John --

> I'm a bit confused here.

You sure are!

> If the formula is for calculating the droop between peaks then shouldn't
> the peak voltage of the transformer be used?

Yes...

> i.e. 9VAC is 9*1.414 = 12.76V peak.  If we are drawing 1A during the
> 8.3ms then we get 8.3mCoulumbs/12.7cV = .652mF or 652 uF.  I would think
> that using 1000uF per 1Amp does work out nicely.  Then even with the
> diode drops it works out with the tolerance of the capacitor.

Where's your allowance for the output voltage?

The peak transformer voltage needs to be:

  output voltage + regulator overhead + diode drop(s) + ripple voltage

which in this case (5V, 1A, 1000uF/A) works out to

  5.0 + 2.0 + 1.2 + 8.3 = 16.5 V

This means that the RMS value needs to be at least 11.67 V. Hence, the
selection of a 12VAC transformer.

-- Dave Tweed

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