Canon MF4690 - ImageCLASS B/W Laser Service Manual page 91

From the start of temperature control until 1.47 sec
Normal times
During normal temperature control
During temperature control at sheet-to-sheet interval, during cleaning mode
After the thermistor temperature exceeds 50 deg C
From the start of power distribution to the fixing heater until 23 sec
While a paper is nipped in the fixing assembly
2. Protection function with the fixing heater safety circuit
The circuit always monitors voltage of the fixing heater temperature detection signal sent from the thermistor. If the fixing temperature greatly exceeds the specified
value, it blocks the power distribution to the fixing heater regardless of the instruction from CPU.
If the temperature of the fixing heater abnormally increases and the voltage of the fixing heater temperature detection signal sent from the thermistor becomes less
than about 0.57V (equivalent of 265 deg C), the output of the comparator becomes 'L' and the transistor turns OFF. As a result of that, the relay is turned OFF to
block the power distribution to the fixing heater regardless the relay drive signal sent from the CPU.
3. Protection function with the temperature fuse
If the temperature of the fixing heater abnormally increases and the temperature of the temperature fuse exceeds about 230 deg C, the temperature fuse meltdown
to block the power distribution to the fixing heater.
Failure of fixing assembly
T-8-4
Thermistor temperature
Less than 55 deg C
230 deg C or above
Less than 100 deg C
Less than 55 deg C
Less than 20 deg C
Less than 100 deg C
Less than 210 deg C
Less than 145 deg C
Heater temperature detection time
0.15 sec in a row
1.2 sec in a row
1.2 sec in a row
0.3 sec in a row
30 sec in a row
30 sec in a row
Chapter 8
8-5