Hitachi RAS-10XH1 Service Manual page 72

9. Dirt sensor (gas sensor) circuit
When set to "dirt monitor": Approx 5 V
Microcomputer
IC601
Dirt sensor
2
power supply
Dirt sensor ａ
97
input
・The dirt sensor (gas sensor) takes advantage of changes in resistance in the sensor gas device
heated by the heater due to dirt (gas) in the air. (Detection of dirty air will lower the sensor resistance.)
・The dirt sensor is activated only when the "dirt monitoring" is configured by remote control. When
"dirt monitoring" is configured, the control of the microcomputer (IC601) turns on the Q661,
energizing the heater of the dirt sensor (GS831) and making the sensor usable. Note also that it
takes up to several minutes for the internal heater temperature to rise, resulting in the sensor device
becoming stable, because of the sensor characteristics. (The time varies with the operating status of
the sensor.)
・When the dirt sensor detects dirt in the air, the sensor resistance will change, resulting in a rise in the
voltage of the dirt sensor input of the microcomputer (IC601). This will enable the microcomputer to
recognize the dirt in the air. Note also that it will detect the voltage decline, resulting in the air having
been purified again.
・The output voltage of the dirt sensor will change finely due to the air temperature, humidity, air flow,
and other conditions even when the air is clean. Dirt in the air is therefore detected by monitoring
voltage changes per unit hour.
5V
R667
R666
Ｑ661
Dirt sensor
GS831
R669
０ Ｖ
Fig. 9-1
[Sensor output characteristics]
a
Stable time
0V
Set to "dirt monitoring" Air contaminated
– 89 –
Voltage at point
Air purified again
Fig. 9-2