Canon ImageCLASS MF4690, Service Manual

Page: 89 / 182

Chapter 8
8-3
8.2 Various Control Mechanisms
8.2.1 Controlling the Temperature of the Fixing Unit
8.2.1.1 Fixing Temperature Control
0016-1859
With this control, the surface temperature of the fixing heater is detected, and also the drive signal of the fixing heater is controlled to maintain the target temperature
of the fixing heater.
The temperature control of the fixing heater is executed with the following 3 modes:
Temperature Control at Startup:
The temperature control at startup is executed from the entry of print command to the DCNT board until the temperature of the fixing heater reaches the fixing
target temperature.
The fixing target temperature is set to execute fixing with appropriate fixing temperature in accordance with paper type, size, thickness, and shape.
Temperature Control at Normal Times:
The temperature control at normal times is executed during printing to maintain the temperature of the fixing heater as its target temperature.
Temperature Control at Sheet-to-sheet Interval:
The temperature control at sheet-to-sheet interval is executed to keep the temperature lower than the target temperature in order to prevent the temperature raise at
the area between a preceding sheet and a succeeding sheet in case of the envelop mode and the small size mode.
The temperature of the fixing heater is detected with the thermistor located on the fixing heater. When the surface temperature of the fixing heater increases, the
resistance value of the thermistor is decreased, and the voltage of the fixing heater temperature detection signal of the thermistor is decreased.
The CPU (IC902) on the DCNT board monitors the voltage of the FSRTH signal. By outputting the fixing heater drive signal (FSRD) in accordance with the voltage,
the CPU makes the phototriac coupler to turn ON/OFF, and it controls the temperature of the fixing heater to be the specified value.
The CPU starts the fixing heater temperature control to be 100 deg C when the power is ON.
Because the viscosity of grease inside of the fixing film unit is high, melting the grease fully prevents feed fault inside of the fixing assembly or faulty image.
Fixing Mode:
In this machine, the fixing mode is determined by detecting paper size based on the time that a paper needs to go through the paper leading edge sensor, the paper
width sensor, and the delivery paper width sensor.
The relationship between paper size being detected and the fixing mode is shown below. (From A to D indicate paper size.)
F-8-3
T-8-2
Paper length Paper width sensor
PS802 Delivery paper width sensor
PS804 Fixing mode
A
Less than 267.4 mm Detect Detect Envelop
B
267.4 mm or above Detect Detect Normal*1
C
267.4 mm or above Not detect Not detect Small size paper
D
Less than 267.4 mm Not detect Not detect Envelop
A
B
C
D
267.4 mm
PS802 PS804