Sharp SF-1120 - Manual (Page 131)

(6) Heater lamp control circuit
1 General
The heater lamp control circuit detects the heat roller surface
temperature with the thermistor, converts it into a voltage, and out-
puts it to the CPu analog input pin.
The CPU converts analog signals into digital signals, compares them
with the value set by test commands to turn on/off the heater lamp,
maintaining the heat roller surface temperature at a constant level.
The higher the heat roller surface temperature is, the greater the
thermistor resistance is, and vice versa.
Therefore, the lower the heat roller surface temperature is, the higher
the thermistor pin voltage is. The thermistor pin voltage is inputted to
the analog port of CPU, which controls ON/OFF of the heater lamp
according to the input voltage level.
[High temperature protect circuit when the CPU is hung up (out
of control)]
1) The IC20 8 pin voltage (reference voltage) is divided by R91 and
R93. The thermistor pin voltage is inputted to the IC20 9 pin.
When the voltage at 9 pin becomes lower than the voltage at 8 pin
(the heat roller surface temperature about 240V), the IC20 14 pin
is driven to LOW, pulling down the PR signal to the GND level,
turning off the AC power of the heater lamp.
2) Since the OP-latch signal is fixed to HIGH or LOW, the collector of
Q7 is open. The voltage at IC20 10 pin is higher than the voltage
at IC20 11 pin (reference voltage), and IC20 13 pin is driven to
LOW.
With the above operations, IC9 13 pin is opened to turn off Q5,
turning off the power relay. As a result, the AC power of the heater
and the copy lamp is turned off.
* The double protection with 1) and 2) is provided.
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
O1
O2
O3
O4
O5
O6
O7
SK
I1
I2
I3
I4
I5
I6
I7
G
14
8
9
D6 D7
IC20C
NJM2901
R91
8.2KJ
R87
1MJ
R89
4.7KF
D3 8
MA700
R9 2
47KF
C55
22000PF
R93
680J
13
11
10
IC20D
NJM2901
R67
1KJ
R6 8
5.6KJ
C7
0.22µ F
R7 5
75J
R84
10KJ
R7 1
47KJ
R54
5.6KJ
R5 3
47KJ
+24V
Q 5
2SA573
GATE T1
T2
AC PWB
PR GND
AC PWB
PR
L2
N1
R35
270J
1/4W 10K
2.2K
Q3
DTA123Y5
+5V2
PR
HL
R32
2.7KJ
CPU
AN1
ML
ML
200V AREA
MLSW
MLSW
ML : 1KW
IC9
ULN2004
PR
8 ~ 20
D16
DSM101
D41
Q 7
2SC945 C6
0.22µ F
R80
24KJ
OP-LATCH
CPU 17PIN
+5V1
Vref
AC
Heat roller
12 – 12