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Just read this archive about charging a HP-25 safely with a simple mod :

Captain Zener

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Why zener? Why not just a bunch of 1n400[x]? Or even some smaller ones will probably do just fine. Avoiding leakage and generally it'll be much more robust. It's not like there isn't room for them in there. Make a nice package in thin heat shrink.

When dealing with such small voltages and much space I'd personally just go for a bunch of ordinary diode. Each having ~0.6 v drop. Hey if you want reverse protection too then just put in either a serial or shunt whichever you prefer and is practical in the specific case. Analog to a zener that'll be a shunt.

Hello there,

Thank you for your replies and help.

It seems however that the topic turned into AC charger/adapter issues. Beeing an B.Sc in electrotechnis I may fully agree with all of you, since NiCd's are almost history now, and also the D-R circuit does not really protect for overvoltages. However, I have used my HP-25 together with my brother and sister for more than 15years, in this time I had to buy another replacement NiCd pack (memory effect) and allways charged them with my original AC/Adpater w/o any problems, until this same went broken/burnt and then lost somewhere.

I would like to have the original AC adpater mainly for two reasons:
1) to complete the HP-25 package
2) because I could use the original cable/plug and design/build a more clever charger which could accomodate even NiMHs perhaps.

Anyway, even if i have not posted my original topic in the advertisment area, I would appreciate, if anyone has some "weird" parts to pass/sell over.

Thank you