Using non-rechargeable batteries on old HP calculators

[jebem]

(12-11-2014 08:08 PM)Dieter Wrote:  

(12-03-2014 11:16 PM)Katie Wasserman Wrote:  I use Energizer lithium cells in and they're up to 1.7 volts per call, so quite a lot higher than nicad's. I've never had a problem doing this with any HP calculator: classics, woodstocks, spicse, stings, etc.

Finally I would like to ask if someone has looked at the schematics of a Classic, Woodstock or Sting series calculator, especially at the voltage supply, and may dare an educated guess from there regarding the maximum "safe" battery voltage. I understand that even Lithium cells obviously do no harm, but maybe this observation can be confirmed by a closer look at the schematics. ;-)

Others can add more insight on this subject, but from what I know from my HP-25, the IC's runs on higher voltages than the one supplied from the batteries, and to achieve this, a regulated DC-DC converter is employed.
It means that the input voltage is not that critical at it seems at first look.

In this example, the nominal supply input voltage is around 2.5VDC when using the original two NiCd rechargeable batteries, while using replacement nominal 1.5VDC non rechargeable cells gives about 3VDC. We are talking here on a excess of just 0.5VDC.

Considering that the average input current on these LED calculators is around 140mA to 170mA (from it, one not so small part is wasted in the DC-DC converter due to efficiency limitations), a little more input voltage is even welcome.

Jacques Laporte shares plenty of insight on these classic machines that I enjoy to revisit from time to time.