Canon L120 Series Service Manual page 44

Chapter 2
The surface temperature of the fixing heater is monitored by the thermistor (TH1) that remains in contact with the fixing heater. The resistance of the ther-
mistor decreases in response to increases in the surface temperature of the fixing heater, causing the voltage of the fixing heater temperature detection
single (FSRTH) to change.
The CPU (IC902) found inside the DCNT board monitors the signal (FSRTH), and generates the fixing heater drive signal (FSRD) according to the voltage
of the signal operating in conjunction with the CPU. The fixing heater drive circuit, on the other hand, uses the signal to control the fixing heater so that
the temperature of the fixing heater remains at a specific level.
The machine executes the following 4 control mechanisms according to the temperature of the fixing heater and the size of the selected paper:
1) start-up temperature control; from when the print command arrives at the DCNT board to when the fixing heater reaches the target temperature.
2) paper passage temperature control; during printing so that the temperature of the fixing heater is identical the target temperature.
The target temperature may be any of 5 settings, used according to the selected fixing mode. Any of these modes is selected according to the settings of
the paper size and the number of printouts.
The following shows the relationship between the fixing mode and fixing target temperature:
3) sheet-to-sheet temperature control; between sheets during continuous printing so that the temperature of the fixing heater is lower than the target
temperature, thus preventing overheating of the fixing film.
4) down sequence; during continuous printing so as to prevent overheating of the edges of the heater.
In this control, the CPU of the DCNT board checks the length and width of paper; if the size of the paper matches a specific set of conditions, the machine
is shifted to long narrow mode (Note) or small paper mode regardless of the settings of the driver.
In this mode, the CPU forces the printing speed to decrease conversely increasing the distance between sheets, thereby increasing the sensitivity of the
thermistor located in the middle of the fixing heater and, consequently, preventing the edges of the heater from overheating.
The following shows the relationship between the printing speed and the conditions used to make a shift to this mode:
Down-sequence mode
No down sequence (normal
state)
Small size paper mode
Long narrow paper mode
2-8
Power supply PCB
Fixing control circuit
Fuse
(FU101)
+3.3V
SSR101
Q102
Fixing heater drive circuit
J303-3
Thermistor (TH1)
Fixing film unit
Thermal fuse (TP1)
Fixing heater (H1)
Pressure roller
Paper type selected for
driver
Plain paper
Heavy paper
Heavy paper H
Transparency, plain paper L 150 deg C
Envelope
Paper length
267.4 mm or more 197 mm or more
267.4 mm or less
267.4 mm or more 197 mm or less
Fixing heater safety circuit
+24V
Relay
(RL101)
+24V
Q103
IC501
-
+
J303-1
Q104
J302-4
J302-3
F-2-6
T-2-6
Fixing target temperature
165 to 175 deg C
180 to 190 deg C
180 deg C
165 to 175 deg C
T-2-7
Power width Printing speed (ppm)
Number of printouts according to
paper size
197 mm or less 8-->6-->4
3
DCNT board
FSRD
J201-19 J902-5
IC902
CPU
FSRTH
J201-20 J902-4
RLYD
J201-21 J902-3