Brother MFC-9420CN Service Manual page 69

<Temperature Control and Safety>
The detection of the fusing temperature is performed by the thermistor and transmitted to
the temperature control circuit. The temperature control circuit makes the comparative
control with the basic control temperature signal and ON/OFF control over the triac (FLS)
so that the temperature of the heat roller will be the preset temperature (PS).
The temperature control is made in terms of the both hardware and software, and detects
any error with the following detection meanings. Upon the detection of such error, the
machine stops and the error message is displayed on the operator panel.
H0: If the wire-cut occurred in the temperature sensor thermistor (TH) or the
breakdown such as wire-cut occurred in the circuit, the wire-cut detection
signal (THERR) is outputted. This signal works to turn off the relay (RY-1)
and the triac (FLS) and subsequently stops the machine.
H1: If the surface temperature of the heat roller continues to be higher than the
preset value of abnormal temperature (TA) for more than the specified time,
CPU forces the HON signal and ACOFF signal to turn off according to the
timer control so that the machine stops.
H2: If the surface temperature of the heat roller does not reach to the preset
temperature (TS) in the specified time after power-on, CPU forces the HON
signal and ACOFF signal to turn off according to the timer control so that
the machine stops.
H3: If the surface temperature of the heat roller continues to be lower than the
preset temperature (TS) for more than the specified time, CPU forces the
HON signal and ACOFF signal to turn off according to the timer control so
that the machine stops.
H4: If the surface temperature of the heat roller continues to be higher than the
preset temperature (TS) for more than the specified time, CPU forces the
HON signal and ACOFF signal to turn off according to the timer control so
that the machine stops.
HA: If the temperature of the heat roller becomes abnormal due to the break-
down of the control circuit etc., the temperature detection circuit (CM2)
transmits the ACOFF signal. This signal turns off the relay (RY-1), and
simultaneously makes the CPU stop the machine.
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