Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!execu!sequoia!memqa!r91400
From: r91400@memqa.uucp (Michael C. Grant)
Newsgroups: sci.electronics
Subject: Re: Life of LCD Panels
Message-ID: <3922@memqa.uucp>
Date: 28 Jun 90 14:26:46 GMT
References: <25269@unix.cis.pitt.edu>
Organization: Memory R&QA, Motorola SPD
Lines: 25

In article <25269@unix.cis.pitt.edu>, nadst2@unix.cis.pitt.edu (Nilanjan Adhikari) writes:
>   The above is something I heard/read several times about, including at least
> once in the Modern Photography. This is something expected in theory, having
> to do with dc leakage or something ( I may be wrong, being too lazy to go read
> about it ). I am curious, how much of it has been proven correct in real life ?

LCD is difficult to work with, because it must be driven by an AC voltage,
or else it will quickly degrade.  The diagram below illustrates how a pixel
                                             in a typical LCD setup is driven.
    Cnt	--+---+                    +         'Osc' is a square wave (say, 60Hz)
          |XOR|---(Cnt XOR Osc)-----(CELL)   and Cnt is 0 when the pixel is to
 Osc---+--+---+                       | -    be off, and 1 when it is to be on.
       |                              |      The resulting voltage difference
       +------------------------------+      across the terminals of the pixel
                                             is, IN THE PERFECT CASE, 0 when
Cnt=0, and a square wave (at twice Osc's amplitude) when Cnt=1 (try it!)

	The problem is that no XOR gates (or square waves) are perfect, and
some DC potential is applied to the cell.  So, the potential across the cell
isn't quite 0 when Cnt=0, and isn't quite 2*Osc when Cnt=1.

	Sure, this DC potential is quite small, but I assume that it is
enough to cause the degredation of the cells over a long period of time.

Michael C. Grant