Hitachi RAM-18QH5E, Service Manual

Page: 54 / 78

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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 falls under the
set value, microcomputer enters defrost control mode.
●
During defrost operation, microcomputer transfers indoor unit defrost condition command from IF transmission
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 value of the relationship between outdoor temperature and voltage are shown below.
<Reference>
When the 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
3.22
40
3.62 2.75
30
Fig. 10-1