Canon LBP-1210 Service Manual page 95

Paper size recognition
Paper length (mm)
Plain Paper
Transparencies
Special Paper 1
Special Paper 2
c. Protective function
The printer utilizes the three protective functions performed by the following parts to prevent the
excursion of the fixing heater:
1. CPU
2. Fixing heater safety circuit
3. Temperature fuse
Each function is explained in the following.
1. Protective function by the CPU
The CPU monitors the output voltage of the FIXING HEATER TEMPERATURE DETECTION
(FSRTH, FSRSTH) signals output from the main thermistor and sub thermistor. When the fix-
ing temperature vastly exceeds its specified value, the CPU cuts off power supply to the heater.
When the voltage level of the FSRTH signal from the main thermistor drops to about 0.75 V
or less (equivalent to 240 °C) due to the abnormal temperature rise of the fixing heater, the CPU
sets the FIXING HEATER DRIVE (FSRD) signal "L" to turn the photo triac coupler (SSR101) off
and shuts off the fixing heater.
Likewise, when the voltage level of the FSRSTH signal from the sub thermistor rises to about
2.93 V or more (equivalent to 320 °C) due to the abnormal temperature rise of the fixing heater,
the CPU sets the FIXING HEATER DRIVE (FSRD) SIGNAL "L" to turn off the photo triac coupler
(SSR101) and shuts off the fixing heater.
2) Protective function by the fixing heater safety circuit
This circuit constantly monitors the output voltage of the FIXING HEATER TEMPERATURE
DETECTION (FSRTH, FSRSTH) signals output from the main thermistor (TH701) and sub ther-
mistor (TH702). When the fixing temperature vastly exceeds its specified value, the circuit shuts
off power to the fixing heater regardless of the output from the CPU.
If the voltage level of the FSRTH signal from the main thermistor decreases to about 0.75 V
or less (equivalent to 240 °C) due to the abnormal temperature rise of the fixing heater, the out-
put of No.1 pin on the comparator (IC301) goes "L" and the transistor (Q103) is turned OFF. This
turns OFF the relay (RL101) to interrupt power supply to the fixing heater regardless of the
RELAY DRIVE (RLYD) signal output from the CPU.
Likewise, if the voltage level of the FSRSTH signal from the sub thermistor increases to about
2.93 V or more (equivalent to 320 °C) due to the abnormal temperature rise of the fixing heater,
the output of No.7 pin on the comparator (IC201) is set "L" and the transistor Q104 is turned
OFF. This turns OFF the relay (RL101) to interrupt power supply to the fixing heater regardless
of the RLYD signal output from the CPU.
3) Protective function by the thermal fuse
Table 2-2-1
Target temperature
Normal Small
356 - 271.4 271.4 - 127
170 - 180˚C
150˚C 170˚C
180 - 190˚C
Print speed (page/min.)
Normal
271.4 - 249
356 - 271.4
8 6
14
CHAPTER 2
Small
249 - 127
3.9 6 3.9
2 - 9