The fixing heater safety circuit always monitors the output voltage of the main/sub thermistors (FSRTH1, FSRTH2). If the voltage of the FSRTH1 is approx.

1.15V and lower (equivalent to 250 deg C and higher), or the voltage of the FSRTH2 is approx. 0.44V and lower (equivalent to 290 deg C and higher), the circuit

determines the fixing unit failure and the program runs along the following sequence.

1. The fixing heater safety circuit makes the MAIN THERMISTOR RELAY DRIVE signal (RLD-) inactive.

2. The fixing heater safety circuit makes the SUB THERMISTOR RELAY DRIVE signal (RLD+) inactive.

3. The relay drive circuit releases the relays (RL3901 and RL3902) to interrupt power supply to the fixing heater.

3) Protective function by thermoswitch

When the temperature of the fixing heater rises abnormally high and the detected temperature of the thermoswitch (TP) exceeds approx. 250 deg C (see Note),

the thermoswitch breaks contact of the line and the power supply to the fixing heater is immediately cut off.

The actual temperature of the fixing heater is about 40 degrees higher than that of the thermoswitch, as the thermoswitch does not make direct contact with the

fixing heater with the spacer.

2.6.3.2 Failure detection

/ i-SENSYS LBP5360 / i-SENSYS LBP5300

The CPU determines the fixing unit failure, stops the printer engine, and notifies the video controller of "fixing unit failure", when it encounters the following con-

ditions, 1) to 6).

1) Start-up failure (Warm-up failure)

The CPU determines the start-up failure:

1. If the detected temperature of the sub thermistor does not reach 130 deg C within 40 seconds of heater energization during WAIT period.

2. If the fixing unit does not enter READY within 40 seconds of heater energization at print operation start.

2) Abnormal low temperature of main thermistor

The CPU presumes the main thermistor has no conduction:

1. If the detected temperature of the main thermistor is kept below 40°C for 4 seconds continuously, from heater energization until residual paper detection is

completed during WAIT period.

2. If the detected temperature of the main thermistor is kept below 70 degC for 4 seconds continuously, from heater energization until paper reaches the fixing

unit at the start of print operation.

3. If the detected temperature of the main thermistor is kept below 130 deg C for 0.5 second continuously, from paper reaches the fixing unit until heater is turned

OFF during PRINT period.

3) Abnormal high temperature of main thermistor

The CPU determines the abnormal high temperature of main thermistor, if the detected temperature of the main thermistor is kept higher than 240 deg C and

over for 0.1 second.

4) Abnormal low temperature of sub thermistor

The CPU presumes the sub thermistor has no conduction at following conditions

1. If the detected temperature of the sub thermistor is kept below 60 deg C for 4 seconds continuously, from heater energization until residual paper detection is

completed during WAIT period.

2. If the detected temperature of the sub thermistor is kept below 90 deg C for 2.5 seconds continuously, from heater energization until paper reaches the fixing

unit at the start of print operation.

3. If the detected temperature of the sub thermistor is kept below 120 deg C for 0.5 second continuously, from paper reaches the fixing unit until heater is turned

OFF during PRINT period.

4. If the detected temperature of sub thermistor is kept below 50 deg C for 5 seconds continuously during STBY period.

5) Abnormal high temperature of sub thermistor

The CPU determines the abnormal high temperature of sub thermistor, if the detected temperature of the sub thermistor is kept higher than 280 deg C and over

for 0.5 second.

6) Drive circuit abnormality

The CPU determines the drive circuit abnormality, if the ZERO CROSSING signal (ZEROX) is kept on being out of frequency range 40Hz to 70Hz for 5

seconds.