Measuring inductance - Page 1
Forum section: electronics / beginners

author: nilswe
author_rank: Newbie
timestamp: July 26, 2022, 06:29:13 pm
content: HiI have a small question:for university, I needed to measure inductance within this circuithttps://imgur.com/a/9A9gYHWFor that, we used this formula: L = (UL1*R)/(UR*2pi*f)When I Google another method, I stumbled across this linkhttps://www.daycounter.com/Articles/How-To-Measure-Inductance.phtmlCan someone tell me the difference between these methods ? Why seems to be daycounter more popular ?Thank you very much

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2022, 06:59:49 pm
content: The circuit you posted is specifically for DC use. If you are to solve it your instructor wants, it's a question of measuring the inverse exponential current increase in the inductor.There are better ways to do this, but if that's what he/she wants...

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2022, 05:37:21 pm
content: This is not the DC method shown in the diagram. The fomula uses the ac reactance to work out the indcutor value using the fact Xl=2pi()fL, or L=Xl/(2pi()f.This is how inductance meters and network analysers obtain the value. Read up on network theory.

author: Benta
author_rank: Super Contributor
timestamp: July 27, 2022, 07:53:12 pm
content: Quote from: Terry Bites on July 27, 2022, 05:37:21 pmLook at the schematic. All values are capital/underscored which (in German notation at least) means DC.That the problem is idiotically drawn or phrased is a different thing.

author: MrAl
author_rank: Super Contributor
timestamp: July 28, 2022, 09:48:42 am
content: Quote from: nilswe on July 26, 2022, 06:29:13 pmHello,Is there a difference? What difference do you see there?I see they both use AC to measure the inductance, which is for low power inductors mainly.

author: nilswe
author_rank: Newbie
timestamp: August 20, 2022, 10:25:54 am
content: Ahh, sorry for the late reply and thank you for all the answers.The main difference I see is that you have to tune the frequency in method by daycounter. In the method by my instructor, the measurement can be done at any frequency.So do my question is exactly do you have an advantage of tuning the voltage over the resistor/coil to 1/2 of the input voltage.And does anyone have a better way to measure the Inductance ?Another way I stumbled on is to measure the frequency of an RLC resonant circuit.So, which one is the best method to obtain the InductanceTanks

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2022, 11:45:05 pm
content: The 4th way is to measure the phase instead of the amplitude, which can generally be more accurate.

author: MrAl
author_rank: Super Contributor
timestamp: August 21, 2022, 01:44:06 am
content: Quote from: nilswe on August 20, 2022, 10:25:54 amHello again,Do you have an oscilloscope? That makes it easier.Here is a formula for the voltage across the inductance:VL=(Vin*w*L)/sqrt(R^2+w^2*L^2)Solving for L we get:L=(Vout*R)/(2*pi*f*sqrt(Vin-Vout)*sqrt(Vout+Vin))where Vout is the voltage across the inductor.This allows just about any f or Vout or R, but you should use reasonable values. The reason Vout=1/2 of Vin for one of your tests is because that is a very reasonable set of values. if you input Vin=2 volts and you only get out 0.1 volts, it is harder to get accurate. if you get 1 volt out though that is more reasonable.Also, if Vout=1/2 of Vin the formula changes to:L=R/(2*sqrt(3)*pi*f)and so that's a little simpler but you can still change R and f as you need.With a scope and a square wave generator you can test in the time domain by looking at the waveform and doing a little calculation.BTW power supply chokes usually require some current level like 100ma or even 1 amp for the bigger ones. You cant get a good test with very low current. The best way is to use the inductor in a buck circuit and then look at the waveforms and do a little calculation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2022, 04:06:17 am
content: For the general case (magnitude only, no phase information, any ratio), use this formula:https://www.seventransistorlabs.com/Calc/RLC.html#rlcHit F12, view scripts, to see the formula. The derivation is left as an exercise to the reader, of course.Tim