Hitachi Summit RAC-18YH6, Service Manual, Page: 35 / 96

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5. Initial Setting Circuit (IC401)
• When power is supplied, the microcomputer reads the data in IC401 or IC402 (E
2
PROM) and sets
the preheating activation value and the rating and maximum speed of the compressor, etc. to their
initial values.
• Data of self-diagnosis mode is stored in IC401 or IC402; data will not be erased even when power is
turned off.
6. Power Supply
First, 35V power which operates the indoor unit is generated by the power source section of the outdoor unit and
supplied to the indoor unit through the C and D lines of the connecting cable.
Second, use the DC/DC converter and the 35 V power supply from the outdoor unit to generate 12 V control
power, which drives the stepping motor during the operation.
In addition, use the regulator IC 121 to generate 5 V power required for driving the micro computer and controlling
fan motor.
If the terminal block was overheated due to a connecting cable improper connection, the thermal fuse built in the
terminal block will burnt to shut off the 12 V line and stop the operation of the indoor unit. Then, the outdoor unit
cannot be communicated with the indoor unit and a communication error occurs (the outdoor LD301 will blink 9
times), stop all operations.
Fig. 6-1
Fig. 5-1
Microcomputer
External ROM
SCL
SDA
0V
0V
0V
5V
5V 5V
C401
IC401 or IC402 (E
2
PROM)
R404 R403
27
26
1
2
3
4
8
7
6
5
}