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HP Forum Archive 13

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Advice on HP 25 Charging
Message #1 Posted by G Evans on 25 July 2003, 9:23 a.m.

The HP25 manual warns against using the battery charger without batteries fitted to the calc.

Can any one say if damage for this reason is common ? The 35 and 45 can be used with chargr alone.

In another thread I requested advice on a 25 that may have its RAM chip damaged ( no STO or RCL but ok otherwise).

Could damage to the RAM have resulted for not observing the above ?

George (UK)

Re: Advice on HP 25 Charging
Message #2 Posted by Victor Koechli on 25 July 2003, 9:49 a.m.,
in response to message #1 by G Evans

In contrast to the Classic models, operating Woodstocks on the charger alone (without a good battery installed) is officially discouraged by the manual an can result in permanent damage to the calculator. This seems to be true especially for the HP 25C and 29C with continuous memory as their memory is always connected to the power and even connecting a charger without turning the calc on can damage it!

And: Yes, it may well be that your machine was damaged this way.

Cheers, Victor

Re: Advice on HP 25 Charging
Message #3 Posted by Vieira, Luiz C. (Brazil) on 25 July 2003, 4:21 p.m.,
in response to message #2 by Victor Koechli


For sure! Voltage supplied by power components to main circuitry when charger is connected and batteries are not may dammage them. Many threads were developed here containing information about this particular subject. Norm Hill found a zener diode that is suitable for this case when correctly connected to batteries' terminals, mostly if this is done inside the calculator. This zener has an id code and Norm is the best person to talk about this. He is the one who researched and found it.

Of course, using this zener diode is enough to prevent such bad situations. Also, this is not a definitive, permanent solution to allow using AC adapter without batteries in Woodstocks, instead it prevents damage when this happens, for example, in case of bad contact.


Luiz (Brazil)

Re: Advice on HP 25 Charging
Message #4 Posted by Albert on 26 July 2003, 1:45 p.m.,
in response to message #3 by Vieira, Luiz C. (Brazil)


I agree with all these stuff. I put a zener and my HP-25C didn't suffer any problem. I also cut off the plastic soldering under the keyboard (to clean key contacts), and calculator works fine again. Bye.

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