There have been several good points made on this thread, and some omitted.

Some of the good points:
   Make sure to account for the cap tolerance.
   Droop is the difference between the transformer max output
      voltage and the required input voltage to the regulator
      circuit (call it VDCinMin) which includes the regulator
      overhead (VinMin-Vout) and diode drop(s)

Earlier in the thread, it was stated something like assume the cap is
instantaneously charged and the period between charges is 10ms (50Hz) or
8.3ms (60Hz). This is an allowable assumption, but...

Omitted:
   The cap only needs to provide power for that part of the rectified
   AC cycle where the transformer voltage is below VDCinMin (as
   specified above)

   If VDCinMin is equal to rated voltage (RMS) of the transformer, then
the cap only needs to provide power for half the time.
   If VDCinMin is less than the rated RMS voltage of the transformer,
then the cap provides the power less than half the time.
   If VDCinMin is greater than the rated RMS voltage of the transformer,
you can not guarantee VDCinMin will be met will an infinite capacitance
since the transformer (by spec) does not provide enough amps to move the
required coulombs into the cap during the period the transformer output
voltage is (supposed) to be above VDCinMin.


Also Omitted:
    If a transformer is operating at less than its rated current, its
output voltage may exceed the calculated peak value. For example, 12V
RMS transformer should have ~17V peak, but at light loads this voltage
may be greater (even considerably greater depending on the transformer
quality).


David W. Gulley
Destiny Designs

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