Mark Morissette wrote:

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5 Years of electronics courses are paying off.. :-)
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Works like a charm - the PDA now successfully powers the GPS for about
2 hours and 20 minutes on a full charge. A flip of the swich, and the
PDA and GPS is powered by the cigarette lighter again, and the PDA
hapilly charges itself.

Problem solved!

Wouldn't a single diode been cheaper and easier or were you afraid of
the voltage drop across it?