Toroid Capacitance Equations

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Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


To All -

Over the past years I have collected about a dozen various equations for the
capacitance of a toroid. The accuracy of these equations vary greatly and
none of them are very accurate for a particular toroid. I have found that
having only a single equation for all sizes of toroids that give accurate
results requires a very complicated equation. In order to simplify the work
of calculating the toroid capacitance I have found that 4 simple equations
can give acceptable results.

The 4 equations are for toroids with minor diameters of 3", 6", 12", and
20". The equations do not give the exact capacitances that Antonio's INCA
program (meters) shows but they will be very close. In fact the equations
will give good accuracies for minor diameters that are between the 3,6,12,20
inch if you extrapolate between the capacitances obtained for these minor
diameters.

d = Minor diameter in inches
D = Major overall diameter in inches
Note that D must be at least twice d.

    d = 3"    pf = 1.6079*(D^0.8419)
    d = 6"    pf = 2.0233*(D^0.8085)
    d = 12"   pf = 2.0586*(D^0.8365)
    d = 20"   pf = 2.2628*(D^0.8339)

The Toroid Capacitance Graph in my TC Notebook shows four capacitance curves
that were made before INCA's exact capacitances became available. The
Notebook curves were made from equations that are similar to the up-to-date
ones shown above.

If you have any questions please let me know.

John Couture

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