Re: HP41 Power Supply Message #11 Posted by Steve (Australia) on 22 Sept 2001, 9:02 a.m., in response to message #10 by Erik van der Meulen (Netherlands)
Remember -- no guarantees!
Look at the battery contacts. Is one covered by a piece of plastic with a contact on it?
Look at where the charger connects. Is there is small piece of metal pressing against the centre connector.
If the answer to both of these questions is YES, then you have the older model which will send a rather larger voltage to the calculator if the batteries go open circuit.
If the answer to both of these questions is NO then you have the newer version which will not kill the calculator if the batteries go open circuit.
I believe that you probably have the latter one, because the former will still provide power to the HP41 even if the batteries all go short circuit.
Measure the voltage across the outer 2 contacts without the charger applied (you will probably get a small voltage -- but maybe 0)
Then connect the charger. The voltage should rise to approx 1.2, 2.4, 3.6, or 4.8 volts. These voltages indicate 1, 2, 3, or 4 cells not shorted. (note that the voltages may vary somewhat from these figures depending on the state of charge on the good cells).
If you see a change in voltage when you add power, then the charging circuit is probably OK.
Now clip the connection between the two centre cells. Apply power to the charger and measure the voltage again. It should be approximatly 6V.
If the voltage is between 5.5 and 6.5 volts then all is probably OK to use this as a power source.
But before you do, place some insulation tape over the centre connectors of the nicad pack to prevent any chance of the HP-41's battery contacts from linking the nicads back again.
Insert into your HP41, and see if it runs while on the charger.
Note that removing the charger will cause the calculator to lose power, but if you're careful you won't lose memory if you don't leave it for too long in this state.
Note that with this charger being used like this, the maximum current that can be provided to the calculator will be insufficient to power the card reader or the wand.
Depending on the current required by the calculator, even operation of the calculator may be marginal. Watch out for the BAT indicator which will certainly come on in this case.
Ah! even if there is insufficient power to run the calculator, it will CERTAINLY provide plenty to keep the memory alive while you charge the nicads you're using.
The other option is to get a set of N cells to use while you're charging the nicads. Used this way they'll probably last for years :-)
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