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'[EE]: Interesting LCD observation...'
2002\08\11@125258 by Pic Dude

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There was a post yesterday about "leaking" LCD's, and I just
remembered this...  Had a Sony CD walkman many years
ago (one of the first very slim units) that had an LCD display.

Left it on the car dash once and when I came back, the unit
was really hot, and the display was completely black.  I tried
some buttons, etc and nothing.  Thought it was hosed, but it
came back after it cooled off.  And this happened many times
since that, but it never did any permanent damage to the
LCD.

Wonder what was going on here?

Cheers,
-Neil.

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2002\08\11@125934 by Spehro Pefhany

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At 11:51 AM 8/11/02 -0500, you wrote:

>Left it on the car dash once and when I came back, the unit
>was really hot, and the display was completely black.  I tried
>some buttons, etc and nothing.  Thought it was hosed, but it
>came back after it cooled off.  And this happened many times
>since that, but it never did any permanent damage to the
>LCD.
>
>Wonder what was going on here?

LCD displays (especially multiplexed variety of same) will
increase in "darkness" at higher temperatures. You can see
this effect especially with highly muxed displays such as
the ubiquitous HD44780 controlled character displays, which
is one big reason they need the pot, or a temperature
compensation circuit connected to that pin.

Taken to extremes, the display turns black even with no
AC voltage applied.

Unfortunately, when my son left my prize Mandarin-speaking
calculator in the back window of my car, it just went black
and never did come back.  8-(

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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2002\08\11@140851 by Dave Dilatush

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Pic Dude wrote...

>There was a post yesterday about "leaking" LCD's, and I just
>remembered this...  Had a Sony CD walkman many years
>ago (one of the first very slim units) that had an LCD display.
>
>Left it on the car dash once and when I came back, the unit
>was really hot, and the display was completely black.  I tried
>some buttons, etc and nothing.  Thought it was hosed, but it
>came back after it cooled off.  And this happened many times
>since that, but it never did any permanent damage to the
>LCD.
>
>Wonder what was going on here?

The liquid crystal material in LCD displays has an upper
temperature limit above which it "switches" (I forget the proper
technical term for it) just like the liquid crystal goop inside
one of those fever thermometer strips.

The transition from clear to opaque is surprisingly abrupt; years
ago I was doing stress-testing on a design of mine that included
an LCD display, and found that the span of temperature over which
the display turned from clear to black was barely two degrees C.

Provided you don't take the LCD up to a temperature a LOT high
than this, the effect is harmless and reversible with no apparent
effect on the unit's life.

In short, what you saw was normal.

DD

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2002\08\12@123728 by Peter L. Peres

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On Sun, 11 Aug 2002, Pic Dude wrote:

>There was a post yesterday about "leaking" LCD's, and I just
>remembered this...  Had a Sony CD walkman many years
>ago (one of the first very slim units) that had an LCD display.
>
>Left it on the car dash once and when I came back, the unit
>was really hot, and the display was completely black.  I tried
>some buttons, etc and nothing.  Thought it was hosed, but it
>came back after it cooled off.  And this happened many times
>since that, but it never did any permanent damage to the
>LCD.

The malfunction and the blackness are different items. The blackness is
caused by the LC material being pushed beyond the temperature where it can
be a 'crystal'. This will turn on all segments and more (if the liquid is
not constrained in cavities). The heat can make normal circuits refuse to
work.

Peter

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