72 Theory of operation Q2431-90912
Fuser over-temperature protection
The fusing heater safety circuit is located on the power supply and constantly monitors the fusing
temperature.
To protect the fuser from excessive temperatures, the printer has the following three protective
functions:
● The CPU monitors the voltage of the thermistor. If the fuser temperature reaches
240° C (464° F) or higher, the CPU turns off the relay (RL101) to interrupt the power
to the fusing heater.
● If the temperature of the fusing heater continues to rise abnormally and the
temperature of the thermistor (TH1) exceeds about 250° C (482° F), the relay 1
(RL101) opens up to cut off the power supply to the fusing heater.
● When the temperature of the heater exceeds about 250° C (482° F), the thermal
switch (TP1) is turned off to cut off the power supply to the fusing heater
.
Figure 9. Fuser over-temperature protection circuit block diagram
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