Re: Capacitance of a coil
> Original Poster: Reinier Heeres <rwh-at-worldonline.nl>
> OK coilers,
> please help me on this: I'm writing a Tesla coil calc program (well,
> just for me to understand it better and get familiair with the calcs),
> but I don't know how to calculate the capacitance of a coil. I could of
> course use Medhurst's formula (is there a table for H/D ratio > 5?), but
> I wanted to know if there's another way to calc it too.
You can calculate it considering it as a tapered transmission line, or as a
series of parallel capacitors consisting of two annular rings with the size
of the outer ring gradually increasing. Either way, you get a nasty
integral (blows up at the bottom) that is best integrated numerically. If
you are going to go to the trouble of numerically integrating, you might as
well use an empirical approximation (which is what Medhurst is) that gets
you within a few percent, particularly for a TC, where there are lots of
other design unknowns as well.
If you come up with a good analytical expression for the integration, I'd
like to see it. I suspect that there is some standard formulation that will
work fairly well (possibly a series expansion), but I checked all the
standard tables (Grashteyn and Ryzhik (sp?), and the CRC math tables) and
couldn't find something useful.