Circuit Descriptions
5–28
1502C MTDR Service Manual
The display heater circuitry regulates the application of power to the display heater
(see Indium Tin Oxide Heater later in this chapter for more information). When the
display thermistor divider senses the display temperature has dropped below
+10° C, the heater can be turned on if the control signal HEAT ENABLE
is not
asserted. For reasons of power economy, the chart recorder and display heater are
not allowed to operate concurrently. The processor does this by asserting HEAT
ENABLE while making a chart recording. When HEAT ENABLE is low,
N–channel FET Q2020 is off, making the voltage on the + side of the comparator,
U2020A, approximately +5 VDC. This will allow the + side (chart recorder) to
always be greater than the – side (display thermistor divider voltage). The output
of the comparator will be +16 VDC, which turns off P-channel FET Q1020. This
turns off the power to the display heater..
When HEAT DISABLE
is high, Q2020 will turn on and the voltage on the + side
of the comparator will be approximately 2.5 volts. When the display thermistor
divider voltage (– side) is above 2.5 volts (about +10° C), the comparator output will
be 0 V, which will turn on Q1020. This will turn on the heater. As the temperature
rises above +10° C, the thermistor divider voltage will be less than 2.5 V and Q1020
will turn off, shutting off power to the heater.
The LCD drive voltage compensation circuitry adjusts the drive voltage (contrast)
to assure a constant display contrast within the operating temperature range of the
instrument. The display thermistor is attached to the LCD and forms the sensor in
the display thermistor divider circuit. Its output is a voltage related to the display
temperature. This voltage is read by the processor through the analog-to-digital
converter, U2023. The processor uses this voltage value to determine a drive
voltage. This is sent to digital-to-analog converter U2021 via the data bus. The
output of the DAC is amplified to op-amp U2010A and applied as the LCD drive
voltage. As the temperature of the display (thermistor divider voltage) changes, the
processor modifies the drive voltage via the DAC. In this manner, the drive voltage
is compensated due to variations in display temperature. Trimmer potentiometer
R1011 is used to offset the drive voltage produced by U2010A to compensate for
variations in display cells and thermistors.
Display Module
The display module consists of the following:
H
LCD Cell
H
Row Driver/Controller Board and Column Driver Board
H
Electroluminescent Backlight
H
Indium Tin Oxide (ITO) Heater
H
Mechanical frame, which supports the above subassemblies
Display Heater Circuitry
Display Temperature
Compensation
Introduction
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