Sharp FACSIMILE FO-4500 Service Manual page 117

IC703
VIN
9. Heater circuit
Though heater output is powered from AC line, the circuit is config-
ured to allow one side of AC to be turned on and off by TR701.
When heater ON/OFF terminal is turned to LOW, the current will flow
to the photo diode of PC702 to trigger the photo triac on the primary
side and also turn on TR701.
This will activate the heater output. On the contrary, when the heater
ON/OFF terminal is turned to HIGH, any current will not
flow to the photo diode of PC702 to turn off TR701.
This will deactivate the heater output.
If TR701 is broken in the short mode, any current will not flow to the
photo diode side of PC703 to turn off the photo transistor on the
secondary side, and Q703 will not be turned off.
Q703 is connected to the gate cathode of SR701, and SR701 is
usually turned off. However, when Q703 is turned off and the heater
ON/OFF control terminal is further turned to HIGH, SR701 will be
brought into the conductive state to cutoff the power supply to the
heater circuit on the primary side.
F701
AC-L
AC-N
S701
F706
HEATER L
1
CONSTANT CONSTANT
VOLTAGE VOLTAGE
CIRCUIT CIRCUIT
OVERCURRENT
DETECTION F/F
CIRCUIT
OVERHEAT
PROTECTIVE
CIRCUIT
RL701
R728
C712
R716
TR701
R729
C713
PC703
HEATER N
PWM COMP.
O
OSC-
R
ILLATOR
S
2
COM
Fig. 23
R725
VM
(+24V)
D711
SR701
C729
Fig. 24
5 – 27
3
VOUT
ERROR AMP.
– 4
OADJ
+
VREF
PC702
R733
R730
R731
C728
PC703
Q703
R732
FO-4500H
HEATER
ON/OFF