Temperature Detection Circuit - Hitachi RAS-60YH5 Service Manual

9. Temperature Detection Circuit

O.H. thermistor
CN8
DEF. thermistor
CN9
Outdoor temperature
thermistor
CN10
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 voltage at
G
pin of microcomputer is increased.
62
Microcomputer compares the voltage present at pin
G
set value 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 microcomputer starts defrosting control.
During defrosting operation the microcomputer transfers the defrosting condition command to the indoor
G
microcomputer via the circuit interface.
The microcomputer always reads the outdoor temperature via a thermistor (microcomputer pin
G
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 9-1
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
7
+5V
6
+5V
5
voltage (V)
5
– 87 –
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
Fig. 9-1
with the internal set value, if it is exceeded the
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-10 0 10
1.19 1.69 2.23 2.75
HIC
62
O.H.
Microcomputer
63 DEF.
64
Outdoor
temperature
of microcomputer
63
20 30 40
3.22 3.62
– are approx. 0V;
62 64
), and
64