Fig. 3-24 1/1 Zero-Cross Circuit - Canon NP-70 Service Manual

3—s2
NP -70
3) OPERATION — 1/1 ZERO -CROSS CIRCUIT
This portion of the "Zero-Cross" heater control circuit provides the triggering to operate the
heaters continuously as follows:
a) When the heater temperature is low, TH1 resistance is high. Due to the design of the
comparison network, Q235 02 is higher than Q235 03 under this condition, and output
©
becomes approximately OVDC.
b) The "LO" output applied to the 1/1 zero-cross circuit enables the alternate ON-0FF ac-
tion of transistor Q231 to occur due to the rectified AC voltage (from D231) being ap-
plied simultaneously to the base of Q231.
c) Due to the full-wave base voltage of Q231
(which doubles the conduction periods
when compazed to Q233) and the lack of
resistance in the chazge path of C232 (which
shortens the charge time when compazed to
C234), one spike is developed in TP231 for
each half cycle of AC voltage.
Fig. 3-24 1/1 Zero-Cross Circuit
d) The frequency of the triggers developed by the 1 / 1 zero-cross circuit is, thus, four times
the rate of those developed by the 1 /4 zero-cross circuit. This results in the heaters ope-
rating continously until the temperature has risen to the correct level (180 ± 3°C).
e) When temperature reaches the correct level, Q235 applies a constant +20VDC to - the base
of Q231, to keep the latter continuously ON. With continuous conduction, Q231 prevents
the charging of C232 and, thus, the generation of spike pulses in TP231.
Second Printing-June, 1974
©CANON INC.