Kyocera FS-1050 - B/W Laser Printer Service Manual page 81

Heater lamp control circuit
Activation of the heater lamp is dominated by the HEAT signal which is derived by the engine CPU
(U01) at its pin #1. When its level is high, transistor QA03-a turns on, photo-triac PC2 and triac
TRC1 turn on simultaneously, and the heater lamp is applied with the primary AC voltage in turn.
Switching of triac TRC1, as affected by the HEAT signal is made in synchronization with the zero-
cross signal ZCROSS which is generated by the power supply unit. The zero-cross signal detector
watches the transition of alternating plus and negative current and detects the zero crosses. This
detector derives the resultant ZCROSS signal at its pin #43 of the engine CPU (U01). Since abrupt
change in the current flow can be significantly avoided by synchronizing triac TRC1 with the zero-
cross signal, the possibility of noise due to the primary AC supply is greatly reduced.
P10/ANI0
CPU
U01
P00/INTP0
Thermal cutout
Heat roller
Heater lamp
Thermistor
FS-1050
33
THERMA
GND
1
HEAT
P50/A8
43
ZCROSS
AC input
Figure 4-3-5 heater lamp control circuit
Engine board
Heater lamp control circuit
+5V
U04-1
R52
2
− 1
393
3
+
R56
GND GND
Power supply unit
Zero
cross
signal
detection
circuit
+24 V
(PC2)
Triac Photo-triac
(TRC1)
4-22
+5V
+5V
Q11
QA03-b
QA03-a
Bias
board
GND
High
voltage
board