- #1
learner1
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How can I get the parasitic capacitance of a inductor? I have a scope, current probe, function generator.
Thank you.
Thank you.
How to measure parasitic capacitance of inductor
- Thread starter learner1
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- Capacitance Inductor Measure
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Discussion Overview
The discussion revolves around methods to measure the parasitic capacitance of an inductor, exploring various techniques and considerations in both theoretical and practical contexts. Participants share their experiences and suggestions for measurement approaches using oscilloscopes, function generators, and other tools.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants suggest measuring the inductor's capacitance by treating it as a capacitor at high frequencies and using impedance graphs for curve-fitting.
- One approach involves connecting the inductor to a signal generator through a resistor and measuring voltage across the inductor to find the resonant frequency, which can be used to calculate capacitance.
- A participant notes that the datasheet of the inductor may provide the parasitic capacitance directly, but emphasizes the importance of considering the limitations of the measurement setup, such as the oscilloscope probe's capacitance.
- Another participant describes their measurement process, reporting a calculated capacitance value and seeking validation of its reasonableness.
- There is a correction regarding the units of capacitance, with one participant clarifying that capacitance should be measured in picofarads (pF) rather than picohenries (pH).
- One participant proposes using the output impedance of the function generator to find capacitance through phase shift measurements.
- Another participant advises measuring capacitance effects in isolation versus in the actual circuit, highlighting the impact of nearby objects in RF applications.
Areas of Agreement / Disagreement
Participants express various methods and considerations for measuring parasitic capacitance, with no consensus on a single best approach. Some methods are endorsed while others are questioned, indicating a range of opinions and techniques in use.
Contextual Notes
Participants mention potential limitations such as the frequency range of the signal generator, the influence of measurement setup on results, and the need for careful consideration of nearby objects affecting capacitance measurements.
- #2
Averagesupernova
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What is the value of incuctance?
- #3
Nam_Sapper
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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.
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.
- #4
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 [tex]\omega_0[/tex].
Since [tex]\omega_0 = \frac{1}{\sqrt{LC}}[/tex] you can calculate the capacitance.
Varying the frequency of your sinusoid will change the amplitude of the signal. The amplitude will be minimal at the resonant frequency [tex]\omega_0[/tex].
Since [tex]\omega_0 = \frac{1}{\sqrt{LC}}[/tex] you can calculate the capacitance.
- #5
berkeman
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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.learner1 said:
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-
- #6
learner1
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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.
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.
- #7
Averagesupernova
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It is totally unreasonable because capacitance is not measured in pH. 
I suspect you mean pF. Anyway, seriously I think it is probably correct. Your approach is the way I would do it.
I suspect you mean pF. Anyway, seriously I think it is probably correct. Your approach is the way I would do it.- #8
learner1
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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.
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.
- #9
Averagesupernova
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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.
- #10
Nam_Sapper
- 26
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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.