Re: Reading this makes me weep. Message #6 Posted by robert rozee on 9 Feb 2011, 7:53 a.m., in response to message #5 by Eric Smith
given (i assume) the calculator runs on a pair of AA ni-cads assembled into a battery pack, the solution is a chunky 3.3 volt zener diode connected directly across the battery contacts. a 1 watt suitable component is the 1N4728A, listed from farnell (their p/n 1467572) for the princely sum of 20.4 cents(nz) each in quantities of 1. no additional series resistor is needed - see below.
now, the ni-cad cells would have had a capacity of perhaps 700mA/h back then, and the charging technology of the day would dictate a maximum safe charging current of 1/10th of that for a 10 hour rate, or 70mA. 70mA x 3.3 volts = 0.23 watts dissipation by the zener (worst case), well within the power rating. even charging at a 3 hour rate, it is still only 230mA x 3.3 volts = 0.76 watts.
note that the zener would ONLY have any effect if the batteries were NOT plugged in. with a battery pack present it would dissipate nothing, with the maximum voltage across a pair of fully charged ni-cads falling well below the 3.3 volt clipping voltage (ni-cads top out at about 1.5 volts at the peak of their charge cycle).
cheers,
rob :-)
Edited: 9 Feb 2011, 8:07 a.m.
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