Here's something from an old Signal Transformer catalog:

Capacitor Selection

For low current supplies (less than 1A out), capacitor selection is
relatively straight-forward. Capacitance is found by tthe formula:

C=I*6e-3/dV

where C is capacity in Farads, dV is peak to peak ripple voltage, and the
ripple frequency is assumed to be 120Hz.

Harold

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