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.
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.
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.
- For displays requiring -5VDC, R3 should be connected to -5,
VSS to ground.
- Typical termistor value 15k @ 25C, B=4300
- R1 and R2 values are selected based on required VO level. See
- R1 and R2 can be variable resistor for manual control.
- Vz value = 4.5V for1/8 & 1/11 duty cycle displays: 5.0V
for 1/16 duty cycle; 10V for modules using +/-5V supply.