Alarm Circuit - OKIDATA ML320 Service Manual

Page: 29
Service Guide ML320/ML321
Chapter 2 Principles of Operation

2.3.06 Alarm Circuit

Drive Circuit Fault Alarm Circuit
This is a protective circuit which causes the AC line fuse to open when a fault occurs in the printhead
drive circuit, the spacing motor drive circuit, or the line feed drive circuit. The fault alarm circuit monitors
the drive time of each of these circuits and produces the ALM signal if any drive time exceeds its
specified value. The ALM signal activates SCR ON, which causes the secondary coil (Rev A, B, and
C: 40 volts. Rev D: 50 volts) of the transformer to short-circuit. This creates an over-current condition,
which causes the AC line fuse to open.
Head Overheat Alarm Circuit
In order to protect the printhead coils, this circuit uses the printhead's built-in thermistor to monitor
printhead temperature.
Printhead temperature rises when the unit is printing. When printhead temperature reaches 110
degrees Celsius, the HEAD OVERHEAT alarm is generated. Printing stops when this alarm is
detected. This allows the printhead temperature to fall. When the printhead temperature falls
below the alarm detection level, printing is resumed.
Here is a description of the alarm detection.
When the temperature of the printhead rises, the resistance of the thermistor decreases and the
potential falls at the negative input of comparator Q6. Q6 is located on the main controller board.
This inverts the comparator output, causing the HEAD TEMP signal to be sent to the motor control
LSI.
Paper End Detection Circuit
When the printer runs out of paper, the photosensor on the main control board is deactivated.
This causes the signal PAPER END to become low. This signal is fed to a pin of the motor
control LSI. Printing stops and the ALARM lamp lights.
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