Overcurrent Overvoltage Protection The Fusing Heater Safety Circuit is located on the Engine Controller Board, and constantly monitors the fusing temperature. In case of abnormality, the output from this circuit turns off the relay (RL101) and interrupts the power to the fusing heater. If an increase in the temperature of the Fusing Heater causes the Thermistor Output voltage to exceed about 1.2V (240° C), the output of pin #13 of the comparator (IC301) becomes “L”. When the output of pin #14 becomes “L” and Q302 is turned off, the relay (RL101) is turned off. To protect the Fuser from excessive voltages, the printer has the following three protective functions:
G The CPU monitors the voltage of TH901. If it is abnormal, the CPU assesses fusing heater failure and turns off the relay (RL101). At the same time, it reports the fusing heater failure to the Formatter. G If the temperature of the fusing heater rises abnormally and the voltage of TH901 falls below about 1.2V (240° C), the fusing heater’s safety circuit interrupts the power to the fusing heater, regardless of CPU outputs. G If the temperature of the fusing heater rises abnormally and the temperature of the Thermal Fuse exceeds about 230° C, the temperature fuse melts, which interrupts the power to the fusing heater. Power Supply System 5-3