Hitachi RAC-18SX8 Service Manual page 155

4. Temperature detection circuit
OH thermistor
DEF thermistor
External temperature thermistor
・An OH thermistor circuit is used to detect the surface temperature at the compressor head, and a DEF thermistor circuit is
used to detect the defrost operating temperature, and an external temperature thermistor circuit is used to detect the
outdoor temperature.
・The thermistor is a negative resistance device. It will decline in resistance when the temperature rises, and increases in
resistance when the temperature goes down.
・When the compressor overheats, the OH thermistor will decline in resistance. The + 5 V is divided by using the OH
thermistor and R301, resulting in a rise in the voltage at pin *5 of the microcomputer.
・The voltage at pin *5 of the microcomputer is compared with the setting stored inside. If it exceeds the setting, the product
will judge that the compressor has overheated and stop running.
・When the outdoor heat exchanger gathers dew, the heat exchanger will go down quickly in temperature. For that reason, the
DEF thermistor will increase in resistance, resulting in a decline in the voltage at pin *4 of the microcomputer. When this
voltage goes below the setting stored inside, the microcomputer enters defrost control.
・When the microcomputer is in defrost mode, it will transfer a defrost condition command from the SDO pin IF transmission
output (pin ^0 of the microcomputer) of the interface into the indoor unit.
・The product will constantly detect the outdoor temperature by using the external temperature thermistor (the voltage at pin
*6 of the microcomputer), transfer it to the indoor unit, and control the compressor speed and switch operation states in dry
mode (such as turning on and off the outdoor fan) according to the settings made in the indoor microcomputer. Following
are the representative values for the external temperature and voltage.
Table 4-1
Voltage at both ends of R303 (V)
＜Reference＞
When the thermistor is open, in an open state, or has a wire break, pins *4 to *6 of the microcomputer will become about 0 V.
When the thermistor is short-circuited, pins *4 to *6 of the microcomputer will become about 5 V, with the LD301 blinking 7
times.
However, the OH thermistor is judged to be in an error only when it is short-circuited. More than 12 minutes after compressor
startup, the product will enter blinking mode.
＋5V
CN8
1
2
＋5V
CN9
1
2
＋5V
CN10
1
2
External temperature (℃)
H Ⅰ C
R203
9
R301
0V
R239
10
R302
0V
R240
11
R303
0V
Fig.4-1
−10
0 10
1.19 1.69 2.23
– 188 –
OH
85
C202
Microcomputer
Normal: 2.4 V or more
AX-8W00
0V
DEF: 2.4 V or less
Reset: 2.9 V or more
84 DEF
C216
0V
86
External temperature
C217
0V
○
20 30 40
2.75 3.22 3.62