Hitachi RAC-25YH4, Service Manual, Page: 81 / 121

– 79 –
7. Temperature Detection Circuit
The Over heat thermistor circuit detects the temperature at the surface of the compressor head, the Defrost.
thermistor circuit detects the defrosting operation temperature.
A thermistor is a negative resistor element which has the characteristics that the higher (lower) the temperature,
the lower (higher) the resistance.
When the compressor is heated, the resistance of the Over heat thermistor becomes low and voltage to a
pin (62) of microcomputer is increased.
Microcomputer compares the voltage at pin (62) with the internal set value, if it is exceeded the set value
microcomputer judges that the compressor is overheated and stops operation.
When frost forms on the outdoor heat exchanger, the temperature at the exchanger drops abruptly. Therefore
the resistance of the Defrost. thermistor becomes high and the voltage at pin (63) of microcomputer drops.
If this voltage becomes lower than the set value stored inside, the microcomputer starts defrosting control.
During defrosting operation the microcomputer transfers the defrosting condition command to the indoor
microcomputer via the circuit interface.
The microcomputer always reads the outdoor temperature via a thermistor (microcomputer pin (64) ), an
transfers it to the indoor unit, thus controlling the compressor rotation speed according to the value set at the
EEPROM in the indoor unit, and switching the operation status (outdoor fan on/off, etc.) in the dehumidifying
mode.
The following shows the typical values of outdoor temperature in relation to the voltage:
<Reference>
When the thermistor is open, in open status, or is disconnected, microcomputer pins (62) (64) are
approx. 0V; when the thermistor is shorted, they are approx. 5 V, and LD301 blinks seven times.
However, an error is detected only when the OH thermistor is shorted; in such a case, the blinking mode
is entered 12 minutes after the compressor starts operation.
Fig. 7-1
Table 7-1
MAIN P.W.B.
R303 Voltage(V)
Outdoor temperature (
°
C)
C304
C303
C302
RAS_25_35_AW_055-086 2003.12.31, 10:59 AM 79