HP82143A Powering a 41 _Extend Your HP41_

[Diego Diaz]

Hi all,

This post was supposed to be published in a few weeks, but since the subject has been brought up this could be a good place/moment to share.

More power (literally) for the HP-41.

Over the years I've seen several ideas (some of them really ingenious) to provide a more convenient, rechargeable, up-to-date battery pack or some type of powering connection for our beloved HP-41s.

I've done a few myself, USB cables and refurbished 82120A with NiMH cells. The first one has the problem of filling one I/O port, while the second offers a very limited capacity and requires the use of the HP transformer.

Also, most of these solutions were just DIY projects and didn't manage to reach a 'production' status and fulfill the needs of the user's community.

Last but not least, the alkalines LR1 type cells are becoming both scarce and expensive and, regrettably, still are the cause for a number of corrosion cases.

In the last couple of months I've been working on a new project to overcome these problems and offer a rechargeable Li-Po battery pack which will, hopefully, become a serious replacement for the alkalines and the venerable 82120A.

The specs of this new power pack are as follows:

- Same dimensions as the original battery holder.

- Same insertion extraction procedure.

- Slightly higher power than the alkalines; 7,000mW vs 6,000.

- Rechargeable while in use via USB-C type connector, thru the existing charging port; or while removed.

- Steady voltage output of 6v. 

- Dual output protection: Insertion switch and fuse.

- Up to 1A continuous current. Well enough to keep the Card Reader running as needed.

Materials:

The pack includes a 2,000mA/h 3.7v Li-Po battery with built-in charge/discharge protection circuit.

USB-C charger with current and voltage limiter IC.

Step-up DC-DC converter slightly modified to provide an output of 6v.

A safety microswitch to disconnect the negative terminal when the pack is removed from the HP-41. This prevents the terminals from accidentally getting short circuited if they both touch a metal surface.

Anyhow (since we've learnt a few lessons from Chernobyl), a 1Amp fuse is connected to the positive terminal. The fuse is placed in a way that is easily accessible so most anyone with average solder skills can replace it if the need arises; and two spare fuses will be provided with the pack.

Seriously, if you blow three fuses, please stay away from any electrical source; for your own safety. :-)

Two drawbacks:

- The principle of operation of the DC-DC converter keeps the voltage in the nominal 6v until the battery is discharged, hence no 'BAT' indicator.

- Some USB-C connectors are built with a metal casing, this type of connectors must not be used with early HP-41C including the golden ball contacts.

So far only the concept and dimensional drawing have been done. First serious test to be run by mid next month, and (if it works as expected) first ten units will hopefully be ready by end July or early August, and before you ask, tag is expected to be around one hundred.

Comments, questions and suggestions are always welcome.

Best wishes from the Caribbean.

Diego.

   
   

PS. The 3D render is by all means inaccurate ans messy