HP V8.5 manual Thermal Printhead, Automatic Head Temperature Feedback

T H E O R Y O F O P E R A T I O N

5.4THERMAL PRINTHEAD

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The thermal printhead is comprised of a row of 1728 heater elements (nibs). Each bit within a scan addresses an individual nib. Plot patterns are generated one scan at a time, each scan consisting of a horizontal row of dots. A nib produces a dot if the corresponding scan bit in the buffer is a logic level “1”. Nibs in the printhead are heated, causing a thermo-chemical reaction with the coating on the media. The intensity or period for which the nib is heated is called the burn strobe.

Automatic Head Temperature Feedback

The thermal printhead rises in temperature as it plots images on paper, thus, less energy is required to activate each nib as the plot proceeds to create an image of the same intensity. A thermistor embedded in the head measures the head temperature and provides feedback information to the main logic board. The main logic board decreases the burn strobe accordingly to keep the contrast of the plotted image consistent as the plot proceeds.

If the printhead temperature reaches 60º C, the Automatic Head Temperature Feedback circuit stops the plotter in order to let the head cool down. The control panel display reads hot (Ht) and the error light flashes. When the thermistor measures that the printhead temperature has cooled to 45º C, the plot resumes with no loss of data.

V8.5eRev2.2.2002 5 -9