Measuring Inductors with an Impedance Analyser - Page 1
Forum section: electronics / projects

author: trewmac
author_rank: Newbie
timestamp: June 28, 2013, 02:53:39 am
content: Have you ever gone to measure an inductor, only to find that it has a negative value? In this post we are looking at real world behaviour of inductors using a TE3001 Network Analyser with a tweezer attachment. Below is a frequency sweep of the measured inductance of a TOKO 470uH wire wound inductor shown above.It starts off as expected with a reasonable inductance value of 432uH.At 4.4MHz something very strange occurs, and above this frequency it has negative inductance???No the analyser is not broken!!!4.4MHz is known as the self resonant frequency of this inductor where it is effectively an open circuit. Above their self resonant frequency, inductors appear capacitive (hence the negative inductance).What causes this strange behaviour?This behaviour is caused by the action of unwanted stray reactances inherent in the physical construction of the component. We can’t make it go away, but we can characterise it, and we can create an electrical model to better describe its behaviour in the real world.First we need to do some theory, and then we’ll show you how to measure all the parameters in the model using the TE3001 analyser.Real world inductor modelInductors are formed by coils of wire that inherently contain unwanted resistance. There is also unwanted capacitance between the windings.When operating at high frequencies, it is necessary to develop an electrical model that includes these stray elements to accurately describe the inductors behaviour.There is no limit to how complex these models can be but below is the simplest model that is sufficient for most LF-VHF purposes.Low frequency responseAt low frequencies (far below the self resonant frequency) the impedance of C is so high it is effectively an open circuit and the model can be simplified to just R and L:R and L can be measured by placing the analyser in series RLC mode, or by viewing the low frequency portion of the series RL chart on the software.Reading from the charts;R=2.96 Ohms is better than the stated specs.L=432uH is within the 10% manufacturing tolerance.High frequency responseAbove the resonant frequency the impedance of L approaches an open circuit and C dominates the response. Again the model can be simplified, but this time to just C.Of course this C is real, and can be measured using the equivalent circuit format at a frequency well above the resonant frequency. Series or parallel format should yield the same result for C, because R is negligible in either case.I prefer to measure values directly from the LCD.Choose a frequency well above the resonant frequency, and change the display format to RLC to measure C.The screen shows 2.92pF measured at 100MHz.Self resonanceAt the self resonant frequencyf, the measured reactance is zero.This occurs at 4.42MHzAt this frequency the reactance of the stray capacitance cancels out the reactance of the inductance.Using the formulas for impedance of an inductor and capacitor this meansWhich can be rearranged to give the calculated value of this stray capacitance C as a function of L andfThis formula can be used to test how accurate our model and measurement technique is.Plugging in our values of L andf, we getNot far off the measured value of 2.92pF.So now we have a real world model for this inductor that more accurately describes its behaviour at high frequency:To summarise the technique:O0 Hope it was helpful! We will follow up with measuring capacitors and resistors.....

author: sorin
author_rank: Frequent Contributor
timestamp: January 14, 2014, 05:30:57 am
content: Thanks for sharing..

author: Rerouter
author_rank: Super Contributor
timestamp: January 14, 2014, 07:31:43 am
content: and a tool that is way above my price range for toysthough looking around did point me to this,http://icom.hsr.ch/fileadmin/user_upload/icom.hsr.ch/publikationen/RLC_Meter_EN.pdfwould i be right to assume it uses something similar? 2 adc's computing the ratio of voltage and the phase shift between measuring points, and possibly a DDS to drive the test frequency cleaned up with a number of filters?

author: BravoV
author_rank: Super Contributor
timestamp: January 14, 2014, 09:45:53 am
content: Thank you, great post especially for hobbyist like me.Noob question, say for a power inductor in switching power supply, what if the switching frequency at the inductor's self resonant frequency ? Magic smoke ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 14, 2014, 10:06:50 am
content: Quote from: BravoV on January 14, 2014, 09:45:53 amprobably, probably not. other elements wil dampen it, but i think the performance will be screwed at resonance. the universe is so loving that does not allow magic spark everytime an inductor is released from a current flow.

author: BravoV
author_rank: Super Contributor
timestamp: January 14, 2014, 10:38:48 am
content: Quote from: Mechatrommer on January 14, 2014, 10:06:50 amSo should I toss a coin ?Geeez Mech, have mercy, as I said that is a noob question, your answer there that involving coin tossing and has to do with the love of the universe thingy, is way out of my poor noob brain to understand it.But thanks for the reply

author: owiecc
author_rank: Frequent Contributor
timestamp: January 14, 2014, 12:54:26 pm
content: Quote from: trewmac on June 28, 2013, 02:53:39 amThis is why I prefer a log-log plot of impedance. You can easily see what is what:Best plotted on a funky paper with L and C scales: