Temperature Detection Circuit - Hitachi YUTAMPO Series Service Manual

4 Electrical checks of main parts
Outdoor unit

4.1.5 Temperature detection circuit

OH thermistor
DEF thermistor
Outside air
temperature
thermistor
• OH thermistor circuit detect the temperature at the surface of compressor head, DEF thermistor circuit detect the
defrosting operation temperature (evaporation temperature).
• A thermistor is a negative resistor element which has characteristics that the higher (lower) the temperature, the lower
(higher) the resistance.
• When the compressor is heated, the resistance of the OH thermistor becomes low and
thermistor and R301 and the voltage at pin 74 of microcomputer.
• Compare the voltage at microcomputer pin 74 and setting value stored inside. If the value exceed the set value,
microcomputer will judge that the compressor is overheated and stop the operation.
• When frost is formed on the outdoor heat exchanger, the temperature at the exchanger drops abruptly. Therefore
the resistance of the DEF thermistor becomes high and the voltage at pin 70 of micro computer drops. If this voltage
becomes lower than the set value stored inside, microcomputer will enter the defrost control.
• During defrost operation, the microcomputer will transfer the defrosting condition command to indoor unit via SD0 pin
of interface of IF transmission output.
• The microcomputer read the outdoor temperature by outside air thermistor and transfer it to the indoor unit, thus
controlling the compressor rotation speed according to the set value in the EEPROM of indoor unit and switching the
operation mode (outdoor fan on/off etc.) to DRY mode.
• Below table show the typical values of outdoor temperature in relation to the voltage.
Outside Air Temperature (°C)
Voltage at both side of R303 (V)
?
N O T E
• When the thermistor is open, open condition or disconnect, microcomputer pin 70-74-75 are approx. 0V;
• When thermistor is shorted, they are approx. 5V and LD301 will blink 7 times.
• However, an error is detected when only the OH thermistor is shorted and will enter blinking mode after 12 minutes start the compres-
sor operation.
40 SMGB0135 rev.0 - 06/2020
HIC PCB
10
11
12
-10 0
1.19 1.69
74
70
75
5V is divided by OH
+
10 20
2.23 2.75
OH
Micro computer
DEF
Outside air
temperature
30 40
3.22 3.62