Revised 5/8/06

Diane—Goddess of Printing

Like their predecessors, the HP 9820A and 9810A, the third-generation HP 9825A and 9815A desktop calculators would have internal, strip printers based on thermal printing technology. A thermal printer creates dots on a chemically treated paper by heating localized areas on the paper (dots) with a thermal printhead. The chemical infused into the thermal paper is thermochromic—it changes color when heated. The printhead essentially consists of several thin-film resistors. When enough current flows through one of the resistors, it heats up sufficiently to initiate the thermochromic reaction in the thermal paper. The result is a blue or black dot on white paper. Once a line of dots is formed, the printer mechanism advances the paper under the print head so that another line of dots can be formed. With enough blue dots on the paper, characters form.

Because the HP 9825A and 9815A were physically much smaller than their predecessors, they could not use the relatively large printing mechanism used in the earlier machines. The earlier printing mechanism used in the HP 9810A and HP 9820A employed an expensive stepper motor to move the thermal paper past the print head. The stepper motor paper-advance mechanism had all the traits of an HP product: it was precise, quiet, large, and expensive. The reduced size and cost of the third-generation machines mandated lower-cost, more-compact printer mechanicals. The printer project for the HP 9825A and 9815A was code named Diane and Roger Edrinn, who had developed the strip printer mechanism for the HP 9810A and 9820A, was the mechanical engineer assigned to the task of designing the printer mechanism for the third-generation calculators. Edrinn developed several functional prototype printer mechanisms at great expense, but they were all too complex, expensive, and power hungry.

About the same time that Edrinn was developing Diane prototypes, Don Morris’ old boss Rex James convinced Morris to take on a tooling engineer named Ken Johnson. Johnson knew nothing about dynamic machinery or electrical motors but Morris assigned him to Edrinn’s project. Morris explained how an electric motor worked to Ken, who summarized the electric motor explanation “as an electromagnet pulling on a piece of iron at certain times.”  “Yeah, that’s it Ken,” said Morris, who wasn’t expecting much to come out of this new team member that Rex James had been so eager to discard.

A week or so later, Johnson asked Morris for some tooling money. It was peanuts with respect to the cost of Edrinn’s prototypes so Morris gave his OK. About a week later Johnson asked Morris to look at a small mechanism that he’d built that had the ability to pull paper past a printhead. Johnson made his paper-advance mechanism work just by manually touching a wire lead from the mechanism to a power supply. Amazingly, Johnson’s two-week wonder worked! Both its mechanical design and the electrical drive signals it required were much simpler than a stepper-based design.

Ken Johnson’s printer mechanism used a solenoid-activated plastic ratchet that significantly lowered the cost of the HP 9825’s printer. Photo courtesy of Fred Wenninger

Printing faster

Several of the thermal printer’s characteristics set limits on the drive levels for the thermal resistors in the printhead. The low-end operating temperature for the two calculators (not called computers for truly political reasons) set the minimum amount of current needed to heat the thin-film printhead resistors to the transition temperature that caused the thermal paper to change from white to blue. The maximum printing speed was set by the calculator performing continuous printing while operating at maximum ambient temperature. This condition caused the printhead’s backing plate to slowly heat up to a maximum operating temperature that caused the thermal paper to start turning blue all over, but still resulted in adequate print contrast. A simple wait loop set the maximum print speed and controlled the printhead’s maximum operating temperature. Another critical factor in the printer’s operation for both the 9815A and 9825A was the need to limit the printer’s duty cycle to prevent heat dissipation in the power supply from exceeding its limit.

As a result of all these conflicting limitations, only one printer was originally designed and it printed at the same rate in both the HP 9825A and the 9815A. However, Morris’ competitive nature would not tolerate being hamstrung by the HP 9815’s limitations. Everything about his HP 9825A had to be better! Something had to be done.

Adding a temperature-sensing thermistor to the printhead’s backing plate in the HP 9825A printer provided the needed temperature feedback that allowed full drive current to be applied to the print head at cold temperatures and permitted drive levels to be automatically reduced as the ambient or printhead temperature increased. At low temperatures, the HP 9825A’s power supply was in no risk of overheating so the printer ran at full speed. As temperatures increased, the power delivered to the head could be reduced while still allowing the printer to operate at full speed. The added thermistor not only enhanced printing speed, it stabilized the printhead’s operating temperature during continuous printing operations at any ambient operating temperature.

Helping out the power supply

Throughout the design of the HP 9825A, the demands on the power supply increased as more capabilities were designed into the calculator. The additional power demands caused the power supply’s heat dissipation to rise. Because no additional cooling was available, the HP 9825A’s internal power supply was getting hotter and hotter. Heat equals death for electronic components so the heat had to be reduced.

The HP 9825A’s power supply is built on a single board attached to an aluminum bracket that serves as the major heat sink for all of the high-power components on power supply board. The power supplies in the HP 9825A included +5V for the logic circuits, +15V and -15V supplies for the tape cartridge, and +20V for the printer. The 20V power supply ran both the thermal printhead and Ken Johnson’s “flapper” mechanism for advancing the paper. Reducing the power dissipated by any of the power supplies on the board would reduce the heat and improve power supply reliability.

By far, the largest power supply in the HP 9825A is the 5V supply. It also generates the most heat. When the HP 9825A’s printer was in constant use, the 5V power supply tended to exceed its power levels. So a neat little trick was added. Normally the circuitry driving the printer’s flapper mechanism would have to withstand an inductively generated negative voltage spike when power was removed from the flapper solenoid (which was an inductive device). Usually, a “catch” diode would be placed across the inductive solenoid to safely dump power from the spike into the power supply’s ground return.

The catch diode actually dissipates a great deal of energy, which had been stored in the flapper magnet while it was energized. The energy dissipated in the catch diode represents wasted energy. By using a Zener diode as a catch diode and returning the flapper solenoid’s energy to the +5V power supply, almost 75% of the stored energy was returned to the machine’s main power supply.

The HP 9825A’s 5V supply was very well regulated and the spikes of returned power delivered through the Zener diode were always small relative to the power being consumed, so the 5V power supply stayed well regulated and operated under less load when the printer was operating due to the extra energy being returned from the flapper solenoid. This scheme worked so well that the power saved in the printer mechanism was used elsewhere in the HP 9825A’s design.

References

Information on this page came from Don Morris.