20
\$\begingroup\$

In truth, I've only been told this anecdotally by an instructor, but can someone explain the physics at play?

I have been told that if an inductor is driven at a high enough frequency, it will begin to behave as an capacitor, but I cannot figure out why.

\$\endgroup\$
36
\$\begingroup\$

An ideal inductor would not behave like a capacitor, but in the real world there are no ideal components.

Basically, any real inductor can be though of an ideal inductor that has a resistor in series with it (wire resistance) and a capacitor in parallel with it (parasitic capacitance).

Now, where does the parasitic capacitance come from? an inductor is made out of a coil of insulated wire, so there are tiny capacitors between the windings (since there are two sections of wire separated by an insulator). Each section of windings is at a slightly different potential (because of wire inductance and resistance).

As the frequency increases, the impedance of the inductor increases while the impedance of the parasitic capacitor decreases, so at some high frequency the impedance of the capacitor is much lower than the impedance of the inductor, which means that your inductor behaves like a capacitor. The inductor also has its own resonance frequency.

This is why some high frequency inductors have their windings far apart - to reduce the capacitance.

\$\endgroup\$
  • 1
    \$\begingroup\$ Perfect, thankyou. So at some frequency where omega L = 1 / {omega c}, the inductor will resonate with itself (the ideal inductor with the parasitic capacitance). thanks for the input. \$\endgroup\$ – Michael Jan 26 '12 at 7:31
  • \$\begingroup\$ As long as we're enumerating the parasitic elements of an inductor, it's probably worth adding that an inductor (effectively) has a resistor in parallel with itself. There's not a physical resistor, but the losses in the core due to eddy currents and hysteresis are commonly modeled as such, for example in SPICE. \$\endgroup\$ – scanny Jul 3 '16 at 1:47
16
\$\begingroup\$

What Pentium100 said. I can only add an illustration. Please excuse my hand drawing skills.

enter image description here

\$\endgroup\$
2
\$\begingroup\$

Capacitors have two conductive plates separate by an insulator. The turns of wire in a coil can also create a capacitor because between each turn of wire there are two conductors separated by an insulator, which can be air, enamel, ceramic, etc. When the right frequency is applied to the inductor, the inter-turn capacitance can create a resonant circuit. This inter-turn capacitance only happens with AC and not DC because inductors are a short with DC.

\$\endgroup\$
  • \$\begingroup\$ The other reason why air inductors have wider spaced windings is to handle higher RF power levels. \$\endgroup\$ – MarkSchoonover Jan 26 '12 at 6:53

protected by Olin Lathrop Nov 11 '16 at 19:30

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.