Heater Lamp Control Circuit - Kyocera FS-1030D Service Manual

2G6

(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 #1. When
its level is high, transistor Q10 turns on, photo-triac PC2 and triac TRA1 turn on simultaneously, and the heater lamp is
applied with the primary AC voltage in turn.
Switching of triac TRA1, 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 TRCA with the
zero-cross signal, the possibility of noise due to the primary AC supply is greatly reduced.
CPU (U1) selectively switches among those variations for applying voltages to the heater lamp according to the THERM
signal which appears at pin #33 as feedback.
A fraction of THERM is applied to pin #2 of comparator U4-1. The comparator maintains comparison of the potential at pin
#2 and pin #3 which gives a reference for the possible anomaly in the heater temperature (bred by resistors R44 and R40).
Should the voltage at pin #2 exceed that at pin #33, the level at pin #1 becomes low. Since pin #1 is wired (via transistor
array QA4) to the output line for the HEAT signal, the HEAT signal is enforced to be low regardless the behavior of CPU
(U1), thus preventing possible heat overrun.
CPU
P00/INTP0
Thermal cutout
Heat roller
Heater lamp
Fuser thermistor
2-3-4
33
THERMA
P10/ANI0
U1
GND
1
HEAT
P50/A8
43 ZCROSS
AC input
Figure 2-3-4 Heater lamp control circuit
Engine PWB
+5V
Heater lamp control circuit
U4-1
R44
2
1
-
3 393
+
R40
GND
Power supply unit
Zero
cross
signal
detection
circuit
+24 V
(PC2)
Triac
Photo-triac
(TRA1)
+5V
QA4
Q10
GND
GND
Bias PWB
High voltage
unit
ZCROSS
HEATN