Protection Function - Canon LBP-1120 Service Manual

3. Protection function

The printer contains the following three protective functions in order to prevent excursion of the fixing heater.
◆ The CPU monitors the voltage of TH701. When detecting an abnormality, the CPU assesses a fixing heater failure
and sets the FIXING HEATER DRIVE signal (FSRD) to "L" and the /RLYD signal to "H" as to turn OFF the relay
(RL101), shutting off power to the fixing heater. At the same time, the CPU notifies the failure to the interface
controller.
◆ When the fixing heater temperature rises abnormally and the thermal fuse exceeds 228°C, the thermal fuse melts,
cutting off the power supply to the fixing heater.
◆ When the fixing temperature rises abnormally and the voltage of TH701 falls below about 0.67V (244°C equivalent),
the fixing heater safety circuit interrupts the power supply to the fixing heater regardless of the output from the
CPU.
4. Detecting Fixing Heater Error
The CPU assesses fixing heater failures and notifies the failure to the interface controller under the following conditions:
a. Within 500 msec after the start of the initial temperature control, the temperature does not reach 20°C.
b. During the fixing heater temperature control, the temperature remains above 250°C for more than the specified
period of time.
c. During the print or warm-up period, the temperature does not reach 100°C, even though the power has been
supplied for the specified period of time.
d. After the 500-msec fixing heater temperature control at the powering-ON, the CPU starts the normal temperature
control and monitors the temperature every 135 msec. During this normal temperature control, the temperature
sequentially drops to less than 20°C twice.
e. During the normal temperature control, the fixing heater temperature falls below 100°C for the specified period of
time.
Note:
The fixing film may be damaged if rotated at a temperature of 100°C or below due to the high oil
viscosity. Therefore, the fixing heater is required to rise above 100°C as to melt the oil before
rotated.
When determining a fixing unit failure, the CPU performs the following operations:
1) Cuts off the power supply 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 of ongoing paper.
5. Pressure Roller Dirt Prevention Function
This function prevents toner from adhering to the pressure roller and building up on it.
As the temperature goes down after the completion of printing, the toner remaining on the nip adheres to the
pressure roller. As the time goes by, the toner develops gradually to form a lump. The lump sticks to the fixing film,
causing the paper to be dirty. To prevent this, the CPU performs the following operation:
1) After the printing is completed and the main motor stops, the CPU heats up the fixing heater with high temperature
for five seconds.
2) When detecting that the thermistor drops to the specified temperature, the CPU drives the main motor for 500 msec
to move the nip location.
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