HP41C "Power Module"


Ever since the 41C has been built discussions came up because of the power

supply. Although the standard machine is very "green" and has a minimum power

consumption users get nervous when devices are connected that do not fit in that

line. The first to mention is the magnetic card reader which eats lots of energy

and renders a BAT signal pretty fast after not too abundant usage.

With the rise of the 41CL the energy question has become even more pressing,

first and foremost if you want to perform writing operations on flash.

To make long things short, I pursued the idea to feed the 41C "backwards". The

IO ports have connectors over which an external power source may supply the

calculator, thus being independent of battery power (Figure 1).

Figure 1

I (ab)used a simple memory module and withdrew the memory chip for the purpose

to solder wires on the respective port outlets and connect them to a 6V DC

supply (with VBat being positive and GND negative polarity).

Any other module would work just as fine but I think that a memory

module is the most easily obtainable and cheapest. The power supply of a cell

phone was a good solution for me. The voltage is stable and the output current

of 800 mA is enough to drive a magnetic card reader and is sufficient to warrant

accurate flash writings in the 41CL (Figure 2).

Figure 2

The battery compartment can be left operational while the external power source

is used. I measured a "loading current" of about 2.5 µA when the power supply

was used in parallel. A such a low current does not produce any heat even if the

calculator is active over night and does not harm the batteries as far as I made

the experience. Of course, the battery compartment may be pulled out while the

external source is used. Caution has to be exercised if the external power

supply is taken from the socket. I made the experience that in that case the

batteries feed the voltage converter, in my situation with a current of about 8 mA.

Interestingly, this solution works well on all 41 fullnuts (HP41C/CV/CX) but not

so well on the halfnuts (41CX with rounded display corners). The main problem in

the halfnut models is that the external power supply appears not to render

enough power to the circuitry to shut off the low bat annunciator (BAT). Even a

rise of up to 8V (I didn't dare more) was not able to toggle off the BAT signal,

i.e. Flag 49 was still active. However, the halfnut performed well with

exception of the CLOCK (resp. SHIFT ON) operation - which of course is a default

behavior when the battery is low.

I do not know if this kind of solution has been suggested before and if I have

"carried coals to Newcastle" (the German proverb I am familiar with would be "to

carry owls to Athens"). I have used this form of external power supply for a

couple of months now and I am very satisfied with it. I think that this may be

generally a good solution for people who do not have a battery pack with

rechargeables as it is a "minimal invasive" way to supply the HP41 with external

power. The basic prerequisites are a free physical port, some basic tinkering

and the exertion of blasphemy on an original HP41 module.


Well done.
Not so different from Diego's solution: he uses an USB cable connected to an empty module, so you can attach it to any free port or to a wall adapter. I have to try if it gives the problems you mention when attached to an halfnut.



I have to try if it gives the problems you mention when attached to an halfnut.

Yes, confirmed. You're right!


so you can attach it to any free port or to a wall adapter

Or to a small USB LI-Ion battery pack. There are several makes of battery packs that are charged using a standard usb cable. I have the Energizer one which came with a zipper case that holds all sorts of adapters. The battery is 2 A capacity. I also have a smaller 1.25 A battery by Duracell.



Ok, so someone needs to get set up making some of these. People donate some cash and a memory module :-) and get a power hookup back.


Gene, you got me thinking.

I think we could make up a small PCB that would have contacts that connect to the 41 and include either a micro or standard USB connector on the other side. We could then make up a shell using rapid protyping plastic. That way you could utilize any off the shelf cables or wall warts.


The battery is 2 A capacity. I also have a smaller 1.25 A battery by Duracell.

...2 Ah, 1.25 Ah??


I did some poking around inside my half nut and it would seem that the battery voltage is being monitored differently. My best guess would be that in the half nuts the battery voltage is monitored at the battery terminal. With +5VDC connected to Vbat on the I/O port the "BAT" indicator does come on however all the other voltages inside are normal and the calculator seems to operate just fine. There are a number of steering diodes inside to protect the calculator from reverse voltage and also to isolate the battery terminals from Vbat on the I/O ports, because of these diodes when you apply 5VDC to the Vbat connection the voltage on the battery terminal is about 1V, however if you apply 5VDC directly to the battery terminals the calculator of course powers on and the "BAT" indicator does not come on.


I confirm your findings for halfnuts. I´m using one of Diego´s USB power modules and definitely the BAT annunciator comes on - even if the calculator seems to work well without batteries.

So yet one more reason to favor FullNuts over their half-board breed, can you spell CL??



I´m using one of Diego´s USB power modules and definitely the BAT annunciator comes on

As testified by my (ugly) picture... ;-)



...can you spell CL??

Eh... yes: C ... L

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