The Museum of HP Calculators
HP Forum Archive 19
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|Classic HP SuperCharger|
Message #1 Posted by Dan W on 7 Aug 2009, 9:08 p.m.
Often when Classic battery packs are rebuilt, we want to use modern high energy NiMH batteries. Problem is, with the original charger at about 55-60 mA, it takes forever to charge these things. Better would be 250 mA.
So for fun, I created an external "Super Charger". This is a small box with a switch that toggles either 60 mA or 250 mA. If 250 mA is selected it turns on a small 40 mm fan that has a blue LED to remind me it's on (I doubt it really needs the fan; I do recommend a visual indicator if the 250 mA is selected).
The box is connected to the accessory external charging cradle through the standard charger cable. But I don't see any problem connecting directly to the calculator since the charge current circuit is independent of the power supply.
The circuit is fairly straightforward. It uses an LM317T to supply the charge current and the switch changes the resistance that governs the output. A 7812 provides the 12V for the fan, followed by a 7805 and a diode to provide the 4.3 Vbsw.
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