CANON LBP800
Service Manual

CHAPTER 2

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 elevation 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. x The CPU monitors the voltage of TH701. When an abnormality is detected, the CPU assesses 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 interface controller of the failure. x 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. x 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 fixing heater temperature control. c. The temperature of 100°C or above is not detected when the power is supplied for the specified 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 temperature 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.

2-9