Temperature Detection Circuit - Hitachi RAS-25CNH1 Service Manual

10. Temperature Detection Circuit

Over heat Over heat
thermistor thermistor
CN4
Defrost
thermistor
CN5
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 e5V is
divided by the over heat thermistor and R301 and the voltage at pin & of HIC.
HIC the voltage at pin & and the set value stored inside, and when it exceeds the set value, the micro
computer 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 * of HIC 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
microcomputer via the SDO pin IF transfer output of the interface.
+5V +5V
R301 R301
+5V
0V 0V
R302
0V
HIC
Normal: 2.7V or less Normal: 2.7V or less
Over heat: 2.7V or more Over heat: 2.7V or more
Reset: Reset: 2.3V or less 2.3V or less
17 17
0V 0V
Normal: 2.4V or more
Defrost: 2.4V or less
Reset: 2.9V or more
18
0V
Fig. 10-1
– 84 –
5 5
Over heat Over heat
Micro computer
(AX-8E21)
5
Defrost