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HP 25 Woodstock - Printable Version

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HP 25 Woodstock - timw - 04-27-2010

I am wondering what the general consensus is regarding the fragility of the HP 25. Does the HP 25 survive an over voltage condition as seen when the charger is used without a battery pack ?

I believe the HP 25 uses PMOS technology which is less sensitive to overvoltage than CMOS or NMOS - is this true ?

I have looked through the archives and found plenty of cases where a HP 29C (and to a lesser extent the HP 25C) is destroyed by the charger over voltage when the battery is not making good contact or is not fitted. Most authors point to the CMOS ROMS as being destroyed, rendering the calculator useless.

I do not recall seeing any examples of a HP 25 or 21 being destroyed in this manner.

What is your experience with the 25/25C/29C ?

Regards, tjw.

Re: HP 25 Woodstock - Jim Horn - 04-27-2010

My HP-25 (non-C) is dead, perhaps for that reason. The charger has been lost for many years but replacing the NiCd batteries with regular AA (alkaline) cells doesn't turn it on. Right now it's in storage so I can't debug it.

My first calculator - wish it still works!

Re: HP 25 Woodstock - Kees van der Sanden - 04-27-2010

From a technical point of view I don't see a reason why PMOS is less sensitive to overvoltage than CMOS or NMOS. The main reason for damage due to overvoltage is gate oxide breakdown. This breakdown depends on the oxide thickness and the quality of this oxide.
The thinkness of this gate oxide is decreasing by each technology generation. This is a reason why the supply voltage of modern chips is also decreasing.
It could be that the CMOS chips used in the C models were made in a new and less mature CMOS process compared to the standard PMOS technology in that time.