# Capacitance effects of coil

1. May 26, 2006

### Jdo300

Hello,

I've learned from my study of oscillator circuits that an inductor has some inherent capacitance in it due to the distance between the individual windings. I've heard somewhere that if you place a permanent magnet close to a coil that it will behave like a capacitor and I was wondering if someone out there could confirm or correct me on that. If this is true, does it also work for air core coils as well?

Last edited: May 26, 2006
2. May 26, 2006

### Hammie

I've never heard that. The coil will behave like like a capacitor, however, above a certain frequency.

3. May 26, 2006

### Staff: Mentor

All inductors have parasitic parallel capacitance, and series resistance. I don't quite understand your magnet question, but I suppose maybe you're asking about the change in complex impedance of a ferrous-core inductor when it is artificially saturated by an external magnet. If so, then yes, the magnetizing inductance will drop, but the parasitic capacitance will stay the same. You can do the complex math to see what that does to the impedance phasor.

4. May 26, 2006

### Jdo300

Thanks for the insight. Would you happen to know how the coil might be affected if it is wound around a non-magnetic core? To better explain my setup, I basically want to wind a coil around something like a dowel rod or anything non-magnetic. Then place a small ceramic magnet on the end of the coil to see how it effects the coil if I were to run an AC or DC signal through it, or pulse it. I'm essentually trying to see if the magnet can enhance the capacitence properties of the coil (without it being wrapped around a ferrous core).

5. May 26, 2006

### Staff: Mentor

No, a magnet will not affect the properties of a coil if there is no ferrous material in the coil construction. Well, if you move the magnet, that will induce a pickup voltage in the coil of course, and if you hold the magnet close to the coil, the ferrous material of the magnet will slightly increase the inductance of the coil. But there will be no effect on the inductance or capacitance of the coil from the presence or absence of the external DC magnetic field.

Last edited: May 26, 2006
6. May 26, 2006

### Jdo300

Okay, thanks for clearing that up for me :-).

7. Jan 31, 2010

### sulabh.dudex

ya there is capacitance in a coil, given by C(pF) = K*diameter(cm) as given by Medhurst formula.
but I was unable to know, how to calculate the value of K. can anyone will give me information about "K".

8. Feb 3, 2010

### sophiecentaur

If you have an oscillator and a scope, you can drive the coil via a high resistance and, when the L and self-C resonate (parallel), you will get a voltage peak (when Xc and Xl are equal and opposite). That will tell you the self capacitance of the coil if you already know the inductance at low frequency.
That's the basis of a Q meter, which used to be a good cheap and cheerful way of measuring component values. Your oscillator needs to provide a high enough output frequency to achieve resonance. If it doesn't, you can get smart and introduce a known value of capacitor and infer the self capacitance by the difference between measured and calculated resonance frequencies.
But if your coil is for audio frequency operation, it will probably have a self resonance in the order of tens of MHz, max -easy to achieve. Look up "Q meter" on the web and you'll find something to suit you. i.e. whatever your level of understanding of the topic, high or low.