Canon LBP-1210 Service Manual page 94

CHAPTER 2
The thermal fuse (FU1132) is installed in the center of the fixing heater. When the fixing
heater temperature rises abnormally, the fuse melts and interrupts power to the fixing heater.
The temperature of the fixing unit, structured with the above mentioned parts, is controlled
by the fixing control circuit on the engine controller on the instruction from the CPU (IC201).
This circuit contains the fixing heater drive circuit and fixing heater safety circuit.
b. Fixing temperature control
This control detects the temperature of the fixing heater surface and controls the fixing heater
drive signal to bring the temperature to its target.
The temperature of the fixing heater is detected with the main thermistor (TH701) and sub
thermistor (TH702) located on the fixing heater. When the fixing temperature increases, the
resistance levels of each thermistor decrease. Then the voltage level of the FIXING HEATER
TEMPERATURE DETECTION (FSRTH) signal for the main thermistor decreases and that of the
FIXING HEATER TEMPERATURE DETECTION (FSRSTH) signal for the sub thermistor increas-
es.
The CPU (IC201) on the engine controller monitors the voltage level of the FSRTH signal. The
CPU outputs the FIXING HEATER DRIVE (FSRD) signal based on the voltage level to bring the
fixing temperature to its target by turning on/off the photo triac coupler (SSR101). The CPU
also monitors the voltage level of the FSRSTH signal and detects the abnormal high-temperature
on the end of the fixing heater.
The fixing film contains oil with high viscosity, so the oil may damage the fixing film. To pre-
vent this, when the power is turned on, the CPU starts fixing temperature control to bring the
fixing heater to 100 °C in order for the grease to melt.
The CPU does not perform fixing temperature control during standby mode. After receiving
a print command from the interface controller, the CPU starts the start-up temperature control
for the fixing heater. When the fixing temperature reaches the target temperature, the CPU
starts normal fixing temperature control.
The CPU performs the following three fixing temperature control based on conditions:
1) Start-up temperature control:
control target temperature after a print command is input to the engine control.
2) Normal temperature control:
control target temperature during a print operation.
3) Between-sheet temperature control:
temperature control target temperature in between sheets to prevent an abnormal tempera-
ture rise in non-paper-path areas.
This printer changes the fixing temperature control and print speed suitable for a print media
according to paper type setting on the page setting tab in the printer driver. The CPU detects
the length of paper with the top of page sensor (PS402) and changes the fixing temperature con-
trol and print speed according to the length also. This prevents an abnormal temperature rise
in the fixing unit during a continuous print operation. Table 2-2-1 indicates the relationship
among paper types, target temperature, and print speed.
If the paper length is between 356 mm and 271.4 mm, the CPU changes the print speed to 3
pages per minute when the detection temperature by the sub thermistor exceeds 235 °C in order
to prevent a temperature rise at the end of the fixing unit.
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Brings the fixing temperature to the normal temperature
Brings the fixing temperature to the normal temperature
Sets the fixing temperature lower than the normal