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HP-41C/CV/CX Battery Pack Circuit (burnt diode) - Printable Version

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HP-41C/CV/CX Battery Pack Circuit (burnt diode) - Al Snyder - 01-24-2008

Hi folks!

As a HP-41CV (since 1980, and lately HP50g) owner, I need your help to solve this puzzle.

One of the diodes in the Battery Pack (HP 82120A) failed. However, HP decided to hide the identification of the components, put his own ID on them. Therefore, I was not able to recognize and replace it. The marks on it are:

(M)K 020 057 218

(Motorola sign)

Does anybody know what this translates into?

I've found a Motorola Diode with very similar looks: 1N 75 1A 615 But, I don't know if this is the same thing.

If someone has the list of components, or better yet, a Circuit Schematic, or a link to a previous post.. I'd highly appreciate it. My searches returned almost nothing.

Thank you and happy HP days,


Re: HP-41C/CV/CX Battery Pack Circuit (burnt diode) - Randy - 01-24-2008

Does anybody know what this translates into?

That would be a 1N5234 6.2V zener diode

They fail when one or more battery cells go open and there is no load on the supply. The diode clamps at 6.2V so it does not overvoltage the calculator. It's really under-rated for the job but it is after all, a fail-safe part. Only comes into action when the battery fails...

PS: If you are in need of one, just email me and I'll mail you one.

Edited: 24 Jan 2008, 1:27 p.m.

Re: HP-41C/CV/CX Battery Pack Circuit (burnt diode) - John Keith - 01-24-2008

You could also use a 1N4735A Zener diode, which has a higher power rating and is only slightly larger. Even better would be a small TVS (transient voltage suppressing) diode, which are available from most electronics distributors. These heavier-duty diodes will not burn out if the open-cell condition recurs, as long as the original charger is used.


Re: HP-41C/CV/CX Battery Pack Circuit (burnt diode) - Randy - 01-25-2008

Use anything you think suitable but keep in mind that the higher the leakage current, the faster you discharge your batteries even with the calculator off.

While most TVS's have low leakage, their turn on range is much wider than zeners, most times about 20% tolerance or more. In addition, don't be mislead about dissipation. TVS's are rated in pulse absorption terms, their intended use. Actual steady state power dissipation isn't any better than zeners for a given physical size.

Spec wise, the 1N5234 is is typically about one half the leakage of the 1N4735A. As with all design, trade-offs must be considered.

Edited: 25 Jan 2008, 5:34 p.m.

Re: HP-41C/CV/CX Battery Pack Circuit (burnt diode) - Al Snyder - 01-27-2008


Thank you very much for your valuable help and comments. I now have a better idea about the design.

Randy, thank you for your kind offer. I recently bought a bunch of components from Ebay. Let me see if I get lucky. If not, I will get back to you.

I have couple more questions about the Card Reader 828104A and the printer HP 82143A, but I better address these issues in new threads.