> I have heard the rule of thumb: 1000 microfarads per amp of power supply
> capacity.

The danger with rule of thumbs like this is that the desired parameters have
been lost in time.  If the relationship between current and capacitance is
fixed, then the droop between power peaks must be constant.  If this cap is
being charged from a full wave bridge with 60Hz input power, then the time
between charges is 8.3mS.

  1A * 8.3mS = 8.3mCoulombs

  8.3mCoulombs / 1mF = 8.3V

So this rule of thumb results in 8.3V droop between power peaks.  That may
be fine in some cases but, is certainly too high for running a 5V regulated
circuit from a 9V AC wall wart.  After accounting for all the diode drops
and some regulator overhead, you would need a 12V AC wall wart.


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