|Re: HP-21 charging voltage, zeners ...|
Message #10 Posted by Steve on 28 May 2000, 7:27 p.m.,
in response to message #9 by Andrés C. Rodríguez
No series resistor. The charger is already current limited by virtue of the fact that nicads must be charged with a current limited (or constant current) supply.
If the charge current is 150mA and the nominal voltage is 2.4V then I would place a 3.3V (lowest voltage zener?) in the circuit. At 150mA a 1W zener would be suitable.
(perhaps I'd place a couple of normal diodes in series to get a lower voltage? - but 3.3V is likely to be safe)
In the HP41 nicad pack, a lower rating is required, 400mW is fine. The charge current is far lower.
I too dislike zeners as a means of regulating voltage sources. A shunt regulator always seemed so wasteful to me. I do have a soft spot for the good ol' 723 though.
These days I find myself using LM317s a whole lot more than anything else.
But, if you've got an existing power supply that's already current limited, then a shunt regulator (expecially when used simply to prevent overvoltage conditions) is so much easier to impliment.
Oh, and be warned that in calculators like the HP21, running off the charger only with a shunt regulator is still problematic because the charger is not filtered. A zener on its own is just there to prevent the premature death of the calculator, but not make it work without batteries.
On the HP41 you _can_ operate the calculator from the nicad pack without nicads because the charger circuit (at least two of them - there are several designs- how many?) rectifies and filters the AC from the charger. Neither of these versions will provide enough current to power the card reader. To do this, one version _requires_ a zener, the other doesn't.