Various Control Mechanisms; Controlling The Temperature Of The Fixing Unit; Protection Function; Flicker Prevention Function - Canon MF8100 Series Service Manual

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The controls and functions in the fixing control circuit are described next.

2.5.2 Various Control Mechanisms

2.5.2.1 Controlling the Temperature of the Fixing Unit

2.5.2.1.1 Heater Temperature Control

This function detects an abnormal temperature rise of the heater and shuts the heater off.

This printer utilizes three protective functions performed by the following parts, to prevent an abnormal temperature rise of the heater:

The fixing heater temperature is monitored by two thermistors: the main thermistor (TH1) and the

sub thermistor (TH2).

The TH1 is to control the print temperature and between-sheets temperature. It is placed contacting inside surface of the fixing sleeve and monitors the sleeve tem-

perature. The TH2 is to control the start-up temperature. It is in contact with the fixing heater and detects the temperature rise at the end. As the surface temperature

of the fixing heater rises, the resistances of both thermistors reduce and the voltage of the FIXING HEATER TEMPERATURE DETECTION signals (FSRTH1,

FSRTH2) varies in analog form.

The CPU (IC101) in the DCNT board monitors the voltage of these two signals (FSRTH1,FSRTH2) and accordingly outputs the FIXING HEATER DRIVE signal

(FSRD). Based on this signal the fixing heater drive circuit controls and maintains the fixing heater at the specified temperature.

This control system is divided into the following five fixing sequences.

1) Start-up temperature control

This control is to determine the start-up temperature of the fixing heater according to its temperature (detected by TH2) upon energization. If the heater is energized

within 30 seconds after a print is completed, the start-up temperature is determined according to the last printing

temperature.

The fixing heater on for a specified time before it drives the feed motor if the heater temperature is below 55 deg C at the start of fixing heater drive (detected by

TH2). It drives the motor after the fixing heater is left on for the prescribed time period.

2) Print temperature control

This control is to maintain the fixing heater at the target temperature during printing. The CPU raises the target temperature in stages in one printing process to

prevent the temperature fall of the fixing sleeve as the paper goes through. The target temperature varies in stages depending on the number of print if continuous

printing and it differs depending on the color of print(monochrome or colors) and the media type.

3) Between-sheets temperature control

This control is to make the temperature of the fixing heater lower than the target temperature

during continuous printing to prevent the temperature rise of the fixing sleeve between sheets.

The between-sheets temperature varies depending on the distance between two sheets and media type.

4) Throughput control

This control is to prevent the overheating at both ends of fixing sleeve unit during continuous

printing of a narrow paper. For a continuous printing, this CPU decreases the throughput by extending the paper pick-up intervals if the sub thermistor reads over

270 deg C and the paper

width is narrower than 210mm, or the sub thermister reads over 280 deg C independent of the paper width.

2.5.3 Protection Function

2.5.3.1 Flicker prevention function

This function is utilized to prevent flicker caused by attaching lighting apparatuses to the ACÅ@power source to which the printer is connected.

The heater drive circuit uses a triac to switch AC power. If the user connects lighting apparatuses

to the power supply, to which the power connector is connected, the current flowing into theÅ@heater increases. This decreases the voltage level of the AC line

and results in flicker.

To prevent flicker, the printer utilizes a zero cross circuit that monitors the AC line voltage. The

CPU detects ZERO CROSS DETECTION (ZEROX) signal to optimize the timing to energize the heater.

2.5.3.2 Protective Function

This function detects an abnormal temperature rise of the heater and shuts the heater off.

This printer utilizes three protective functions performed by the following parts, to prevent an abnormal temperature rise of the heater:

1) CPU

2) Fixing heater safety circuit

3) Thermoswitch

Each protective function will be discussed in the following:

1) Protective function by the CPU

The CPU monitors the output voltages (FSRTH1, FSRTH2) of the main/sub thermistors. If the FSRTH1 is approx. 0.77 V or less (equivalent to 245 deg C or higher),

or the FSRTH2 is approx 2.9 V or higher (equivalent to 290 deg C or higher), the CPU determines a fixing unit failure and executes the following:

Fixing unit

Main

Fixing film

thermistor

unit

(TH1)

Thermoswitch

(TP)

Sub thermistor

(TH2)

Fixing heater