Canon ImageCLASS MF4690, Service Manual

Page: 91 / 182

Chapter 8
8-5
T-8-4
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 Thermistor temperature Heater temperature detection time
From the start of temperature control until 1.47 sec Less than 55 deg C
Normal times 230 deg C or above 0.15 sec in a row
During normal temperature control Less than 100 deg C 1.2 sec in a row
During temperature control at sheet-to-sheet interval, during cleaning mode Less than 55 deg C 1.2 sec in a row
After the thermistor temperature exceeds 50 deg C Less than 20 deg C 0.3 sec in a row
From the start of power distribution to the fixing heater until 23 sec Less than 100 deg C
While a paper is nipped in the fixing assembly Less than 210 deg C 30 sec in a row
Less than 145 deg C 30 sec in a row