Hitachi RAC-18SX8 Service Manual page 133

5. Room temperature heat exchanger thermistor circuit
Microcomputer
IC601
Room temperature
thermistor input
Heat exchanger
thermistor input
The room temperature and the temperature of the indoor heat exchanger pipe
・
are detected by using a thermistor.
A thermistor is a sensor whose resistance changes with the device temperature.
・
Analog voltages obtained by the resistance voltage divided with the fixed resistor
are recognized by the microcomputer (IC601) as temperature signals.
The relationship between the thermistor temperature and circuit voltage is roughly
・
as shown in Fig. 5-2. If it is easier to take actual measurements between the
terminals of CN4, use the chart in Fig. 5-3 "Voltages between thermistor ends."
6. Humidity sensor circuit
・
The humidity sensor changes to a resistance that matches the ambient relative humidity. The voltage divided with R661
changes accordingly and the voltage that matches the humidity is recognized as a signal by the microcomputer (IC601).
Caution
When a DC current is applied, the humidity sensor may deteriorate in characteristics. Do not take voltage measurements
with a DC tester or something similar. Conduct failure diagnosis as follows to make a check:
1. Power switch is turned OFF.
2. After the power switch turned OFF, wait for 10 seconds or so and it is turned ON again.
3. When an initial run begins, press the START button.
4. In the case of a failure, the timer lamp will blink 9 times. Replace the SW P.W.B.
90
R406
91
R407
0V
Fig. 5-1
Microcomputer
IC601
72
Pulse output of humidity sensor
(P1)
92
Humidity sensor input
0V
Pulse output of humidity sensor
71
(P3)
5V
Room temperature
Ｃ Ｎ ４
thermistor
1
2
3
4
Heat exchanger
thermistor
R408
C402
R405
R661
0V
Fig. 6-1
– 166 –
Thermistor characteristics
(microcomputer input voltage)
4.0
3.0
2.0
1.0
0 10 20
Thermistor temperature
Fig. 5-2
Thermistor characteristics
(voltages between thermistor ends)
4.0
3.0
2.0
1.0
0 10 20
Thermistor temperature
Fig. 5-3
CN17
6
Humidity sensor
7
30 40 50
（℃）
30 40 50
（℃）