Hitachi RAC-10G5, Service Manual, Page: 44 / 62

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2. Reset Circuit
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The reset circuit is used to reset the program to its initial settings when the power is turned on or when the
power is recovered after a power failure.
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The micro computer is reset when the reset input is “Lo”, and operation is possible when the reset input is
“Hi”.
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The waveforms at each point when the power is turned on and off are shown in the diagrams.
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When the power is turned on, the voltages of the DC 35V line and DC 5V lines are increased. When the
voltage of DC 35V lines reaches about 27V, ZD201 is turned ON, the potential of Q201’s base rises and
Q201 is turned ON. Since Q201’s collector is set to “LO” at this time, Q202 is turned ON and the reset
input of the micro computer is set to “Hi”. The DC 5V line voltage has already become stable at this time
and the micro computer starts operation.
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When the power is turned OFF, the voltage of the DC 35V line decreases. When it becomes below 27V,
ZD201 is turned OFF, then Q201 is turned OFF, Q202 is turned OFF the reset input of the micro computer
is set to “Lo’ and the micro computer is set to the reset mode.