|Re: Charger for HP19C|
Message #2 Posted by Andrés C. Rodríguez on 19 Apr 2000, 9:50 a.m.,
in response to message #1 by Bo Kristoffersen
There is no reason to concern about polarity with AC adapters that deliver AC on their low voltage side.
The "polarity" may matter, in theory, if the adapter had an autotransformer (I don't know any real-world case); and as a safety issue (ground connection, etc.) Again, this should not be the case.
In the Woodstock series (HP21, 22, 25, 27, 29C, the low-voltage AC is rectified by a diode inside the calculator, and the pulsating DC is applied, via a current limiting resistor, to the batteries. In such manner, the batteries receive a charging current, and also act as a filter and voltage regulator. That is why you should not operate such models with an AC adapter if rechargeable batteries are not present.
The 2.5 VA vs. 3.0 VA issue may have to do with the particular transformer used to build the AC adapter. A 300 mA current at 8 VAC (rms) gives 2.4 VA. I don't think your calculator takes that much current. So the 2.5 VA transformer seems to be OK.
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