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Service Guide ML390/391
Chapter 2 Principles of Operation
2.4 MECHANICAL PRINCIPLES OF OPERATION
2.4.01 Printhead
Operation
The printhead used by the printer is a highly efficient, stored-energy type. Power is not consumed until the
pins are activated. Printhead life is approximately 200 million characters.
The printhead is designed with twenty-four pins, vertically aligned. Each pin is welded to an armature ring.
Behind the armature ring is a spacer ring.
Each armature has a permanent magnet behind it. The magnet attracts the armature, keeping the pin
inside the printhead. Because of the spacer ring behind the armature ring, the armature is attracted toward
the permanent magnet at an angle. A coil is wrapped around each permanent magnet.
When a dot is printed, current passes through the coil, creating a magnetic field which counters the
magnetic field of the permanent magnet. The armature and pin spring forward. A dot is printed on the
paper.
When current is removed from the coil, the magnetic field of the permanent magnet attracts the armature.
The pin is retracted into the printhead.
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