Hi Horta and all,
I will chirp in even if it seems rather late in the thread.
Don't forget the very definition for the unit of capacitance: One Farad
is the capacitance that will change the voltage on it by one Volt, when
a current of one Ampere flows through it during one second.
With this definition, you can easily calculate the capacitance if you
know how much current you will need for how much time, with what
allowable voltage drop. For example, if we assume that you want to make
a 2200V, 500V supply, running from 50Hz, that you are willing to accept
5% ripple, and that your transformer is fairly typical, then the math is
this:
Each half cycle is 0.01s long. You will need the filter to provide the
current during about 80% of this, because during the other 20% the
transformer/rectifier is delivering current. So, you need 0.5A during
0.008s, with a drop of no more than 110V. Applying the definition of
capacitance:
C[F] = 0.5A * 0.008s / 110V = 0.000036F, or 36uF.
Of course, this number varies if your line frequency is different, if
you want more or less ripple, and it also varies a little if the
transformer has higher or lower resistance.
Regards,
Manfred.
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