Hitachi RAC-18SX8 Service Manual page 159

Connector pin No.
Expansion valve control involves detecting the temperature of the compressor head and adjusting the aperture to stabilize it to
the target temperature.
The control period is once every 20 seconds, at which time a few pulses are produced.
・This circuit incorporates 2 shift register latch driver ICs (IC11 and IC12) of the 8-bit type that convert serial signal inputs in
parallel and produces the result.
・ A typical circuit operation when the expansion valve is driven is described by using the time chart in Fig. 5-3.
① The product produces a clock signal (12 periods worth at 750 μs. per period) through pin #2 of the microcomputer. (One
period of clock signal consists of being off for 500 μs. and on for 250 μs.)
② Serial signal #1 of drive pattern is produced through pin #4 of the microcomputer synchronously with the clock signal.
#1: A serial signal is a serialized version of the pattern of energizing 1 or 2 phases of the 4 phases of windings of the
expansion valve. A serial signal is outputted so that the middle of an ON signal (Hi level, 500 μs.) of the serial signal enters
at the rise of the clock signal.
The above actions ① and ② update the contents of the shift register in the IC.
Next, 250 μs after the ON signal output in the 12th period of the clock signal, the following is conducted.
③ The latch signal from pin #3 of the microcomputer will be turned off (Lo level, 500 μs.) and on again (Hi level).
④ After the necessary energization time has elapsed, and if the expansion valve needs to be driven, the product will go back
to ① and repeat this procedure.
The above steps will energize 1 or 2 phases of windings of the 4 phases of the expansion valve, while controlling the
expansion valve aperture according to the contents of the shift registers updated for one energization time. When the
expansion valve aperture reaches its target, step ② above will become a stop pattern. The actions ① to ④ will then be taken
to terminate the expansion valve control.
Pin No. of
Lead wire
CN15
1
④
White ON
③
Yellow OFF
②
Orange OFF
①
Blue OFF
Operation modes
1→2→3→4→5→6→7→8 VALVE CLOSE
8→7→6→5→4→3→2→1 VALVE OPEN
④
③
②
①
50ms.
Table 5-1
Drive states
2 3 4 5
ON OFF OFF OFF
ON ON ON OFF
OFF OFF ON ON
OFF OFF OFF OFF
75ms.
200ms.
Fig. 5-2
– 192 –
6 7 8
OFF OFF ON
OFF OFF OFF
ON OFF OFF
ON ON ON
12V
1.2V