Various Control Mechanisms; Protective Functions; Protective Mechanisms - Canon LBP3200 - LBP 3200 B/W Laser Printer Service Manual

Chapter 2

2.4.2 Various Control Mechanisms

2.4.2.1 Protective Functions

2.4.2.1.1 Protective Mechanisms

The machine is equipped with the following 3 types of protective mechanisms used to prevent its fixing heater from
going awry.
1. protective mechanism by the CPU
2. protective mechanism by the fixing heater safety circuit
3. protective mechanism by the thermal fuse
1. Protective Mechanism by the CPU
The machine's CPU monitors the voltage of the fixing heater temperature detection signal from the main
thermistor and the sub thermistor at all times, and it cuts the power to the fixing heater if it finds that the fixing
temperature appreciably exceeds a specific level.
When the fixing temperature abnormally increases and the voltage of the fixing heater temperature detection
signal from the main thermistor drops to about 0.775 V (equivalent of 235 deg C) or less, the CPU causes the
fixing heater drive signal to go Low and turns off the photo triac coupler, thus cutting power to the fixing heater.
If the temperature of the fixing heater abnormally increases and the voltage of the fixing heater temperature
detection signal from the sub thermistor reaches about 2.779 V (equivalent of 320 deg C) or more, the CPU
causes the fixing heater drive signal to go Low and turns off the photo triac couple, thus cutting off the power
to the fixing heater.
2. Protective Mechanism by the Fixing Heater Safety Circuit
The fixing heater safety circuit monitors the voltage of the fixing heater temperature detection signal from the
main thermistor and the sub thermistor at all times.
If the fixing temperature appreciably exceeds a specific level, the machine cuts off the power to the fixing heater
regardless of the instructions from the CPU.
When the temperature of the fixing heater increases abnormally and the voltage of the fixing heater temperature
detection signal of the main thermistor reaches about 0.775 V (equivalent of 235 deg C) or less, the output of
the comparator turns Low to turn off the relay, thus cutting off the power to the fixing heater.
Likewise, if the temperature of the fixing heater increases abnormally and the voltage of the fixing heater
temperature detection signal of the sub thermistor reaches about 2.779 V (equivalent of 320 deg C) or more, the
output of the comparator turns Low to turn off the transistor, thus turning off the relay and cutting off the power
to the fixing heater regardless of the relay drive signal from the CPU.
3. Protective Mechanism by the Thermal Fuse
If the temperature of the fixing heater increases abnormally and the temperature of the thermal fuse exceeds
about 228 deg C, the thermal fuse starts to melt to cut off the power to the fixing heater.
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