Hitachi RAS-50WX8 Service Manual page 104

8. Temperature Detection Circuit
O.H. thermistor
1
2
CN8
(WHITE)
DEF. thermistor
1
2
CN9
(BLUE)
Outdoor temperature
thermistor
1
2
CN10
(RED)
The over heat thermistor circuit detects the temperature at the surface of compressor head, the defrost thermistor circuit
detects the defrosting operation tempreture.
A thermistor is a negative resistor element which has the characteristics that the higher (lower) the temperature, the lower
(higher) the resistance.
When the compressor is heated, the resistance of the over heat thermistor becomes low and + 5V is divided by the over
heat thermistor and R512 and the voltage at pin
Microcomputer the voltage at pin
judges that the compressor is overheated and stops operation.
When frost forms on the outdoor heat exchanger, the temperature at the exchanger drops abruptly.
Therefore the resistance of the defrost thermistor becomes high and the voltage at pin
If this voltage becomes lower than the set value stored inside, the microcomputer starts defrosting control.
During defrosting operation the microcomputer transfers the defrosting condition command to the indoor microcomputer via
he SDO pin IF transfer output of the interface.
The microcomputer always reads the outdoor temperature via a thermistor (microcomputer pin
to the indoor unit, thus controlling the compressor rotation speed according to the value set at the EEPROM in the indoor
unit, and switching the operation status (outdoor fan on/off,etc.) in the dry mode.
The following shows the typical values of outdoor temperature in relation to the voltage:
Table 8-1
<Reference>
When the thermistor is open, in open status, or is disconnected, microcomputer pins
thermistor is shorted, they are approx. 5V, and LD301 blinks seven times.
However, an error is detected only when the OH thermistor is shorted; in such a case, the blinking mode is entered 12
minutes after the compressor starts operation.
+5V
+5V
+5V
and the set value stored inside, and when it exceeds the set value, the microcomputer
Outdoor temperature ( )
Microcomputer pin voltage (V)
HIC
Normal 2.7V or less
Over heat 2.7V or more
Reset 2.3V or less
7
0V
Normal 2.4V or more
Over heat 2.4V or less
Reset 2.9V or more
6
0V
Refer to the table 8-1
5
0V
Fig. 8-1
of misrocomputer.
-10 0 10
1.19 1.69 2.23
– 144 –
62
O.H.
Microcomputer
(AX-8T11)
63 DEF.
Outdoor
64
temperature
of microcomputer drops.
voltage ),and transfers it
20 30 40
2.75 3.22 3.62
- are approx. 0V; when the