HP Forum Archive 13

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HP-25 internal Power Circuit Charger Message #1 Posted by Albert on 1 Sept 2003, 1:17 p.m. Hi all, Could anybody bring me the HP-25 internal power circuit to charge NiCd batteries? I know is a little bit dangerous to charge batteries inside the machine (I put already the Zenner between contacts), this is the reason why I would like to make an external battery charger using the original HP AC adapter. Thanks in advance, Albert

Re: HP-25 internal Power Circuit Charger Message #2 Posted by Tony Duell (UK) on 1 Sept 2003, 4:21 p.m., in response to message #1 by Albert Sure. The chearger circuit is very simple, it consists of 4 components in series. They are : The AC adapter secondary winding; A diode (try a 1N4007 or similar), a 8.2 Ohm resistor (looks to be at least 1W, maybe 2W), and the NiCd pack. The diode's cathode (banded end) goes towards the +ve terminal of the battery. That's it. The actual 'reserve power pack' is a bit more complicated as it has an LED to indicate the batteries are on-charge, I can dig out a schematic of that if you like.

Re: HP-25 internal Power Circuit Charger Message #3 Posted by Albert on 2 Sept 2003, 12:22 p.m., in response to message #2 by Tony Duell (UK) Thanks a lot, I'll try to find out the components and a single-layer circuit. Albert

Re: HP-25 internal Power Circuit Charger Message #4 Posted by David Smith on 2 Sept 2003, 2:57 p.m., in response to message #2 by Tony Duell (UK) I think they just put the LED/current limiter resistor across the 8.2 ohm resistor. Thats what I do when I modify Woodstock corpses to be external chargers.

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