[Amps] Filter Capacitors

[Manfred Mornhinweg]

Gerald,

> I have not seen the 1v, 1A, 1 second, 1 microfarad theory before. 

It's not a theory. It's the very definition of the unit "farad"!

But it's "farad", not "microfarad"! A capacitor of one microfarad 
instead would gain one volt in one second if charged by one microampere. 
Or if charged by one ampere, it would gain one volt in every microsecond.
If you get confused by where to add "micro" and where not, better use 
the base units only, without any prefix, even if then a typical Pi tank 
tuning capacitor might end up having a capacitance of 0.0000000002 farad.

If you don't believe me about the definition, look it up here

http://searchcio-midmarket.techtarget.com/definition/farad

or here

http://en.wikipedia.org/wiki/Farad

or google for additional sources!

> I will  
> need to look into that as I was under the impression that voltage drop was 
> based  on time constants and they are related to 65% voltage charge or 
> discharge.

Those time constants happen when you charge a capacitor through a 
resistor, resulting in a non-constant charging current. In many 
situations it's good to use those time constants, for example when you 
use a 555 timer chip. But in a power supply filter calculation it's more 
practical to consider the load current as being constant, and thus apply 
the definition of the unit.

In other cases, like RF work, you would typically use neither the linear 
volt ampere second farad relationship, nor the RC time constant, but 
instead you would use the LC resonant frequency, Q factor, and the 
reactance of your capacitor at a given frequency.

For a power supply filter, a calculation using the reactance of the 
filter capacitor isn't really applicable, because the waveform delivered 
by the rectifier contains an infinite series of harmonics, and so a 
given capacitor has an infinite number of different reactances to consider!

So we have to use the most suitable method for each particular situation.

Manfred.

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