Re: HP41 rechargeable packs - another query Message #2 Posted by Randy on 11 June 2004, 9:30 p.m., in response to message #1 by james summers (UK)
The three pin pack disconnects the battery from the calculator when the wall wart connector is inserted.
The small TO-92 three terminal regulator in the three pin version is a 78L05 or 06, I forget which. It provides regulated DC to the calculator while the battery charges, disconnected from the calculator. In the two pin version, the TO-92 regulator is a 78L12 and only provides the charging current to the battery. The calculator is powered from the batteries. In these versions, there is a 6.2V zener across the battery to prevent overvoltage from reaching the calculator. It becomes a fail safe should the battery ever open as the thermal rating is exceeded and it fails shorted.
With a little work, you can convert a three pin to a two pin version and completely eliminate the flex circuit that always gets eaten by the battery leakage. It involves replacing the regulator, the series charge limiting resistor and adding the zener. Yes, a fair bit of work but then you'll have a safer battery with fewer parts in the end.
BTW - tinned beryllium copper is the material of choice for the contacts. I don't know a source in the UK (I doubt RS has it) but here in the US - http://www.smallparts.com/products/descriptions/bcs.cfm is one source of supply.
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