajoining ones also seem to light up faintly. Does this indicate some
sort of short in the interior
connections??

ps>> thanks for the link
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Re: back lit LCD questions



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has physical
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PCB. I have
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eliminates everything
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(that is visable), it is
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segments that do light
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sort of short in the interior
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Not necessarily, there may be some signal getting through to the segments within
the lcd
by capactive coupling. I wouldn't be concerned with this unless the segments are
activated
when they shouldn't.

Each segment of a LCD is XOR'd with the backplane signal, but driving LCDs with
more than one backplane is more complicated. If you have a datasheet of the
display,
you could see if the damaged segments are driven with a common backplane signal.

The pins attached to a LCD are fragile and damage can occur that may not be
immediately visible. If you have a magnifier, check around the pins/glass/track
interface of the damaged segments, as well as the backplane pin.

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Re: back lit LCD questions


It sounds as if you don't know what you are doing, when you say you "mess
around .......... "



This is the first time I really started to mess around with one of these,
but I have a display that is not
working correctly and have some questions as to how this is 'suppose' to
work.

I am measuring 4.15v on all lines for the display, even on the pin that the
segment is 'clear'.
What kind of signal is used to activate the segment as it does not seem to
be DC controlled.

The driver is ok, I swapped in another display and it work fine. Is it
possible that the display is not good (no physical damage, however),
or maybe there is something just 'goofy' with the display? Is it possible
that the connections inside the
display have been servered on the non-working segments?

Not sure if back lit LCD's work the same as standard one (have no experience
with either), but I kind of
assumed that you applied a DC voltage to each segment to activate it (like a
LED), but this does not
seem to be the case in this situation.

I hope that is clear...



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