# [TCML] Measuring capacitance

David Speck dave at davidspeckmd.org
Thu Nov 26 20:27:20 MST 2009

```Brandon,

You can calculate the effective capacitance of series/parallel
arrangements mathematically.

Capacitors connected in parallel are simply additive.  Caps in series
combine as the sum of their inverse values, just like resistors
connected in parallel.

The voltage rating of a set of parallel caps is only equal to that of
the lowest rated capacitor.  If you series equally valued caps (same
voltage and capacitance), the total voltage rating for the string is
equal to the sum of ratings of the individual caps.  If you series
unequal valued caps, then the voltage rating calculation gets rather
complex (something to be avoided).

Before you start building high voltage equipment, you would do well to
get some basic theory under your belt.

Many high end digital meters include a capacitance measuring function.
You can also buy specific cap meters for not a lot.  However, these are
mostly useful for testing that a cap is actually exhibiting its stated
value.  Note, however, that in high voltage use, a cap may seem to have
proper values when tested with a meter, but fail when subjected to high
voltage.

Dave

Brandon Hendershot wrote:
> I'm looking to make a new tank cap this Christmas and I'd like some
> help from a meter to measure the farad rating to aid wiring the in a
> mixture of series and parallel. The only thing I can think of that
> would measure farads is an ohmmeter. Do they make a farad meter? If
> not, how do you convert ohms to farads?
```