Handheld Calculator Power Supplies


I was recently given a TI-55 calculator. One of its interesting features was a dc-to-dc converter module in the battery pack that converted the 2.4V from the two NiCD cells to 9V to run the calculator. The previous owner had replaced the battery pack with an ordinary 9V battery.

I can't recall ever seeing an HP calculator with a dc-to-dc converter in the battery pack. HP's battery packs always appeared to be simple assemblies of NiCD cells. The dc-to-dc converter, if any, was located on the main printed circuit board.

Not having opened many of the calculators that I've owned over the years, I was curious about which HP models ran directly off the batteries and which ones needed a dc-to-dc converter for a voltage boost and possibly voltage regulation.

I've noticed that HP's graphing calculators do not seem to be afflicted with a problem that I've noticed in TI's graphing calculators, an LCD display that fades as the batteries are losing their charge, requiring the LCD contrast to be adjusted. Which suggests that HP uses a voltage booster/regulator and TI doesn't.


All the HP LED display calculators had an internal dc-to-dc converter. I've opened many pre-LCD handheld calculators to fix them and/or replace the nicad batteries. The TI's are the only ones that I've come across with the dc-to-dc converter in the battery pack -- it was a nifty idea. But it had one huge drawback: the charge would drain off exceptionally quickly (much faster than the nicad's self-discharge time) because the conversion circuit would drain the batteries even when the load was zero.

Moreover, aside from the HP's and the TI's very few calculators even had removable battery "packs". The early Sanyo and Sharp models are notable exceptions.


The really huge drawback of all the TI battery packs which used dc-to-dc converters was that Ni-Cad cell leakage or outgassing would destroy the printed circuit paths on the converter board (and in some cases the wiring from the cells to the converter board) with the result that the battery pack could not be repaired by simply replacing defective Ni-Cad cells.


Palmer O. Hanson, Jr. wrote:
> The really huge drawback of all the TI battery packs which used dc-to-dc converters was that Ni-Cad cell
> leakage or out-gassing would destroy the printed circuit paths on the converter board ...

Of course the plastic container would also reduce the corrosion to the rest of the calculator.

You can make a DC-DC converter much more easily than repairing the whole calculator.

I have an HP-67 with immaculate shell (essentially unused), but all the internal components are completely corroded.



Are we talking about the same battery packs? The battery packs used in the TI-55 has two AA size NiCad cells and the dc-to-dc converter which generates the nine volts used by the calculator all inside the battery pack envelope.

The BP-7 which is used in the TI-55, TI-57 and TI MBA has a dippy little three pin connector. To use a standard nine volt battery in those devices the user has to fabricate some sort of adapter. The BP-8 used in the TI-30, SR-40 and Business Analyst I has the same internal configuration as the BP-7 but has a standard nine volt connector.

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