TEMPERATURE COMPENSATION

  High Temperature 
  
  Increasing temperature changes some physical properties of the 
LC material, especially the anisotropy of the refractive index,
 which is responsible for STN-LCD operation. This changes the 
background color from yellow-green to blue at temperature above 
50°C. The result is a decrease of the characteristic excellent
contrast of STN, but even at temperature up to 70°C the 
readability is not completely lost. This effect is reversible, 
and after cooling, the display returns undamaged to its 
original state

   Low Temperature 
   Lower temperature also results in a change of the background
color to red-orange for STN-LCD. There is almost no change in 
the contrast, but the switching times are increased significantly 
as is well known for the standard TN-display. Air bubbles may form
at very low temperature (below -20°C) due to the large temperature
dependent shrinkage of the liquid crystal material. But this behavior
is reversible without damage to the cell.

  Temperature Compensation
  LCD modules have a limited operating temperature range. The fluid
 within the glass is the most limiting factor. Threshold voltage and 
viewing angle are temperature dependent. Temperature compensation of
 the driving voltage is necessary to have good contrast and viewing
 angle of the operating temperature range. This temperature compensation
 circuits depend on the physical properties of the dV/dT parameter which
 varies from 8 to 22 mV/°C-1 depending upon the fluid and duty rate. 
Over the rated range, the bias, or VO voltage required to optimize 
the contrast and maintain a constant viewing cone varies slightly. 
Compensation or adjustment can be achieved manually, with a temperature
 compensation circuit, or a combination of both.

  Manual adjustment involves user accessible control via a potentiometer
or digital device. A standard, negative temperature coefficient 
thermostat serves as an inexpensive, automatic sensor. It should be 
mounted as close to the glass as possible to get an accurate measurement.
A thermistor circuit can be configured to provide automatic temperature
compensations. Each module's specification lists approximate VO voltages
required at the extremes of the temperature range and at 25C. A suggested
circuit is shown below.
  When operating outside of the module's rated temperature range, forced 
air and/or a heater are required to maintain reliable operation. The
heater manufacturer can offer design assistance.

  Note:
  
1.	For displays requiring -5VDC, R3 should be connected to -5, V
     SS to ground. 
2.	Typical termistor value 15k @ 25C, B=4300 
3.	R1 and R2 values are selected based on required VO level. 
     See module specifications. 
4.	R1 and R2 can be variable resistor for manual control