Hitachi RAM-50CNHZ2, Service Manual, Page: 81 / 107

– 89 –
10. Temperature Detection Circuit
●
Compressor head surface temperature is detected by OH thermistor circuit, defrost operation temperature
is detected by DEF thermistor circuit, and outdoor temperature is detected by outdoor temperature thermistor
circuit.
●
Thermistor is a negative resistance element with the following characteristic: Resistance falls when
temperature rises, and increases when temperature falls.
●
When compressor is over-heated, resistance of OH thermistor decreases and voltage at pin
x
of
microcomputer rises.
●
Voltage at pin
x
of microcomputer is compared with set value stored inside: If voltage exceeds set value,
microcomputer judges over-heating and stops operation.
●
If outdoor heat exchanger is frosted, heat exchanger temperature will rapidly drop. In response, resistance
of DEF thermistor increases and voltage at pin
c
of microcomputer falls. When the voltage flls under the
set value, microcomputr enters defrost control mode.
●
During defrost operation, microcomputer transfers indoor unit defrost condition command from IF transmision
output at SDO pin of interface (pin
/
and
X
of microcomputer).
●
Outdoor temperature is always read in (voltage at pin
v
of microcomputer) by outdoor temperature
thermistor, and then transferred to indoor unit side. According to this value, compressor rotation speed
control and operation selection (outdoor fan ON/OFF, etc.) in dehumidifying mode are performed.
Represented values of the relationship between outdoor temperature and voltage are shown below.
(Reference)
When thermistor is open or heat is shut off, pins
x
to
v
of microcomputer are set to about 0 V; when
thermistor is short-circuited, pins
x
to
v
of microcomputer are set to about 5V, and LD301 blinks 7 times.
However, OH thermistor detects only short-circuit as error: It will enter a blink mode after 12 minutes or more
has elapsed from the start of compressor operation.
Table 10-1
Outdoor temperature (
°
C)
Voltage at pin
1
of CN7 (V)
-10
1.19
0
1.69
10
2.23
20
2.75
30
3.22
40
3.62
62
0V
+5V
O.H. thermistor
O.H.
Normal: 2.7V or less
OH: 2.7V or more
Return: 2.3V or less
CN5
R301
R304
R305
R306
0V
63
0V
+5V
DEF thermistor
DEF.
Normal: 2.4V or more
OH: 2.4V or less
Return: 2.9V or more
CN6
R302
0V
64
0V
+5V
Outdoor temperature
thermistor
Outdoor
temperature
Refer to Table 10-1
CN7
R303
C301
C302
C303
0V
Microcomputer
(AX-6M00)
HIC
Fig. 10-1