Canon LBP-800 Service Manual page 83

2. Fixing heater safety circuit
This circuit is located on the engine controller PCB and always monitors the abnormal rise of
the fixing temperature.
When the output voltage from the thermistor falls below about 0.6V (240°C) due to the ele-
vation of the fixing heater temperature, the output of the comparator (IC302) becomes "L" and
the relay (RL101) goes OFF to cut off the power to the fixing heater.
3. Protection function
The printer has the following three protective functions to prevent excursion of the fixing heater.
The CPU monitors the voltage of TH701. When an abnormality is detected, the CPU assess-
es a fixing heater failure and then sets the FIXING HEATER DRIVE signal (FSRD) to "L" and
the /RLYD signal to "H" to turn OFF the relay (RL101). At the same time, it notifies the inter-
face controller of the failure.
When the fixing heater temperature rises abnormally and the temperature of the thermal
fuse exceeds 240°C, the thermal fuse melts and the power to the fixing heater is cut off.
When the fixing temperature rises abnormally and the voltage of TH701 falls below about
0.6V (240°C equivalent), the fixing heater safety circuit cuts off the power to the fixing heater
regardless of the output from the CPU.
4. Detecting fixing heater error
The CPU assesses a fixing heater failure and notifies the interface controller of the failure under
the following conditions.
a. The temperature of 20°C or above is not detected within 500 msec after the start of initial
temperature control.
b. The temperature remains above 220°C for more than the specified period of time during fix-
ing heater temperature control.
c. The temperature of 100°C or above is not detected when the power is supplied for the spec-
ified period of time during printing or warming up.
d. The fixing heater temperature control is performed for 500 msec when the power is turned
ON. During this period, the temperature is monitored every 135 msec. The monitored tem-
perature is below 20°C twice in a row.
e. The fixing heater temperature falls below 100°C for the specified period of time during the
normal temperature control.
Note: The fixing film may be damaged if it is rotated at a temperature of 100°C or below due
to the high oil viscosity. Therefore, the temperature of the fixing heater is required to be
above 100°C to melt the oil.
When a fixing unit failure is assessed, the CPU performs the following operations:
1) Cuts off the power to the fixing heater.
2) Turns off the relay.
3) Immediately stops driving the main motor, scanner motor, and high-voltage system and sets
the printer to the error mode regardless of the presence or absence of paper being printed.
CHAPTER 2
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