Re: chargers for HP calculators... Message #6 Posted by Tony Duell (UK) on 10 Jan 2002, 2:03 p.m., in response to message #5 by David H Thompson
Many of the HP chargers have been 'fathomed out' as you put it. The circuitry is often very simple, but it does depend on the characteristics of things like the transformer.
AFAIK, no HP handheld calculator series ever used 'traditional' voltage regulator chips. The earliest series (the 'classics') has a fairly complicated charger with internal regulation circuirty. Using discrete components. 4 diodes + an electrolytic cap to get about 16V DC from the transformer. Then a classic zener + emitter follower circuit to give a stabilised 4.2V (or so) for the logic. And 2 transistors (the base-emitter junction of one connected across a current sensing shunt resistor, the collector current of this trnansistor then controls the other one) as a constant current supply
to charge the NiCds. That charger could be reproduced I think as all the components are 'known'.
Some later series use the battery as a shunt regulator. The voltage across an NiCd under charge is not going to rise above 1.5V per cell. The 'charger' can be as simple a the transformer (in the AC adapter module) and a diode + resistor in series in the calculator. In fact the Woodstock and Spice series do just that.
The problem is that if the battery goes open-circuit then the terminal voltage rises and this tends to blow up logic chips :-(. In the topcats HP protected against this by a little circuuit that loads the output of the AC adapter if the battery terminal voltage tries to rise above the Vss (+6.2V) line from the logic board power converter circuit. That's why a Topcat with power converter problems will draw around 1A from the battery pack, BTW.
Anyway, these machines (Woodstocks, Spices, Topcats, HP41 printer, HPIL tape drive, etc (which use the Topcat battery, AC adapter and much the same charger circuit) depend on the characteristics of the transformer to work correctly. There's a linear falloff of AC output voltage with increasing load current (I've measured it). You can't use just any transformer and expect it to work (or indeed be safe for the calculator).
HP did sell adapters to charge some of these calculators from a 12V DC input (a car battery, for example). They're quite complicated inside (half a dozen transistors + R's and C's) and have a somewhat strange output characteristic. The one for the
Woodstock series (which I've had in bits on the bench) acts as a constant current source with an output current suitable for charging the NiCd pack until you load it more heavily. Then the output current jumps to a higher (but again constant on further increasing the load current). This second current is enough both to charge the battery and
run the calculator. If you had to make a charger for the Woodstocks (or the Spice series which have essentially the same battery and internal charger circuit) you might have to make something like the 12V input charger and run it from a bench PSU.
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