Konica Minolta bizhub C250 Service Manual page 144

Theory of operation ver. 3.0 Aug. 2006
20.3.12 Abnormally high temperature detection (software protection)
• It detects when the temperature of the heating roller and the fusing pressure roller
increase abnormally high. It informs the error and stops the operation.
A. Detection condition
• When conditions below are protected, it will be judged as "heating roller abnormally high
temperature C3751/fusing pressure roller abnormally high temperature C3752."
When the temperature of TH1 and TH3 goes above
specified degree and stays for more than one sec-
ond, the fusing temperature will be judged to be
abnormally high (heating heater.)
When the temperature of TH2 goes above specified
degree and stays for more than one second before
the fusing temperature control starts, the fusing tem-
perature will be judged to be abnormally high (fusing
pressure heater.)
When the temperature of TH2 goes above specified
degree and stays for more than one seconds after
the fusing temperature control has started, the fusing
temperature will be judged to be abnormally high
(fusing pressure heater.)
20.3.13 Abnormally high temperature detection (hardware protection)
• It forces the heater relay to be OFF to turn the heater lamp OFF when the high tempera-
ture trouble cannot be detected due to CPU run away.
• It will be judged as "fusing unit abnormally high temperature (heating heater) C3751"
when detecting the heater relay OFF.
• Power supplied to the heater lamps can be shut off before the thermostat operates. It
protects the fusing unit from being damaged.
A. Heater lamp OFF control
Control
When the temperature of TH4 and TH5 go over
the temperature by the comparator, the com-
parator (circuit) on the mechanical control
board will turn the heater lamp relay of the DC
power unit OFF, to shut power off for each
heater lamp.
B. Heater lamp relay OFF detection control
• The thermistor detects the comparator, and turns heater lamp relay OFF.
• When the heater lamp relay turns OFF, it stays OFF.
• When the heater lamp relay stays OFF, the OFF signal will be entered.
• When the OFF signal is entered and stays for more than specified period of time, it is
judged to be the fusing unit abnormally high temperature (heating heater.)
C. When to start controlling
• It always watches the heater lamp relay OFF signal.
Control Thermistor
Heating roller ther-
mistor/1
Heating roller ther-
mistor/2
Fusing pressure
roller thermistor/1
Fusing pressure
roller thermistor/1
Thermistor Symbol
Heating roller ther-
TH4
mistor/3
Fusing pressure
TH5
roller thermistor/2
20. Fusing section
Symbol Set temperature
TH1
222°C
TH3
TH2 222°C
TH2 222°C
Specified temperature
by the comparator
232°C
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