Hitachi RAS-25CNH2 Service Manual page 62

10. Temperature Detection Circuit
O.H. thermistor
CN4
DEF. thermistor
CN5
Outdoor temperature
thermistor
CN7
The Over heat thermistor circuit detects the temperature at the surface of the compressor head, the
G
Defrost. thermistor circuit detects the defrosting operation temperature.
A thermistor is a negative resistor element which has the characteristics that the higher (lower) the
G
temperature, the lower (higher) the resistance.
When the compressor is heated, the resistance of the Over heat thermistor becomes low and e5V is
G
divided by the over heat thermistor and R512 and the voltage at pin
Microcomputer the voltage at pin
G
the 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.
G
Therefore the resistance of the Defrost. thermistor becomes high and the voltage at pin
drops.
If this voltage becomes lower than the set value stored inside, the micro computer starts defrosting control.
During defrosting operation the micro computer transfers the defrosting condition command to the indoor
G
microcomputer via the SDO pin IF transfer output of the interface.
The microcomputer always reads the outdoor temperature via a thermistor (microcomputer pin
G
and 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
dry mode.
The following shows the typical values of outdoor temperature in relation to the voltage:
Table 10-1
Outdoor temperature (°C)
Microcomputer pin
<Reference>
When the thermistor is open, in open status, or is disconnected, microcomputer pins
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.
+5V
42
R512
+5V
0V
41
R513
+5V
0V
40
R514
0V
Fig. 10-1
x
and the set value stored inside, and when it exceeds the set value,
v
voltage (V)
– 65 –
Normal 2.7V or less
Over heat 2.7V or more
Reset 2.3V or less
0V
Normal 2.4V or more
Over heat 2.4V or less
Reset 2.9V or more
0V
Refer to the table 10-1
0V
x
-10 0 10 20
1.19 1.69 2.23 2.75
HIC
62
O.H.
Micro
computer
(AX-8J01)
63 DEF.
64
Outdoor
temperature
of microcomputer.
c
of microcomputer
v
30 40
3.22 3.62
x v
- are approx. 0V;
voltage),