Fixing Temperature Control; Protective Function - Canon LBP6670dn Series Service Manual

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Technology > Fixing System > Fixing control circuit > Protective function

■ Fixing temperature control

The fixing temperature control maintains the temperature of the fixing heater at its targeted
temperature.
Block diagram of this control is shown below.
Low-voltage power supply
Fixing control circuit
Fixing heater
+24V
control circuit
Relay
control
circuit
RL101
Fixing heater
safety circuit
The DC controller monitors the FIXING TEMPERATURE (FSRTH) signal and sends the
FIXING HEATER CONTROL (FSRD) signal according to the detected temperature. The fixing
2 Technology > Fixing System > Fixing control circuit > Protective function
Engine controller
DC controller
Frequency
FREQSNS
detection circuit
FSRD
RLYD
FSRTH
Fixing film unit
H1: Fixing heater
TH1 TP1
Pressure roller TP1: Thermoswitch
TH1: Thermistor
Fixing unit
F-2-15
heater control circuit controls the fixing heater depending on the signal so that the heater
remains at the targeted temperature.

■ Protective function

The protective function detects an abnormal temperature rise in the fixing unit and interrupts
power supply to the fixing heater.
The following three protective components prevent an abnormal temperature rise of the fixing
heater:
• DC controller
• Fixing heater safety circuit
• Thermoswitch
1) Thermoswitch
The contact of the thermoswitch is broken to interrupt power supply to the fixing heater
under the following condition:
• Temperature fuse: 228°C (442.4°F) or higher
2) DC controller
The DC controller monitors the detected temperature of the thermistor. The DC controller
makes the FIXING HEATER CONTROL signal inactive and releases the relay to interrupt
power supply to the fixing heater under the following condition:
• Thermistor: 240°C (464°F) or higher
3) Fixing heater safety circuit
The fixing heater safety circuit monitors the detected temperature of the thermistor.
The fixing heater safety circuit releases the relay control circuit to interrupt power supply
to the fixing heater under the following condition:
• Thermistor: 270°C (518°F) or higher
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