Hitachi RAS-25CNH2 Service Manual page 63

11. Reset Circuit
Q302
Reset circuit performs initial setting of the microcomputer program before power is turned on.
G
Microcomputer resets program with reset voltage set to Lo, and program can be operated with Hi.
G
Fig. 11-1 shows the reset circuit and Fig. 11-2 shows waveform at each point when power is turned on
G
and off.
When power is turned on, 12V line and 5V line voltages rise and 12V line voltage reaches 7.2V (Zenor
G
voltage of ZD501), D501 is turned ON, Q301 and 0302 are turned ON and reset voltage input to pin
of microcomputer is set to Hi. By ZD501, reset voltage maintains input of pin
microcomputer rises to 5V to obtain operable status.
When power is shut off and potential of 12V is lowered, ZD501 is shifted to OFF. However, since reset
G
voltage is feed back to Q301 by R312, maintains ON state until 12V line voltage drops to about 7.6V. This
prevents reset voltage from chattering due to voltage change in 12V line.
12
6
0
12
6
0
+12V
+5V
Q301
0V
To trip signal
synthesis circuit
12V line
5V line
Reset voltage
0V
Fig. 11-1
12V line
5V line
Reset voltage
Fig. 11-2
– 66 –
8J-HIC
Micro
computer
(AX-8j01)
48
RES
k
at Lo until Vdd of
Time
Time
k