Jinx wrote:
> For example, enter 10 (volts in), 4.7 (zener voltage), 10 (mA load)
> and click Solve -> 265ohms 0.1W resistor, 0.1W zener

Yes, but the resistor will drop 2.65V all the time when the load is 10mA,
even when the input voltage is the 5.0V it's supposed to be.  That means the
PIC Vdd will be 2.35V with 5V in.  The 10F will operate down to 2V, so maybe
that's OK, but the OP should at least be aware of this limitation.

If this is not acceptable, you could try a 5.6V zener with emitter follower
as a crude regulator.  That requires one additional NPN transistor from the
plain shunt regulator described above, but will only drop about 700mV when
the input voltage is below about 5.7V.  Above that it will regulate
reasonably well to 5V up to the C-E voltage limit of the transistor.  The
extra $.03 for the MMBT4401 or whatever will be worth it in many
applications.

By the way, 10mA is quite high for a 10F itself, although the whole circuit
could easily draw that.  The OP needs to provide some real specs.


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