> Ok...so what is the relationship between the formula and the rule of
> thumb? For instance, if I had a 5 amp load, do I need to increase
> capacitance to 5000 uF to keep 7-8V ripple?

Yes, exactly. Calculating the exact amount of ripple is difficult, because
the waveform is never sinusoidal, there's always resistance and inductance
that can't be ignored, etc. But basically you always have less than 1/120
second when the circuit is powered by the capacitor, so you can use that
figure in the calculation.

The ripple voltage is then very easy to calculate:

(T * I) / C

T being time, in this case .00833 second. I is the load current, and C is
the capacitance in Farads. This gives 8.33 volts ripple for your test
case. But since that assumes a longer time than you will actually have is
why my rule of thumb is 7 to 8 volts.

Cheerful regards,

Bob


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