A Call for Captain Zener ( and others) for HELP!


Hi, all;

I have an HP82120A, serial # 2309Sxxxxx; it's a rechargeable batt pack for the HP41 series. I need help to identify a zener diode directly connected to the batteries terminals.

I measured voltage across it and I got to the (approximate) 6.2Volts as zener voltage. I inspected the circuit and got to the following:

a bridge rectifier (discrete, four diodes)

a 100uF/25V electrolytic (filter) capacitor in parallel with a 10nF ceramic capacitor

a SOT-type (like a BC-series transistor) integrated regulator

a series rectifier

a 365 ohms, 1%, 1/2 watt resistor and

THE zener diode

Voltages measured are when connected to AC adapter are:

about 18Vcc after rectifier bridge
about 12Vcc after regulator
about 6.5Vcc over the batteries

This pack was recovered after being exposed to corrosive battery leakage and I used everything but original bridge rectifiers and 10nF ceramic capacitor. But I cannot red zener specs, it seems battery leakage reacted with surface coat and there is not visible inscriptions over it. It is too small, almost like those pre-SMD 1/12 watt sub-miniature resistors, found in many earlier VCR and a few PC power supplies. I want to know what zener is this one, because it is a lot critical, as it's responsible to be steady for a somewhat fragile component: the HP41 and all attached interfaces. As it is mostly indicated to be used when power demands go beyond battery lifetime, using it supposes other "power-consuming" components are connected to the calculator, like the wand, card reader or 82242 IR interface. And these are not easy to find, low-cost devices, as the HP41 itself.

I hope you all understand my concerns: if batteries are not good and this zener fails...

Luiz C. Vieira - Brazil


It's a technician..... it's an engineer..... it's a Heathkit builder.......

NO! It's Captain Zener! Gliding thru the attic in a tangle of oscilloscope probes and 50 ohm BNC cables, helping to keep people from despairing over their zener diode problems.
Born on planet "HP" the only thing Captain Zener is vulnerable to is a greenish radioactive mineral called "Carlynite" . If exposed to "Carlynite" Captain Zener is reduced to an incapacitated weakling who falls down and barfs into the bathroom john.......


Introductions aside:
Your battery pack is not familiar to me, so when you talk about all this stuff (regulators, resistors, etc) I dont see a circuit diagram so I dont know what its all about.
Without a schematic, it doesn't mean much.

NEVERTHELESS..... you've been clear enough that the zener diode looked corroded by chemicals, BUT, it was functional and it measured 6.2V. You said it was around the size of a baby resistor (1/8W with leads is common in Japanese equipment).

WELL, ask yourself if there is space for a 1 Watt 6.2V zener, a 1N4735A.

YEP, the same unit we've been always chatting about before, the one that is good to rework into the HP-25C.

I'd say, you need space for a 1/4W resistor. If you have that space then the 1W zener will fit. Of course, its good to have extra lead-length to help the heat to spread out.

You may just need a 1N4735A then.
Probably the one that was in there was from the 1/2W zener family. It would be good, if possible, to use the 1W zener instead (more melt-proof).

Let me know if you that is an acceptable part then,
1N4735A, 1W, 6.2V .


Hello, Norm... Oops, Captain Zener, sorry...

Thank you for the handfull concerns.

About tthe "charger" schematics... No big deal, it looks more like a two-stage regulator: first stage with the 12Vcc regulator, that provides an (about 1 to 3% drift) regulated 12Vcc so you can use a resistor-series voltage drop and a zener. Provided you'll have a stable 12Vcc and an expected current consumption, no matter fo final zener stage.

What called my attention was the tinny 6.2 V zener (it's the standard reference, right?). I take as a critical component, and I thouhgt a 1N4735A would do fine. Thank you.

This way nothing else need to change.

I'm still wondering about what zener is that; can anyone check for it, please?

Thanks, Captain Zener; good job!

Luiz C. Vieira - Brazil


Is your pack the one with three pins in the charger plug or two? It may not be a zener, it could be a plain diode.


Hi, David;

the pack is a two-pin AC input type. I use a standard 82143A original AC adapter.

As for the diode, I cannot see it as another type but as a zener diode. It is inversely polarized in relation to the batteries poles. If it is a plain diode, the only application I'd see for this sort of connection would be as a free-whelling for inductive loads.

I did not measure the current intensity at this condition, but it shows a 6.2V breakdown voltage and its size indicates it is a 1/2 watt or 400mWatt zener.

Thenk you, David.

Luiz C. Vieira - Brazil


That sounds just like my battery pack. The three terminal regulator is HP 26-0275 which is a 78L12, this has been verified by Tony Duell, I believe, as being in his HP P/N cross reference list.

I did a quick calculation and the 365 ohm resistor biases the 6.2V zener at about 16 mA, or the charging current for the NiCads, not enough to run a high-drain peripheral - is it even enough to run the calculator while running a program I wonder? And so the zener is dissipating less than 100 mW.

If the zener is working, why change it?


How could 16mA be the charging current
for the NiCads ? It sounds too low for purposes
of charging.

It must serve a different person.
Of course, a schematic would explain all.


Hello, Norm;

if you have a look at the tinny cells... they are 2/3 (or 3/4?) N-cells, NiCad (original pack). It seems they deliver no more than 150mAh, what is good enough for the HP41 and somewhat crytical for "high" current devices (card reader, wand and IR module 82242A).

16mAh is about 10% nominal current, what matches average charging current. It still needs about 10 hours charging time.

Just a few more info.


The cells are 1/2N cells (also called N70 cells). They are rated at 70mAH (later 80maH). They were last made by Sanyo a few years back (and are about impossible to find anymore). The 16ma charge rate is actually rather large for a trickle charge, but does not seem to cause cell heating which is the main killer of overcharged batteries.


Hello, Ellis;

thank you for your comments.

To be honest I do not want to replace it, I want to know who is this guy. I prefer having two or thee as spare and also have a few 1N4735A, just in case... Norm made a good research job, it should not be wasted, don't you agree?

also I'd like to read this zener's specs so I think I will understand a bit of what did HP designers had in mind when they designed this charging circuit.

I know batteries are connected directly to the circuit load (HP41 circuitry) and there should be no noise at all in batteries' poles, but I do not feel comfortable with pure DC for charging batteries. A little PWM signal added to a DC component would act almost the same as inteligent chargers do. But when the batteries are in the circuit there's no way to change it.

Classics chagers have a separate regulated DC source to feed calculator while a non-regulated DC is used to charge batteries. Even being expensive, this sort ot charger is a lot more reliable, I think.

Thank you again, Ellis.

Luiz C. Vieira - Brazil


If you want to double-check zener diode spec's,
goto www.onsemi.com

They are the company that took over after Motorola dumped all that kind of traditional semiconductor stuff, and focused on personal computers instead (blecch).

When you get to their website, type in 1N4735A for the 6.2V zener 1 Watt data sheet.

The data sheet is for a family of parts, so you may see it steering you towards a "base" part number that isn't 1N4735A but you click on the link anyway to see the datasheet.

I'm 90% sure that the 1/2W zener diode (6.2V) would have part number 1N5221B. So you'd type that in, with the same guidelines. Of course, maybe I am not remembering the part number correctly.


Hello, Norm;

the diode I want to know specs about is the unknown guy in the battery pack, because I want to know what HP had in mind. You have already made the hardest part: finding the 1N4735A, and I am 100% confident with your choice, believe me!

About the "strange" guy: I thought about removing it and feeding it with a controlled DC source and check for zener test current so I'd have an idea about it. What bothers me is that I do not have a cotrolled DC source, nor a reliable ampmeter. When I need some stuff like a voltage source, I grab a few components and build it. I had a good lab for researching tasks, but I "left the building" in June 2002. No scopes, no controlled sources so far.

Thank you again. I'll check 1N5221B specs and I'll post again. My only concerns go for what did HP designers consider when placing a single 6.2V zener after a 12V regulator to "clip" voltage over batteries. As we have 4 × 1.25 Vcc NiCad cells, nominal voltage is about 5 Vcc; will the battery pack reach 6.2V when charging?

Best regards.

Luiz C. Vieira - Brazil


The voltage across NiCads that are in good shape doesn't go as high as 1.5V during charging; in fact, as charging finishes (when all the chemicals have been converted), the voltage starts dropping and the temperature start rising. These are the two indications that intelligent NiCad chargers use to turn off the current.

Even if the battery voltage did rise above 6.2V, it wouldn't matter because the total current is limited by the 365 ohm resistor. Even with the batteries out of the circuit, the zener dissipates less than 100 mW.

The heat is created by a reaction to re-combine oxygen which is generated at the positive plate after it is fully charged and electrolysis of water begins to occur (the negative plate is made larger to guarantee that the positive plate will be charged first). The oxygen travels to the negative plate and reacts with cadmium and water to form cadmium hydroxide, which is the reaction that produces heat. By doing it this way (oversized negative plate), the generation of hydrogen is prevented. At least that's what I understand from my Sanyo databook.


First, I just looked at the schematic I sketched of my HP41 rechargeable pack PCB and I had overlooked the diode in series with the 365 ohm resistor, so the current I calculate is more like 14 mA. That might actually be high for the original NiCads, weren't they 110 mA-H? Plus, when good NiCads are in the circuit, dropping 4.8V, the zener is not conducting and the voltage on the 365 ohm resistor is higher and so the current is about 18 mA. Even when the cells have risen to 1.4V each, the current is about 16 mA.

Yes, I appreciate Norm's advice. I'm sorry to hear that Motorola doesn't make zeners anymore and that nobody has taken up the slack (maybe somebody like Oliver Whats-his-name of Germanium Power Devices will come forward.) Motorola's zener databook is what I've always used as a reference and I guess I didn't know how good I had it. If the TL431 came in a TO-220 package I'd feel a lot better! You could use it as is (with two resistors to set the voltage) for at least a 1W zener or heat sink it and use it where you would put a 7800 series (series) regulator. I can't think of any reason why you couldn't turn it upside down and use it instead of a 7900 series (series regulator)(a message from your department of redundancy department). I like shunt regulators because they get warm. Please don't tell the EPA I said that.

My rechargable pack was in pretty good shape and I can read the markings on the parts. The zener is marked "M[Motorola logo]K/020/057/503". The two diodes on the negative side of the bridge are marked "MK/020/965/426". The two diodes on the positive side of the bridge, and the diode in series with the 365 ohm resistor, are 1N4002. The three terminal regulator is marked "M26/-0275". I wonder why they used two different kinds of diodes in the bridge? Maybe that will become evident if we can find out what the two private labeled ones are. I looked in the service manual for the HP3421A and in it, HP consistently used 1901 as a prefix for rectifiers and switching diodes, and 1902 for zeners. It's probably safe to assume that the third line on these diodes is a datecode and they only used the unique digits from the part number, making the 6.2V zener a "1902-0057" and the the diodes in the bridge "1902-0965". Looked at separately, these two zener diodes are connected back-to-back across the transformer secondary where they can clip high voltage transients of either polarity that come through the transformer from the line. Normally, the highest voltage across each diode is twice the peak voltage (capacitor charges to one peak, then transformer lead swings to the opposite peak) so any zener higher than 40 or 50 volts wouldn't normally conduct.

I'd like to read more about the idea that pulsating current is better for charging batteries. I have been studying a fairly old (1988) Sanyo NiCad databook lately (when I was having trouble soldering to the straps on my drill batteries) and it has no mention of the subject. There is a discussion of full wave vs. half wave rectification but the purpose of the discussion seems to be minimizing the weight of the transformer. BTW, this databook has a section on battery packs and they show photos of some examples, one of them is the HP Classic battery pack, 82001(might be A, the opening in the plastic for the terminals is a little shorter than my B).

Also, I looked up a PPC Journal article I had run into and bookmarked: V7N2P25 (Feb.-Mar. 1980), how to modify a Classic recharger (82002A) to use it as an AC adapter for a 41C. It includes a schematic of the Classic recharger as it comes and it shows both supplies - the constant voltage for the calculator and the constant current for the battery - originating from the same rectified and filtered source. BTW, the constant current source compares the voltage drop across a 13 ohm resistor to a transistor Vbe. 0.6/13=46 mA, 0.7/13=54 mA.

The modification for the 41C consists of changing the zener on the base of an emitter follower transistor in the constant voltage supply to a 1N755A (7.5V 1/2W), adding a protective zener (1N754A, 6.8V 1/2W) after the emitter follower, adding a 1N4001 rectifier diode in series with the load, and then building a plug to mate with the little gold contacts that were accessible on the side of the early 41's.

The 6.8V protective zener looks to me like cutting things pretty close. If the 7.5V zener is high and the 6.8V zener is low, it is going to be drawing current and there isn't anything to limit it. The text says the 1N4001 is to prevent discharging the battery on the 41 but I think they also needed another junction voltage drop since they started with 7.5V.

I tried to find an article about the HP41 rechargable pack that might have schematics and part numbers but so far, no luck. However, I did find this: in V13N9P8 there is an article about batteries in general. First there is a lot of discussion from someone at HP about the batteries for the earlier calculators (before the 41). Then starting on page 11 there is a shorter article from a battery shop in California about the special problems of the 41, which have been discussed here before - people who got the card reader found their batteries had a very short life. The introduction to the set of articles states that the 41 article will help people cope until the rechargeable pack is available from HP, so it must have already been announced at that time (Dec. 1986).

The 41 article says the 41C in ' "computing" state ' draws about 6 mA and the card reader draws 250 mA. The article also has general instructions for building an AC adapter for the 41. It does not reference V7N2. It does suggest using a "6.3 or 6.8 V" zener to protect the calculator. I wonder if 6.3 was a typo or if the writer was thinking of the common vacuum tube heater voltage?

I think the function of the 6.2V zener in the 41C rechargeable pack is the same as we have been discussing for the 25, except in the 41, 6.2V is a better fit since the battery has twice as many cells as the 25. The 12V pre-regulator and the low charging current makes it possible to use a small zener.

I have "extracted" the two articles as GIF files and will email them on request. The whole bunch is about 500KB. The Classic recharger schematic alone is about 10KB. I guess I could scan my 41 rechargeable schematic too, it's just hen scratchings on a post-it note.


Hello, Ellis;

I'd like very much having all of the information you have about this subject, meaning schematics, scratch notes, e-catalogs, whatever. Man, I saved your text and I'm gonna print it (as I have one with many others)! It's full of important, valuable information. Thank you very much!

Best regards.

Luiz C. Vieira - Brazil


Actually, Motorola didn't shut down the zener production line. They sold it off to a new entity called "ON Semiconductor". http://www.onsemi.com

So you could say that was charitable of them. Rather than smash all the equipment with hammers, and burn down the building, they just sent it along to somebody else.
I can live with that.

If Carly were in charge, they'd make sure nobody could ever have zeners again. It's not like that. I still preferred seeing everything be "Motorola" and don't know why the big-wigs have to keep divesting themselves of whatever good things built the company originally.


I'd like to see your power supply schematics, too. I don't have a Classic or a 41, but I'm curious anyway.

Maybe you could take your diagrams and earlier power supply info and add it to the Articles Forum. Information like this is too valuable to just let slip into the Forum Archive.

- Michael


I downloaded two datasheets from On Semi, 1W and .5W zeners. I did as Norm suggested and searched for 1N4735A. I tried getting there through the parametric search but never found this family of parts! Then I did the same searching for 1N755A which is used in the PPC Journal article. This is the .5W family.

I believe the diodes on my 41 rechargeable pack are .5W zeners mainly because of the lead diameter. The specs for DO-41 (1W) and DO-35 (.5W) from the two datasheets give a pretty wide latitude for the body size, it can be the same for both packages. But the DO-35 has smaller diameter leads. I measure .020 on the diodes on my pack. If they were 1W they would have thicker leads like the .032 leads on the 1N4002 diodes also on my pack.

Both DO-41 and DO-35 are glass packages. I think I have read that zeners in plastic packages sometimes change their voltage after soldering because of stress on the die from the leads that is felt by the die while the plastic is softened by the heat from soldering.

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