Fig. 3-22 1/4 Zero -Cross Circuit - Canon NP-70 Service Manual

3-60
NP•70
b) The bridge rectifier provides afull-wave rectified signal to the 1 / 1 zero-cross portion of
the circuit and ahalf-wave rectified signal to the 1/4 zero-cross portion.
c) The 1/4 zero-cross circuit responds by producing aspike-like pulse every fourth 1/2 cycle
of the A. C. line voltage to trigger the heaters for low-level operation .
d) If the temperature of the heaters is low, the 1 / 1 zero-cross circuit will be enabled by the
output of Q235. This operation will cause the heaters to operate at high level until they
are sufficiently hot.
e) When the heater temperature is at the correct level (180 ± 30°C), the 1 / 1 zero-cross
portion is disenabled and stops functioning. The 1 /4 circuit, continues to operate.
2)
OPERATION — 1/4 ZERO -CROSS CIRCUIT
This circuit operates continuously to provide the Heater Assembly with triggering for low-level
operation, as follows (reference —Fig. 3-22 & 3-23):
a)
The voltage from rectifier D231 is applied to
the base of Q233 and results in the alternate
ON -OFF action of the transistor.
R238
R24
C233 R
D235
VR231
Q233
C234
R237
Q234
3
TP231
Fig. 3-22 1/4 Zero -Cross Circuit
b) During the periods that Q233 is conducting, the charge of capacitor C234 remains ~on-
stant. However, during periods of Q233 non-conduction, C234 charges at a rate deter-
mined by the setting of variable resistor VR231.
c)
As shown in the waveforms in Fig. 3-23, C234 will be completely charged at the time
the AC voltage has completed two alternations.
d) As the third AC alternation is begun, the charge of C234 reaches the level sufficient to
cause conduction through Q234. The turning ON of Q234 results in the rapid discharge
of C234 through transformer TP231.
e) The flow of current through TP231 appears as a spike-like waveform and causes the
triggering of the heaters by gating ON triac Q236.
Second Printing,lune, 1974
© CANON INC.