Sharp SF-1014 Service Manual page 109

(4) Heater lamp control circuit
1
General
The heater lamp control circuit detects the heat roller surface
temperature with the thermistor, converts it into voltage level, and
outputs it to the CPU analog input pin.
The CPU converts the analog voltage into digital signal level, com-
pares it with the value set by simulations, turns on/off the heater lamp
according to the level, and maintain the heat roller surface tempera-
ture at a constant level.
R145 R146
1K 56KJ
D105
MA700
R147
10K
HL
PR
When the heat roller surface temperature is low, the thermistor resis-
tance increases. When the surface temperature is high, the resis-
tance decreases. The thermistor terminal voltage, therefore, in-
creases when the heat roller surface temperature is low, and
decreases when the temperature is high. The thermistor terminal
voltage is inputted to the CPU analog port, and the CPU controls
on/off of the heater lamp with the input voltage level.
[High temperature protecting circuit in hung up of the
CPU]
The voltage at IC133 3 pin (reference voltage) is divided by R149 and
R150, and the thermistor terminal voltage is applied to IC112 9 pin.
When 9 pin voltage is lower than 8 pin voltage (the heat roller surface
temperature: about 230 degree C), IC112 14 pin becomes LOW. As a
result, HL signal is pulled to the GND level and the heater lamp
lighting signal is not generated. (IC112 output 14 pin is normally
HIGH (Open state).)
VAREF
R149
7.5KJ
9
+
14 FTH
8
-
IC112C
NJM2901M
R150
4.7KJ
RTH
AN4
I/O
CPU
A C
24V
IC102
9
COM
1 16
I1 O7
2 15
I2 O6
3 14
I3 O5
4 13
I4 O4
5 12
I5 O3
6 11
I6 O2
7 10
I7 O1
8
GND
ULN2003A
Heat roller
G N D
AC PWB
(RY201)
-PR
[When the heat roller surface temperature is lower than
the set temperature]
a. Since the thermistor pin voltage is higher than the set level, output
signal HL from the CPU becomes HIGH.
b. HL signal is passed through IC102, TR, and Q110 to the solid
state relay.
When, therefore, HL signal is LOW, the internal triac turns on.
c. When the internal traic turns on, a pulse is applied to the gate of
the external triac to allow a current to flow from the power source
through the heater lamp to the triac, thus lighting the heater lamp.
[When the heat roller surface temperature is higher than
the set temperature]
a. Since the thermistor pin voltage is lower than the set level, output
signal HL from the CPU becomes low.
b. HL becomes low, SSR is turned off, the external triac is turned off,
and the heater lamp is turned off.
[Q110]
Q110 protects the heater lamp from lighting due to trouble in the
harness of the heater lamp drive signal.
12 – 6
24V
Q110
DTA114YK
47K
10K
R105
2.2KJ 1/2W
24V
Q111
DTA114YK
47K
10K
PR
C507
R504
CR501
GATE
T1
T2 TD501
R503
200V AREA
Thermostat
He ater
170˚ C
l ump
He ater
Thermostat
l ump
170˚ C
HL
SSR501