HP 4300 Service Manual - Page 92
Fuser over-temperature protection, Fuser over-temperature protection circuit block diagram
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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: q 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. q 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. q 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 72 Theory of operation Q2431-90912
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