Heater Lamp Control Circuit - Kyocera Mita KM-1500 Service Manual

2DC

(2) Heater lamp control circuit

Activation of the heater lamp is dominated by the HEAT signal which is derived by the engine CPU (U1) at its pin #74. When
its level is high, transistor Q8 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 (U1). 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.
P72/ANI2
CPU
PDL3/AD3
P05/INTP2
Thermal cutout
Heat roller
Heater lamp
Thermistor
2-3-4
98
THERMA
R52
U1
GND
R57
74
HEAT
43
ZCROSS
AC input
Figure 2-3-4 Heater lamp control circuit
Engine board
+5V
Heater lamp control circuit
U6-1
2
−
1
393
3
+
GND GND
Power supply unit
Zero
cross
signal
detection
circuit
+24 V
(PC2)
Triac Photo-triac
(TRC1)
Q8
GND
Bias
board
High
voltage
board