Printhead Operation; Printhead Temperature - Oki Microline 520 Service Handbook

Printhead Operation

When the printhead is idle, the armature is attracted to the permanent magnet. The print wires,
which are attached to each armature, are then concealed inside the wire guide.
When a signal to print a character is received, current flows through a coil. The magnetic field
generated by the coil opposes the magnetic field between the armature and the permanent
magnet. The armature is then driven in the direction of the platen by the force of the armature
spring. The print wire (which is attached to the armature) protrudes from the tip of the wire
guide and strikes the paper through the ribbon. This prints a dot on the paper.
As the armature gets closer to the electrode of the analog sensor, the capacitance between them
increases and a small amount of current flows. This current is amplified and sent to the logic
control LSI to indicate armature activity. In order to attain optimum drive time, this information
is transferred to the MPU. The MPU continually modifies the head gap to maintain the optimum
drive time condition.
The analog sensor element monitors all of the pins. The difference in pin stroke due to the
curvature of the platen is compensated for at the logic control section of the printhead and is not
transferred to the MPU.
After the character has been printed, the permanent magnet attracts the armature and the print
wires are retracted into the wire guide.

Printhead Temperature

If the printhead temperature exceeds 110 degrees Celsius, the printer will switch to
uni-directional print. If the temperature exceeds 118 degrees Celsius, printing will stop. Once
the printhead cools, printing will resume.
Two thermistors are used to monitor the printhead temperature. One is contained in the
printhead. The other is in the printhead driver.
A thermistor in the printhead prevents over-heating of the coil during periods of continuous
bi-directional printing. If the temperature of the coil exceeds the pre-determined limits, the
control circuit detects the thermistor signal (TSD).
Another thermistor monitors the temperature of the printhead driver. If the temperature of the
driver exceeds the pre-determined limits, the control circuit detects the thermistor signal (TSD).
The voltage of the TSD signal is monitored at the MPU A/D port to control the pass number and
print method (uni-directional / bi-directional) at each of seven levels.
Principles of Operation 2 - 22 Microline 520/521 Service Handbook
P/N 59257803