Re: Chargers for HP 25 calculator Message #7 Posted by Hal on 5 Jan 2006, 10:28 a.m., in response to message #6 by Stephen Easterling
Hi Steve,
As I recall, the output of the woodstock AC adapter is on the order of 12vac. It's then rectified in the calculator, and regulated by the battery pack. Hence the danger...if the battery pack develops an open for any reason (bad contact, bad cell, etc.) you will apply overvoltage (around 10-12vdc) to the calc. The only safe way you can use an HP ac adapter is to acquire one of those woodstock external chargers (they look like a miniature calc with no keys or display, and the battery pack goes where the keyboard would be). These external chargers have rectifier circuitry in them, and the ac adapter connects up to them in the same fashion as to a calculator.
In my 29c I just use NiMH rechargables from radio shack, and charge them externally (with a radio shack wall charger). They last a long time and work great. You just have to cut the center rail out of the battery pack to get the cells in and out.
You raise a good point, however, about charging with the HP ac adapter connected to the calculator and just leaving the calculator off. I don't know if this is safe or not. Can anyone else address this issue...can you protect a woodstock from unsafe voltages from the ac adapter by simply keeping it switched off while the adapter is connected? Thanks and best regards, Hal
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