Busting LCDs ( was: Re: just blew up my 17" monitor )

Keith Lofstrom keithl at kl-ic.com
Mon Nov 10 07:02:47 UTC 2003

I'm a bit shy of mentioning it here and giving the virus writers ideas,
but indeed you can trash an LCD with video signals.  A half-field-rate
flashing black-white pattern would do it. 

Physics:  LCDs invert every other field of video data; black areas get
big AC signals, white areas get little AC signals.  The video interface
chips in the LCD panel do resampling, gamma correction, and convert
the "DC" video levels to AC levels that alternate every field.  The
big AC signals align the LC molecules perpendicular to the display,
while little signals let them relax to a horizontal position, so they
can repolarize the light moving from the back polarizer to the front
polarizer, making a path for the light.    Thus, white = lowAC
= relaxed molecules = polarization change = light.  

The problem comes if the drive signal has a DC component.  That will
electromigrate the LCD crystals perpendicularly, towards the face or
the back of the display, and cause them to plate out.  The screen 
turns black, and stays that way.  This could be caused by feeding a
field-synchronized video signal that was black in even fields (making
a positive voltage, say) and white in odd fields (making no voltage).
A few minutes of this will leave a pattern permanently ghosted into
the display.

This might be something for video driver writers to be cognisant of.


Keith Lofstrom           keithl at ieee.org         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs

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