Canon NP6218 Detecting Overheating at the End of the Fixing Heater, Protection Mechanism

CHAPTER 6 FIXING SYSTEM

D. Detecting Overheating at the End of the Fixing Heater

A sub thermistor (TH2) is provided at the end (rear) of the copier’s fixing heater to monitor overheating.

This consideration to prevent damage to the fixing film caused by overheating of the area where paper (smaller than B4) does not come into contact.

When TH2 registers 250°C or more, the distance between copies is increased to facilitate discharge of heat by the fixing heater, thereby preventing overheating.

The copier continues to make copies maintaining such a distance even after TH2 registers a value lower than 250°C.

If the temperature further increases after the distance has been increased and if the value reaches 260°C or more, the ongoing copying operation will be stopped and the power will be cut off; at the times E001 information is stored. (See p. 10-109.)

E. Protection Mechanism

The copier is equipped with the following protective functions to prevent malfunction of the fixing heater:

1. Thermistor (TH1, TH2)

1)The microprocessor on the DC controller monitors the voltage of the thermistor (TH1, TH2); if it detects a high or low temperature error, it stops the power to the heater and cuts off all the power. At the time, E000 thorugh E003 information is stored. (See p. 10-109.)

2)If the overheating detection circuit identifies that TH1 has registered 260°C or TH2 has registered 270°C for 0.3 sec or more, the heater error detection signal (HTNG) turns off the relay (RL101) to stop power to the fixing heater and cut off all the power. (See p. 10-109.)

Caution:

Be sure to disconnect the connector (J4) of the fixing heater and press the service

switch (SW301) on the DC controller when turning on the power.

2. Thermal Fuse (FU1)

If the temperature around the thermal fuse remains 226°C or more for a specific period of time, the fuse melts to cut off the power to the fixing heater.

3. Heater ON Detection Circuit

The period of time during which the fixing heater drive signal (HTRD) remains off is detected; if an error is found, the power is cut off and E004 information is stored at the time.

6-6 COPYRIGHT © 1997 CANON INC. CANON NP6218 REV. 0 MAY 1997 PRINTED IN JAPAN (IMPRIME AU JAPON)