# PFC Formula (was: Re: Power Factor Correction weirdness?)

```Hi Terry,

> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> I looked at my coils and the PFC caps I use and the basic
>principle behind Adam's test seems to be a useful way of
>selecting PFC cap values. Basically just correct for the
>transformer's VA rating.  In the non-linear operation of a TC
>there are a few inaccuracies but they do not seem very
>significant.

[Snip]

> 49.7uF where the actual value is about 60uF.  Not exact but
>close enough. Thus, I would propose the following equation
>(unless everyone has been using it for years and I am the
>last person on earth the figure it out ;-))
>
> Cpfc = ( Vo x Vi ) / ( 2 x pi x f x Vi^2 )

Naw, your not the last person to find it. There will always be coilers,
who enter the scene after you ;o)), so fear not ;o)). I have been
using the same basic equation in my TC proggy. I think I "stole" it
from Jim Lux´s website ;o) and I have seen it mentioned thereafter
in some of the Richard Quick / R. Hull / TCBOR text files floating
around on the internet.

The only *problem* (which really isn´t one) is that this formula
is for purely sinusoidal waveforms. Once you start squeezing an
xformer, like we do in coiling usage, your waveform can become
quite distorted, so the math won´t *truly* fit the real world. But as
this has nothing to do with the output power of a transformer (i.e:
it won´t increase your actual power throughput or spark length), it
doesn´t really matter. It (PFCing) DOES allow you to actually draw
more USEABLE power from a certain outlet as the fuse or c.b. no
longer see an overcurrent situation (in certain "near-the-c.b.-limit"
setups). However, it might (never thought about this until now) might
contribute to a higher inrush of current the moment you flip on the
main power switch. This, of course, only holds true for non-variac
controlled setups. Another *problem* I can see, is if you run your
coil at different voltages (via a variac). You can only determine
(actually better: use) the proper PFC for a given voltage. In other
words a 120V PFC will not be correct for those guys who will
occasionally ramp the variac & xformer up to 140+V.

One last thought: I never quite figured out where to place a PFC:
in front of, or behind the line filter? In front of the line filter
would mean you would need to compensate for the line filter as
well. I.e: it might not "PFC" as well as w/o the line filter. Placing
the PFC behind the line filter would call for a cap that can take
a kickback from the coil (voltage wise). I use the 2-3x figure (in
my TC proggy) on AC voltages, which can lead to mighty big
PFCs as they grow larger to accommodate for their higher
operating voltage ;o(?)

Coiler greets from Germany,
Reinhard

```