Hitachi RAF-25FX8 Service Manual page 120

7. Temperature Detection Circuit
O.H. thermistor
DEF. thermistor
Outdoor temperature
thermistor
The Over heat thermistor circuit detects the temperature at the surface of the compressor head, the Defrost.
thermistor circuit detects the defrosting operation temperature.
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 voltage to a
pin of microcomputer is increased.
62
Microcomputer compares the voltage at pin
microcomputer 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 the circuit interface.
The microcomputer always reads the outdoor temperature via a thermistor (microcomputer pin
transfers it 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 dehumidifying
mode.
The following shows the typical values of outdoor temperature in relation to the voltage:
Table 7-1
<Reference>
When the thermistor is open, in open status, or is disconnected, microcomputer pins
0V; when the thermistor is shorted, they are approx. 5 V, 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.
MAIN P.W.B.
+5V
CN8
1
2
+5V
CN9
1
2
+5V
CN10
1
2
Outdoor temperature (°C)
R303 Voltage (V)
R304
C304
R301
0V
0V
R305
C303
R302
0V
0V
R306
C302
R303
0V
0V
Fig. 7-1
with the internal set value, if it is exceeded the set value
62
-10 0 10 20
1.19 1.69 2.23 2.75
– 138 –
62
O.H.
Microcomputer
63 DEF.
Outdoor
64
temperature
of microcomputer drops.
63
30 40
3.22 3.62
62 -
64 ), an
64 are appro x.