HP LaserJet Enterprise 500 Service Manual - Page 53
Fuser over temperature protection, TH3: 285° C 545° F or higher
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Fuser over temperature protection To protect the fuser from excessive temperatures, the product has four layers of protective functions. If one function fails, the subsequent functions should detect the problem. ● DC controller: When a thermistor or thermopile detects a temperature above a certain threshold, the DC controller interrupts power to the specific heater. Following are the thresholds for each component: ◦ TH1: 230° C (446° F) or higher ◦ TH2: 285° C (545° F) or higher ◦ TH3: 285° C (545° F) or higher ● Fuser heater safety circuit: If the DC controller fails to interrupt the power to the heaters at the prescribed temperatures, the fuser heater safety circuit deactivates the triac-drive circuit and releases the relay, which causes the heaters to stop at slightly higher temperature thresholds. ◦ TH2: 290° C (554° F) or higher ◦ TH3: 290° C (554° F) or higher ● Current detection protection circuit: If current flowing in each triac exceeds a specific value, the current detection protection circuit deactivates the triac-drive circuit and releases the relay, which interrupts the power supply to the heaters. ● Thermoswitch: If the temperature in the heaters is abnormally high, and the temperature in the thermoswitches exceeds a specified value, the contact to the thermoswitch breaks. Breaking this contact deactivates the triac-drive circuit and releases the relay, which interrupts the power supply to the heaters. Following are the thresholds for each thermoswitch: ◦ TP1: 270° C (518° F) or higher NOTE: When the thermoswitches reach this temperature, the temperature on the fuser rollers is about 320° C (608° F). ENWW Engine control system 17
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