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The neglected factor: self-capacitance of coils ?

Hi.

Capacitance exists from every metal surface of a wire to all other metallic surfaces in & around the wire/cables (which is a bundle of wires).

The self-capacitance of a coil of wire is generated by the electroststic coupling btween individual turns, between layers of winding, AND turns with our mother earth.

If self-capacitance is too much, it will reduce the Q of the of the coil/winding & will be worsen via the dielectric or insulation of the wire or interleaving between windings.

In all cases, the self capacitance of a coil has an apparent effect on its true DCR, true inductance & true Q of the coil itself & at frequencies considerably below the self resonant frequency of the coil.

The apparent H, DCR & Q is always larger than their true values, &
the increment is proportional to its true H, DCR & Q value & inversely proportional to any additional capacitance to bring up to the coils resonance frequency.

In multi layer coil winding, like audio transformers, winding capacitance is a huge issue. Various ways of winding, e.g. vertical sectionizing, etc. are employed to minimize the winding capacitance.

To reduce the leakage inductance of an audio transformer, mixing winding of the primary & seconday windings is often used. But at the expense of increasing the winding capacitance.

Quiz: why common mode chokes are typically built of two coils of a few loosely wound wires?

c-J


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