Hitachi RAS-18LH2 Service Manual page 45

2. Reset Circuit
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.
The micro computer is reset when the reset input is "Hi", and operation is possible when the reset input is
●
"Lo".
The waveforms at each point when the power is turned on and off are shown in the diagrams.
●
When the power is turned on, the voltages of the DC 12V line and DC 5V lines are increased. When the
●
voltage of DC 12V lines reaches about 7V, ZD511 is turned ON, the potential of Q511's base rises and
Q511 is turned ON. Since Q511's collector is set to "LO" at this time, Q512 is turned OFF and the reset
input of the micro computer is set to "Lo". The DC 5V line voltage has already become 5V at this time and
the micro computer starts operation.
When the power is turned OFF, the voltage of the DC 12V line decreases. When it becomes about 7V,
●
ZD511 is turned OFF, then Q511 is turned OFF, Q512 is turned ON the reset input of the micro computer
is set to "Hi' and the micro computer is set to the reset mode.
– 50 –