Hitachi RAM-68NP3B, Service Manual, Page: 51 / 79

Fig. 4-1
40
3.62 3.22
30
2.75
20
2.23
10
1.69
0
－
10
1.19
●
The temperatures at narrow pipe 1 - 3 and wide pipe 1 - 3 are read by thermistors, and the amount that electric
expansion valves 1 - 3 are opened is changed to control the distribution of refrigerant.
When a thermistor is open and disconnected, pins - and - on the main microcomputer are
Voltage (V) at both ends of R333
Outdoor temperature (
℃
)
Table 4-1
●
The OH thermistor circuit detects compressor head surface temperature, the DEF thermistor circuit detects defrost
operating temperature, and the outside air temperature thermistor circuit detects the outside air temperature. In
addition, the electric expansion valve thermistor (narrow pipe 1) detects the temperature of narrow pipe going to
indoor unit 1 and (wide pipe 1) detects the temperature of the wide pipe going to indoor unit 1. (Narrow pipe 2) and
(wide pipe 2) are for indoor unit 2. (Narrow pipe 3) and (wide pipe 3) are for indoor unit 3.
●
Thermistors are negative resistance elements. The resistance value grows smaller as the temperature rises, and grows
larger as the temperature falls.
●
If the compressor overheats, the resistance value of the OH thermistor grows smaller. B-5V is divided between the OH
thermistor and R331, and therefore the voltage of pin on the main microcomputer rises.
●
The voltage of pin on the main microcomputer is compared with the value set and stored internally. If the set value
is exceeded it is determined that the compressor has overheated, and operation is stopped.
●
If frost accumulates on the outdoor heat exchanger, the temperature of the heat exchanger will fall rapidly. Therefore, the
resistance value for DEF thermistor grows large and the voltage of pin on the microcomputer falls. If this voltage
drops below the value set and stored internally, defrosting of the main microcomputer will start.
●
Outdoor temperature is read by the outdoor temperature thermistor (voltage of pin on the microcomputer).
Commands from the indoor microcomputer, values read from the outdoor temperature thermistor, and values read from
the OH thermistor are taken into account to control the speed of the compressor and the speed of the outdoor fan.
Typical values that indicate the relationship between outdoor temperature and voltage are shown below.
OVERHEAT
THERMISTOR
DEFROST
THERMISTOR
OUTDOOR AIR
THERMISTOR
OH
DEF
OUTDOOR
Narrow pipe 1
Narrow pipe 2
Narrow pipe 3
Narrow pipe 1
Narrow pipe 2
Narrow pipe 3
Wide pipe 1
Wide pipe 2
Wide pipe 3
Main
Microcomputer
CN11
CN10
CN9
CN8
XA-8P
(RED)
38
35
36
37
39
40
75
67
68
66
77
79
CN12
XA-8P
(WHT)
8
7
6
2
3
4
5
1
B－0 V
B－0 V
Ｒ 369
C339
C341
C343
R338 R340 R342
Ｒ 371
Ｒ 373 B－0 V
B－0 V
Wide pipe 1
Wide pipe 2
Wide pipe 3
43
44
45
76
78
80
B－0 V
B－0 V
Ｒ 370
C340
C342
C344
R339 R341 R343
Ｒ 372
Ｒ 374
B－0 V
B－0 V
4. Temperature Detection Circuit
Electric expansion valve thermistor
Electric expansion valve thermistor
75
8
7
6
2
3
4
5
1
Reference:
approximately 0V. When there is a short-circuit in a thermistor, these pins are approximately 5V, LD351 will lit and
LD352 blinks during stanby mode or running. Except for overheat thermistor, if there is a short-circuit in a overheat
thermistor the LED indication during stanby mode and running will be different as below Table 4-2.
The number of blinks by LD352 indicates the area/portion in unit that thermistor detect have problem.
1 time blinking
LD351 indication
Lit Stanby Mode
Table 4-2
Unit condition
Running
LD352 indication
Off
6 times blinking