Early Electronic Calculator HP-20 and 30 Series Battery Substitute

HP 20-series and 30-series pocket calculators use two NiCd cells for the battery. The battery is an integral part of the power supply, providing filtering and voltage limiting. Using the original wall-wart charger the calculator will not function properly without the battery installed.

The battery can be replaced with a simple 'equivalent circuit', involving a couple of electrolytic capacitors and some diodes. This can be built into the original battery pack case, and can be installed into the calculator without requiring any alterations to the calculator itself. The photos show this done for an HP-21.

Referring to the schematic, the capacitors provide filtering of the rectified DC. The diodes, together with the series resistance of the transformer and the voltage dropping resistor inside the calculator, function as a shunt regulator. The four diodes in series limit the voltage to a nominal 2.8V .

The downside to this scheme is that when the calculator is turned off the shunt regulator will continue to consume energy.