Protection Against Abnormal Conditions - Konica Minolta bizhub c450 Service Manual

Theory of Operation Ver. 1.0 Mar. 2005
20.3.4

Protection against abnormal conditions

• The machine provides protection against abnormally high temperature of the Fusing Unit
in the following three steps.
1. When the Heating Roller Thermistor/1 detects 245
Roller Thermistor detects 205
the temperature control), the machine displays the malfunction code representing
an abnormally high temperature and prohibits the initiation of a new copy cycle.
(This is an ordinary detection of a malfunction of an abnormally high temperature.)
2. A different protection is provided when the CPU overruns, becoming unable to
detect the malfunction of an abnormally high temperature. If the Heating Roller
Thermistor/1, 2 detects 250
°
235 C, the heater relay of the DC Power Supply is turned OFF through the Control
Board. Power supply to each of the heater lamps is then shut down.
If the temperature detected by the Heating Roller Thermistor/1 and Fusing Pres-
sure Roller Thermistor thereafter decreases, the heater relay is turned ON again
and supply of power to the heater lamps is resumed. The sequence of these oper-
ations is repeated until the CPU recovers from the overrun and becomes able to
detect the malfunction of an abnormally high temperature. This control allows the
power supply to each of the heater lamps to be shut down before the correspond-
ing thermostat operates. Heating Roller Thermistor/2 is, however, protected: it
turns ON the heater relay only after the power switch is turned OFF and ON or the
malfunction is reset. Through these control procedures, the power supply to the
heater lamps can be shut down before the thermostat is activated. It thereby sup-
presses damage to the Fusing Unit itself.
3. If detection of the abnormally high temperature through approaches 1 and 2 above
is not possible due to a defective thermistor or other reason, the thermostat comes
into play to shut down the power supply to each of the heater lamps. The Heating
Roller Thermostat is rated at 210
is rated at 180
ature of the corresponding rollers, since the thermostats are installed in a non-con-
tact fashion with respect to each of the rollers. (Ordinary protection against high
temperatures)
Interlock SW
°
C (before the temperature control) or 230
°
C, or the Fusing Pressure Roller Thermistor detects
°
C, while the Fusing Pressure Roller Thermostat
°
C. The ratings do not, however, represent the actual surface temper-
HEATER
5V
DC Power Unit
TS1
SSR
5V
TS2
SSR
HEATER
HEATER_RY
HEATER_RY
5V
TH2
GND
5V
Power SW
HEATER_RY
TH3 Signal
°
C, or the Fusing Pressure
TH1
TH3 Signal
CPU
TH3
5V
5V
GND
20. Fusing section
°
C (during
4037T2C579AA
95