# How to measure parasitic capacitance of inductor

#### learner1

How can I get the parasitic capacitance of a inductor? I have a scope, current probe, function generator.

Thank you.

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#### Averagesupernova

Science Advisor
Gold Member
What is the value of incuctance?

#### Nam_Sapper

Pretend the inductor is a capacitor, and measure its capacitance at high frequencies.

You can do it manually by simply plotting a frequency/Impedance graph, and curve-fitting some sample values of capacitance based upon the 1/2piefc algebraic equation.

S

#### SGT

You can connect your inductor to the signal generator through a resistor and measure the voltage across the inductor with your scope.
Varying the frequency of your sinusoid will change the amplitude of the signal. The amplitude will be minimal at the resonant frequency $$\omega_0$$.
Since $$\omega_0 = \frac{1}{\sqrt{LC}}$$ you can calculate the capacitance.

#### berkeman

Mentor
learner1 said:
How can I get the parasitic capacitance of a inductor? I have a scope, current probe, function generator.

Thank you.
The easiest way will be from its datasheet. There should be a DCR (series resistance) and parallel capacitance listed. Depending on the L and C values, you may have a hard time measuring the resonance with your signal generator. Like if the sig gen only goes to 10MHz, and your inductor is 10uH with 10pF of capacitance, your resonant freq will be around 16MHz. Also keep in mind that your oscilloscope probe has capacitance (probably in the 10pF range), so that will alter your reading and needs to be accounted for.

If you need higher frequencies for your tests, you can try adding a series diode and driving the inductor with a square wave. You would make a low-inductance measuring fixture similar to a Z-lead for your scope probe, and measure the ringout frequency of the inductor and parasitic capacitance (and then subtract the scope probe capacitance and any other fixture capacitance).

Let us know what you come up with, and maybe post a link to the inductor's datasheet so we can check it out. -Mike-

#### learner1

Thanks for all the suggestions. I tried SRF one, and here is the result.
I know the inductance is about 1.4mH. The inductor was directly connected to the signal generator. The voltage and current across the inductor are measured using scope. I increased the frequency of sinusoid until phase between V and I changed from 90 to –90 (impedance from inductive to capacitive). Then I use this frequency (420kHz) to calculate C based on . C is about 100pH.
Is this value reasonable?
I want to try something else to verify it.

#### Averagesupernova

Science Advisor
Gold Member
It is totally unreasonable because capacitance is not measured in pH. :rofl: I suspect you mean pF. Anyway, seriously I think it is probably correct. Your approach is the way I would do it.

#### learner1

Yes, pF. I am thinking about another way to measure it. If the output impedance of the function generator is 50ohms, can I use Xc=50=1/2pifC and find this f? Am I looking for -45 phase shift? Thank you.

#### Averagesupernova

Science Advisor
Gold Member
I would use a series resistor and note the frequency at resonance and several other places. Make sure your calculations all come up with about the same capacitance value.

#### Nam_Sapper

keep in mind the capacitance effects you are measuring should be small, but drastically effected by the presence of nearby objects. So do you want to measure it in isolation, or in the actual circuit you intend to use it? This is not mere philosophy. In RF circuits it is critical, and usually some method of 'tuning' the inductor must be provided.

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