HP 3500 Series

ENWW Engine control system 121
d. The relay drive circuit cuts the relay (RL301) OFF and interrupts the current to the fuser
heater.
● Fuser heater safety circuit: The CPU monitors the output voltages of the main and sub
thermistors (MAINTH and SUBTH). It cuts the relay (RL301) off and interrupts the current to
the fuser heater when the MAINTH voltage is under approximately 1.0 V (240° C, 464° F) or
the SUBTH voltage exceeds approximately 3.0 V (290° C, 554° F).
● Thermoswitch: When the thermoswitch reads a temperature exceeding 250° C (482° F), it is
disconnected, interrupting the current to the fuser heater.
Note The thermoswitch is some distance away from the fuser heater. The actual temperature of the fuser
heater is about 50° C (122° F) higher than the temperature read by the thermoswitch.
Failure detection
If the CPU judges the fuser faulty, it performs the following tasks:
1 It stops the output of the FUSER DRIVE signal (FSRD) (turn “L”).
2 It sets the RELAY DRIVE signal (RLD) to stop (turn “H”).
3 It sends an error message to the formatter.
The following conditions cause the CPU to judge the fuser to be faulty.
● Set-up failure: The temperature of the main thermistor does reach 40° C (104°), 0° C (32° F)
when power is turned on, within 1.7 seconds from when the heater energizes, or the
temperature of the main thermistor never becomes 5° C (41° F) lower than the target
temperature during the paper passing.
● Abnormally low temperature of the main thermistor (Break in the main thermistor): The
temperature of the main thermistor stays at approximately 70° C (158° F), equivalent to
approximately 3.17 V, or lower for 4.0 seconds or longer.
● Abnormally high temperature of the main thermistor: The temperature of the main
thermistor stays at 230° C (446° F), equivalent to approximately 0.92 V, or higher for 1.0
second or longer.
● Abnormally low temperature of the sub thermistor: The temperature of the sub thermistor
stays lower than 90° C (194° F) for 2.5 seconds or longer.
● Abnormally high temperature of the sub thermistor: The temperature of the sub thermistor
stays at 280° C (536° F), equivalent to approximately 2.8 V, or higher for 0.1 second or longer.
Fuser detection
The printer detects the presence of the fuser.
The fuser is detected by the DC controller during the WAIT period. The DC controller determines
that the fuser unit is not installed if it detects the following 2 conditions and then sends an error
message to the formatter.
1 The FUSER DELIVERY PAPER DETECTION signal (/AFFPSSNS) indicates the presence of
paper (“L”).
2 The temperature at the sub thermistor does not exceed 40° C (104° F) after 500 ms of fuser
temperature control.