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This calculator was developed fron the QT-8B portable calculator and uses the same chip set.
The journal "Electronics" reported[1]:
"How to cut a pocket calculator in half
Limited battery power was the biggest problem faced by Sharp Corp. engineers when they were told to repackage the company's Micro-compet electronic desk calculator into a case with about half the volume of that formerly used. The new case has only enough room for six penlight-size rechargeable nickel cadmium cells, giving a total of 450 milliampere-hours compared with the 1,200 milliampere-hours available in the earlier model. Even the smaller cells are a tight fit: the, case measures 2.76 inches high by 4.02 inches wide by 6.46 inches long. Weight including cells is only 1.59 pounds.
Redesign was quite successful - display drain was reduced to 460 milliwatts from 1,480 mW and logic circuit drain was reduced to 100 mW from 375 mW. As a bonus, the redesign eliminated the separate high-voltage power supply for the display.
The redesigned calculator, called ELSI-8 around the Sharp plant, uses the same four large-scale-integration arrays made by North American Rockwell Electronics. These arrays feature a 4-phase logic scheme in which there is no steady-state dc power drain. Only the small current-1 to 2 mW-needed to charge the small gate capacitances of the driver transistors flows in the logic circuits.
The lion's share of the power charged to the logic circuit is actually dissipated in the clock generator. The new clock generator reduces drain by three quarters and has better frequency stability. It consists
of a static cross-coupled reset-set multivibrator circuit operating at twice the 50-kilohertz clock frequency, followed by logiccircuits for generating the clock signals. An external resistor and an external capacitor set the
frequency. The former arrangement was less stable because it used circuit resistance and stray capacitance. The double frequency arrangement cuts the number of logic inverters needed from 250 to 50.
Display circuit power drain was reduced, and the need for a separate power supply eliminated, by a circuit change that at first glance appears to be a step backward. Instead of using the time division mode pioneered by Sharp several years ago, the new calculator drives each segment of each display tube separately in a static mode. The big virtue of time division is large reduction in driving circuits, but the big disadvantage is that the rapid switching dissipates large amounts of power in load resistors.
Today, the saving in components is far less important than when the
time division mode was adopted. In the new calculator, segment drive is by MOS FETs with nine driving circuits per package. Nine packages are sufficient to drive all eight digits and decimal points. Load resistors are completely eliminated because the display tubes segments serve as loads on the driving circuits.
What's more, because the display tubes are operated in a static mode, the power supply voltage can be reduced from the 55 volts used in the time division mode to 25 V, the same voltage used for the logic circuits. With time division operation, a high supply voltage is needed for high instantaneous brightness to obtain high average brightness because of the short duty cycle."
Rockwell collaborated with Sharp in developing calculator integrated circuits (ICs). A year or two later Rockwell produced its own range of calculators and sold the ICs to many other manufacturers.
For further information about Sharp Corporation and its calculators visit the Calculator Companies section of this site.
References
- "How to cut a pocket calculator in half", Electronics, Feb. 7 1971.
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