HP41 Power Supply


Hi, I am new to this forum and have gone trough the Archive, but might have missed things. I would like to assamble a Mains supply for my HP41 (both CV and CX).
I have the following items:

- A recargable battery pack and the original HP Charger. Unfortunately this is dead to the point where the machine will not even operate when the Charger is connected to the mains.

- A few standard battery packs. I have purchased NiCad (GP-Silva) cells but they do not seem to pull it off in the calculator. I assume the voltage (1.2 instead of 1.5) is too low. I get the 41 to come on with 'bat' indicator, but failing to operate the card reader at all.

I have learned that the 41 needs 6V DC, so the standard Charger is not much use without building a regulator, but I suppose that I can find some old 6V DC Adapter around somewhere. Further, I suppose that I could operate on one of the regular battery packs to accomodate for a plug which would lead to the connectors inside the 41.

Question is: has anyone done this, or does anyone have a better solution to operate the HP in a more economic way than purchasing the standard cells?

Thanks a lot, looking forward to any suggestions!



There was a recent thread here with much good info on rebuilding the rechargeable pack (i.e. what part number nicads to use).

I have used "N" nicads in the regular battery holder, charged externally, with success, but I haven't used the card reader. I know it is a heavy load but I have always heard that it is the main reason many people would use nicads - to save money on alkalines. Maybe you aren't charging them completely? BTW, I have never seen another product that uses "N" cells (well, OK, the HP28C/S) and I think HP41 (and 28) users must be keeping them in production!


The last Duracell "N" cells I bought had a logo in the blister about medical uses; perhaps hearing aids or some kind of cardiac monitor uses them.


Correct, Andrés. I have seen old-fashion hearing aids and portable FM tuners. Also, take some mini toys, too.


Totally off-topic, but somewhere I have a hearing aid that uses an AA cell and a battery, about the size of an AA cell but sqaure in cross-section giving 22.5V. The active
components are valves (tubes), of course, IIRC 2*DF64 for the first 2 stages and a DL64 in the output stage.


The HP rechargeable battery packs use four small nicad cells. The 41's should definitely work just fine on four "N" cell nicads in the standard alkaline holder (HP even sold these as an option). The Nicads can provide MUCH more current than the alkalines and drive the card reader quite well. In fact this was the main use of the HP rechargeable pack. Alkalines will last over a year of normal use, but the reader will suck them dry in short order.


Thans for all responses to my querys. I have given my NiCads a second chance after the suggestions that the voltage should not be a real problem and... they seem to work fine now!
What remains is a completely dead HP41 rechargable pack and a charger. I would like to be able to exchange those dead cells and use the pack in combination with the charger/adapter. It does not seen easy to open the pack, no sight of screws or clips. Does anyone have tips or experience operating that thing?

Much appreciated.




I have once opened a battery pack by using sort of a cave-man procedure :-(

I had to press (using fingers) right and left sides and carefully separate both up and down halves of the battery case by using a thin screwdriver, as you hear some snaps indicating they are getting apart. If you are patient enough, it’s harmless.

After inspecting the little PC board inside it, I had to remove a lot of chemical dust by using acid (warm lemon juice) and cleaning with water.

At that time (8 years ago) I had found a pack of rechargeable batteries composed by three NiCad, fortunately the same size used in the 41´s pack. This pack was easily found at that time cause it was used in some PC Boards for XT-type computers and were fairly available, at least here in Brazil. I´m sure I still have one at home, and I'll search for it and check for brand and type.

Rebuilding was easy, but the batteries did not rest for more than 1 year, and died too. The pack I used was somewhat old and I was not aware of it.

Well, the experience was good and I still have both halves of the pack holder, but I have lost both batteries and rectifier/regulator PC Board.

Somewhat vague, I know... Maybe it helps.


I have just looked at the circuit diagram of both versions of the nicad pack that I have circuit diagrams for (are there more?)

It seems to me that your nicads have cone short circuit (maybe only a couple of them).

One version if the pack will work quite nicely if you clip the link between 2 of the nicads (and thus run from the charger alone)

The other version will probably kill your calculator if you do so :-(

(You can actually tell by looking at the charger connector or the battery terminals).

It seems you have the calculator running with other batteries, so there's no need to investigate this further.

However if you do go on to open the pack and replace the nicads, I have a fairly simple fix to prevent the possibility of the pack delivering too high a voltage. It costs less than a dollar, and if you're inside the pack, that's the time to do it :-)

If I can find the time I'll try to post the procedure as an article.


My Pack is a 82120A with sn 3233S00204. I recall having bought it as a replacement (and having payed much for it!).
If I look at it from the top (I call the end that goes in towards the top of the calculator the top), I see the four barreries. The outside ones have the clip that connects to the calculator, the inside ones are connected by a small welded strip of metal.
If I understand well, there is a chance that the external power supply will work if I disconnect that strip? That would be good enough for me, because I can use the NiCad's in the regular pack normal, and have this solution when I need to charge the NiCad's.
Are you aware of a way to test this without running the risk of sending the 41 up in smoke?

Thanks a lot.



Remember -- no guarantees!

Look at the battery contacts. Is one covered by a piece of plastic with a contact on it?

Look at where the charger connects. Is there is small piece of metal pressing against the centre connector.

If the answer to both of these questions is YES, then you have the older model which will send a rather larger voltage to the calculator if the batteries go open circuit.

If the answer to both of these questions is NO then you have the newer version which will not kill the calculator if the batteries go open circuit.

I believe that you probably have the latter one, because the former will still provide power to the HP41 even if the batteries all go short circuit.

Measure the voltage across the outer 2 contacts without the charger applied (you will probably get a small voltage -- but maybe 0)

Then connect the charger. The voltage should rise to approx 1.2, 2.4, 3.6, or 4.8 volts. These voltages indicate 1, 2, 3, or 4 cells not shorted. (note that the voltages may vary somewhat from these figures depending on the state of charge on the good cells).

If you see a change in voltage when you add power, then the charging circuit is probably OK.

Now clip the connection between the two centre cells. Apply power to the charger and measure the voltage again. It should be approximatly 6V.

If the voltage is between 5.5 and 6.5 volts then all is probably OK to use this as a power source.

But before you do, place some insulation tape over the centre connectors of the nicad pack to prevent any chance of the HP-41's battery contacts from linking the nicads back again.

Insert into your HP41, and see if it runs while on the charger.

Note that removing the charger will cause the calculator to lose power, but if you're careful you won't lose memory if you don't leave it for too long in this state.

Note that with this charger being used like this, the maximum current that can be provided to the calculator will be insufficient to power the card reader or the wand.

Depending on the current required by the calculator, even operation of the calculator may be marginal. Watch out for the BAT indicator which will certainly come on in this case.

Ah! even if there is insufficient power to run the calculator, it will CERTAINLY provide plenty to keep the memory alive while you charge the nicads you're using.

The other option is to get a set of N cells to use while you're charging the nicads. Used this way they'll probably last for years :-)


Are you thinking of adding a zener diode? I've never understood why HP didn't put one in the calculators that need the battery pack in place, to keep the voltage from getting too high when the battery terminals get corroded. I'm convinced this is the reason my first several HP25s (not 25C) had bad storage register/program memory chips.

Well, I can think of two objections: First, the zener will have some leakage and will shorten the time between chargings, although this should be a very small current if the diode voltage is above the battery voltage (while still low enough to do any good.) And second, when the battery is out of the circuit, all the charging current will go through the zener, possibly creating a thermal problem. 50 mA for AA cells (I think) times 3 V (reasonable for a 2 cell battery pack application) equals 150 mW - that doesn't seem so bad but who knows?

Another possibility is UL requirements regarding batteries. I doubt if the calculators themselves (at least the handhelds) were UL but a company like HP would be very aware of and sensitive to the requirements. I remember when the IBM PC/AT first appeared with its backup battery, there was a blocking diode and a 1K resistor in series with the battery. The compliance guys where I worked said that was for UL, a failsafe in case the battery load got shorted. It didn't matter to UL if there was a diode, but there had to be two things in series with the load. Putting a zener directly across a battery might be a UL no-no.


Yes, I place a zener across the output.

However this will not cause significant additional leakage. I use a 1W 6.8V zener, and although the coltage rises to 6v during charge, the nominal voltage for the pack is 4.8v and this is quite a long way below the knee of this device.

In the worst case, the (batteries going open circuit) the dissipation would be lower than during a charge because the zener has a higher voltage across it than the batteries, thus the current through the curent limiting resistor is lower. Thus lower current from the same DC rail means lower power dissipation.

Assuming a constant 16mA charge current, the dissipation of the resistor is almost always 0.15w (580R). The zener would dissipate 0.11w.

The later version of the charger circuit DOES have a zener across the output, and charges the nicads from a regulated 12v rail via a resistor,

What is UL???


UL stands for "Underwriters Laboratories", an US based (I think private) organization, which audits and approves electric security of devices (not only electronics). I think it all started with fire security of appliances, and its beginnings were driven by insurance concerns.

AFAIK, UL requirements for low voltage devices are very simple, that is why the UL marks usually appears on the AC adapters or AC powered products (it makes sense form the "fire" safety point of view, although rechargeable batteries may be harmful).

UL, and similar agencies in other countries, use a FMEA approach (Failure Modes and Effects Analysis), where the manufacturer should prevent the consequences of *each* *single* component failure; that is why a series resisitor may appear connected to the needed diodes, for instance. This will limit currents to safe levels in case the diode gets "shorted"


Underwriters Laboratories, an independent company that provides product testing and follow-up inspection of manufacturing facilities. Most electrical equipment, and many other kinds of products, sold in the USA have their approval. A competitor of theirs is ETL, I have only seen one piece of consumer equipment (a food dehydrator) with the ETL mark. I once saw a computer that had been submitted to UL for testing, they had opened the hard disk drive and immobilized the platters with duct tape to see how it would behave with a stalled rotor.


I just pulled out my rechargeable pack, which is in pieces, scrubbed and waiting for me to find the right nicads. I had sketched a schematic "from life" (I like that term, I got it from Robert Pease, the analog guru at National Semiconductor) and see that mine has a zener diode across the battery!

Connected to the AC adapter there is a rectifier bridge, a filter cap, a TO-92 device that would appear to be a 78L12 (from the voltages measured) HP P/N 26-0275, then in series with the battery a forward biased diode and a 365 ohm resistor, and in parallel with the battery a reverse biased diode that measures 6.7 V across it (no nicads). So I guess I could use my nicad-less pack to run the calculator

I have a set of 1/3AA nicads I remember ordering from a DigiKey catalog some time ago based on the dimensions and my attempt to measure the rechargeable pack plastic and my hope that the nicad dimensions in the catalog were maximums (maxima?)! In the "2/3AAA NIMH" (approximately) thread, someone said the 1/3AAs could be made to fit with perseverance, so I will have to put myself to the task.


Yes, that is the newer version.

1/3 AA can be made to fit, but you will need to make the holes in the bottom of the battery pack larger, and make holes in the side too. The strip between the batteries and the charger must be thinned down. Lastly the plastic on the top side (that which is on the outside of the calculator) will need to be made thinner.

The pack ends up being a slightly tighter fit, bit has worked fine for me.

BE VERY CAREFUL inserting the pack before you have the case secured. If you do this, the pack goes in, but WILL NOT come back out again. (embarassing)


Thanks for the details, you have saved me a lot of head scratching.

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