HP LaserJet 5000 Service Manual - Page 96

Overcurrent Overvoltage Protection This circuit located on the DC controller PCA always monitors an abnormal rise of the fusing roller temperature. If an error occurs, this circuit outputs a signal to turn the relay (RL1) off and interrupt the power to the fusing roller heater. The circuit operates as follows. When the fusing roller heater temperature rises and the output voltage of the thermistor exceeds about 0.5 V (230°C), pin 7 of the comparator (IC304) goes "L." IC304 pin 1 goes "L," Q4 turns off, then the relay (RL1) turns off. The printer has the following three protection functions to prevent wrong activation of the fusing roller heater. q The CPU monitors the thermistor (TH1301) voltage. If it is abnormal, the CPU identifies the fusing roller heater error, turns the relay (RL1) off, and reports it to the Formatter. q If the fusing roller heater temperature rises excessively and the TH1301 voltage falls below about 0.6 V (220°C), the safety circuit in the fusing roller heater interrupts the power to the fusing heather regardless of the CPU output. q If the fusing roller heater temperature rises abnormally and the thermal fuse temperature exceeds 240°C, the thermal switch turns off and interrupts the power to the fusing roller heater. EN Power Supply System 5-5