Hitachi RAM-90QH5, Service Manual, Page: 63 / 83

– 72 –
Fig. 9-2
With expansion valve control, opening degree is adjusted to stabilize target temperature, by detecting temperature of
compressor head.
The period of control is about once per 20 seconds, and output a few pulses.
3 pices (IC11-IC13) of 8 bit type shift resistor latching driver IC is using in this circuit, which convert the serial input
in to parallel output.
Explain the circuit operation of the electric expansion valve using the Fig. 11-3, 4 time chart
1
Clock signal (750
µ
s/12cycle) output from Micon pin (32), (29)
(1cycle of clock signal = off : 500
µ
s, on : 250
µ
s)
2
Driving serial signal #1 output from Micon pin (34), (31) same phase as clock signal
#1.... Serial signal is the method of current conducting only 1 or 2 phase out of 4 phase of windings of the electric
expansion valve.
Due to the above
1
,
2
operation the internal shifting resistor value be updated.
After 250
µ
s of the output ON signal of 12th cycle of clock signal.
3
Latch signal from the Micon pin (33), (30) be OFF (LOW level 500
µ
s), and again be ON (HI level)
4
After the necessary current conducting time if the electric expansion valve operation is necessary return to
1
and
repeat this operation.