Hitachi RAC-25JX4, Service Manual

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9. Dirt Sensor (Gas Sensor) Circuit
Dirt Sensor
(Gas Sensor)
GS421
3
2
1 4
R311
C305
R312
5V
0V
a
64
Microcomputer
IC501
Dirt Sensor
Input
<Sensor Output characteristics>
a Point Voltage
Stability
Time
0V
Dirt Sensor to
be turned on.
Dirty Air Return to Clean Air
The Dirt Sensor(Gas Sensor) works by making use of the change of resistance value which occurs with the gas-sensing
element heated by the heater, depending upon the dirtiness of the air (gas).
(When the dirty air is detected, the resistance value of the sensor changes to be lower.)
When electricity is turned on to the heater of the Dirt Sensor, the sensor will be in a useable mode. Due to the
characteristics of the sensor, it takes up to some several minutes before the temperature of the inner heater rises and the
conditions of the gas-sensing element become stable.
(Time varies, depending upon how the sensor is used.)
When the Dirt Sensor detects the dirtiness of the air, the resistance value of the sensor changes, and the voltage of the Dirt
Sensor Input of the microcomputer (IC501) becomes higher.
By this, the microcomputer recognizes the air being dirty, and by detecting the drop of the voltage, it also recognizes that the
air has turned clean again.
As the output voltage of the Dirt Sensor will change delicately even in the conditions of clean air, depending upon
temperature, humidity and the flow of the air etc., the detection of the dirtiness of the air is done by observing the change of
the voltage per hour.
Fig. 9-1