Canon MF4500 Series Service Manual Download Page 49

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Technical Overview > Fixing System > Various Control Mechanisms > Protective Functions

to the fixing heater.

3) Protection function by temperature fuse (TP1)

When the fixing heater temperature rises abnormally and temperature fuse (TP1)
temperature reaches approx 226 deg C, TP1 opens to shut the power supply to the fixing
heater.

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Failure detection

With this machine, 8 types of failure detection are available.

1) Initial startup failure detection

When temperature of the Fixing Assembly does not exceed 50 deg C within 1.48 sec after
the start of temperature control performed at initial rotation, it is judged as initial startup
failure.

2) Initial Thermistor open detection

When temperature of the Fixing Assembly does not exceed 35 deg C within 1.48 sec after
the start of temperature control at warm-up rotation temperature control or at cleaning
mode, it is judged as Thermistor open failure.

3) Short circuit of Thermistor/abnormal high temperature detection

When temperature of the Fixing Assembly is 220 deg C or higher for 30 consecutive times
by monitoring the temperature for every 5 msec, it is judged as Thermistor short circuit
failure. In addition, this function also doubles as abnormal high temperature detection
function of the Fixing Assembly.

4) Abnormal low temperature detection

After temperature of the Fixing Assembly exceeds 50 deg C at least once, monitor
temperature of the Thermistor for every 5 msec. It is judged as heater abnormal low
temperature when the following condition is detected 240 consecutive times: the
temperature is 100 deg C or lower while paper is at fixing nip, or it is 55 deg C or lower
during paper interval temperature control or at cleaning mode.

5) Thermistor open detection

After temperature of the Fixing Assembly exceeds 50 deg C, monitor temperature of the
Thermistor for every 5 msec. When it is detected that the temperature is lower than 20 deg
C for 6 consecutive times, it is judged as Thermistor open failure.

6)Startup failure detection

If temperature of the Fixing Assembly is 100 deg C or higher when the machine becomes in
pickup enabled state after entering fixing low voltage inlet sequence, it is judged as startup
failure.

7) Thermistor low temperature detection 2)

Temperature of the Thermistor is monitored for every 200 msec when paper is at fixing
nip during the Fixing Assembly control. When the temperature is lower than 135 deg C
(120 deg C), increase the value of low temperature detection counter by 1 (+1). When it
is 135 deg C (120 deg C) or higher, decrease the value by 1 (-1). When the value of low
temperature counter b150, it is judged as Thermistor low temperature failure. The
default and minimum value of low temperature detection counter is 0.

8)Frequency detection circuit error (230V models only)

When frequency measurement is not completed within 3255 msec after the completion of
previous frequency measurement, it is judged as frequency detection circuit error.

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Processing after failure detection

If the Main Motor is in driving state when Fixing Assembly failure is detected, rotation of the
motor is maintained for 300 msec after the machine moves in Fixing Assembly failure state.
Then, drive systems (Main Motor system, Laser/Scanner system, high voltage system, and
fixing system) are stopped immediately and the machine moves in failure state.
If the Main Motor is not in driving state, drive systems are stopped immediately and the
machine moves in failure state.