> What is the purpose of the diode parallel to the regulator ? Is it
> necessary ?

Not strictly necessary in this circuit, but it is recommended in the
regulator specs. If the regulator is supplying a large filter cap and
the regulator input is shorted, the diode dumps the filter cap's
energy into the short. Otherwise the output terminal of the regulator
may be higher than the input (because of the short) and that's not
good. If the regulator blows closed when the short is removed the
circuit could then see unregulated voltage. And that's not good
either

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