Measuring inductance with an oscilloscope and signal generator

[@JohnSmith-iu8cj]

This video and the link with the formulas helped me a lot to get back into the topic, I had forgotten almost all the formulas and the calculations and after this I feel like I am ready to continue!

[@Tony-mq5yo]

Measuring inductance with ordinary meters can be deceiving unless it is air core inductor. This video did great job on inductances measurement

[@ajayjoseph4498]

What a fantastic video! Didn't realise measuring inductance can be so easy.

[@nevmarr]

Thanks Scott, that's brilliant. I tested two inductors I wound on two T50-6, they were wound per the toroid specs to give me 3.98uH on one and .13uH for the second. Using your method the first came out at 3.95uH. The second was too small for my scope & FG upper limits. I then modified the formula per your comments to 1/4 & sqrt(1/15) to try to estimate the smaller inductor value. The 1/4 volts was right on the limit of my gear and the math came out at .09uH. This has given me confidence that my winding calcs have created inductors pretty darn close to my target. Until I found your video I was progressing with fingers crossed, now I'm progressing with a lot more confidence. Cheers!

[@ScottMarley]

Really glad it was useful, I've never wound my own inductors but maybe I should give it a go!

[@farzadbakhtiar]

Hi Neville. Can you point to any good resources online to wind my own coils? TIA.

[@nevmarr]

@@farzadbakhtiar Hi Farzad. My go to site for toroidial coils is kitsandparts.com/toroids.php Click on the part number eg T50-2 for a whole world of info. You'll notice there is a blue menu at the top of the linked page. It has some really useful info too. Sing out if you get stuck or have any other questions

[@ftmmrbs1996]

Hello @Neville would you please explain how you came up with 1/4 and sqrt(1/15)? for some inductors the voltage is never more than a certain value (600mVmax for 3 V input) no matter what the frequency. is it because of the core I'm using that is getting saturated? was this your problem too? is that why you used 1/4 and sqrt(1/15)?

[@nevmarr]

@@ftmmrbs1996 I'm sorry but I cheated with the 1/4 and sqrt(1/15) ... I got those numbers from Scott's reply to @Suresh Kumar's question in the comments. If you're good at maths the detail is in the link in the description. I'm useless at maths but maybe 1/6 and sqrt(1/35) may work for you??? [I went for 1/6 as your 600mV "limit" is 1/5]. I have no clue why you cannot exceed 600mV but I wonder if the inductor value is so small it cannot be assessed by this method?? I used 1/4 and sqrt(1/15) because my function generator could not generate a frequency high enough to get 1/2 3V.

[@vinceotten129]

This is fascinating! The simplest method I’ve seen for using a function generator and a scope to determine inductance. Saved & noted.

[@acestudioscouk-Ace-G0ACE]

I'm not the best with maths but you explained this so clearly, I followed easily. Thank-you. I like your calm and clear presentation style.

[@calculatingcapibara2914]

Yes, the style of presentation is great plus here.

[@computerchemist]

@@calculatingcapibara2914 I'll third that. Or is it the square root of a third? :D

[@lancetollenaar7905]

Another way that I like to use is one that works well if your function generator is crappy and doesn't necessarily have a nice linear output impedance (e.g. the FG-100 DDS generator). You connect up the resistor (in your case the 178.2 ohm resistor) as a shunt resistor, and use a 2-channel scope to measure voltage across the inductor, and voltage across the (known) resistor - from which you can infer the inductor current. Note that you would want a low ESL resistor, or at least ESL orders of magnitude above what you're measuring. You also measure the delay between the "current" & voltage waveforms, from which you calculate phase shift (along with freq.). From there you can use Ohm's law to calculate the magnitude of the impedance, and with the phase shift you can calculate the complex impedance. Take a few different data points and plot them in Excel, and compare with the theoretical calculated impedance :) An interesting thing you can see with this method, is the ESR of an inductor shows up as a linear offset to the calculated theoretical impedance!

[@ScottMarley]

Great bit of theory there, thank you!

[@noa97]

The great truth, I know other methods with AWG and oscilloscope, but this is the fastest to measure an inductance, thank you very much for your contribution.

[@PayneDK]

If you adjust the frequency until the voltage is 0.71 x generator voltage (in this case 2.2V), then the coil impedance XL is the same as the generator output impedance. Hereby the inductance is calculated as: XL=2pi*f*L=50.5 Ohm and L=50.5/(2pi*f)

[@andrewscottpersons]

sqrt(2) / 2

[@jeff-73]

Thank you soooo much for this. This was the first video I found on youtube where you SHOW the wiring setup. cheers

[@johnpearcey]

I think it is much more useful to derive those formulas that just pull them out of a hat. Edit: I didn't see the link in the description - well worth taking a look.

[@yamakawa511]

This is a really practical method, well explained.Thanks for the link to Ronald's page, it's well worth reading. Y

[@pedrogodoycruz]

Your method works perfectly. But, I usually test the resonant frequency. Place a known capacitor (measured value, not just from the specs) and connect it to any voltage source. Disconnect the voltage and look at the resonant frequency. You only need an oscilloscope (and a good tester to measure the actual capacitor value). Also, voltage measurements are usually not very accurate with regular oscilloscopes, but frequency/time measurement are spot on, even for cheap oscilloscopes.

[@ScottMarley]

Very useful information, thank you!

[@patrickmonnier2193]

Not sure to have understood your method. Do you mean, you connect L and C in parallel and connect them to a power supply (maybe 3V). Then with your scope setup for a single trig open the circuit and measure the ring frequency?

[@pedrogodoycruz]

@@patrickmonnier2193 Correct. Set the oscilloscope trigger to capture the self oscillations when you disconnect from a low voltage power supply. It allows you to calculate the inductance within 5% in a few seconds if your oscilloscope has good resolution for the frequency.

[@FirstLast-om8li]

​@pedrogodoycruz I think that's what those little testers are doing too actually.

[@PbPilotnl]

Thank you for the video. It helped me confirming my LCR meter was crappy with higher inductance. I did had some problem with my Uni-t 962 Function Generator set to 50 ohm, i changed to HighZ (scope was on M as well) and everything was fine.

[@elizabethjohncollins9363]

If you have an oscilloscope and a known size of capacitor you can deternine the inductance in a perhaps more straightforward fashion. The idea is to constuct a series LC circuit and find the resonant frequency. Then use that frequency to calculate the inductance from the formula for series LC resonance. Put the inductor and capacitor in series, add a resistor (e.g 50 ohms) to stop the function generator from shorting out at low frequencies. Find the value of frequency which minimizes the p-p voltage,. Use this frequency you found, the known capacitance, to calcuate the inductance using the formula for resonant frequency of an LC circuit.

[@srtamplification]

That's the way I do it.

[@Felipe2077tv]

Can you draw a basic schematic visualizing what you just said?

[@MrEdwinHubble]

One addition. Series LC(R) circuit will have a resonant frequency when the measured voltage on R has a maximum when xL and xC are the same.

[@Regulatorrobb]

Awesome!!! thank you for the extra link in the description.

[@jomosoft]

Very nice ! I'm automating this procedure with python/SCPI commands which is great fun to do !

[@Stephens8x6Workshop]

Thank you so much for this. As a newbie Radio Homebrewer this has helped me greatly. Much appreciated. Liked subbed and rang your bell too!

[@ScottMarley]

Appreciate it chap!

[@rZERO_game]

1 khz to 600khz output amplitude at no load position does amplitude remain same?

[@aronhighgrove4100]

At 4:17 you said it was 3.06Vpp but later in your calculations at 9:30 it became 3.08Vpp. Did you manually adjust it to get an output impedance closer to 50Ohm?

[@izsakitt3711]

S11 Smith chart can be set to indicate the combination of resistance and inductance (capacitance) of a component as a direct reading. The impedance of a component may widely vary as a function of frequency and also with excitation level (volt seconds that is, flux density, gauss or tesla) The VNA is a great tool to plot the component impedance as a function of frequency. - for the case in this video, a 10 fold change in the capacitor value likely computes a different inductance value

[@Lee-pj4be]

Great video, and thanks for the link to some additional info!

[@carl112466]

I think I am missing something! I take the unloaded voltage and it was 3v peak to peak. I connect the inductor but still get the same voltage value of unloaded generator

[@vienlacrose]

you're making a high pass filter with the signal generators built in impedance and manually sliding to the cutoff point to measure the value of the inductors, right?

[@tonyrebeiro]

Very nice, neat explanation and video.👍

[@AlexA-ei4js]

Thanks Scott for the video. Could you please provide me the link where to purchase the M Tester the same as on your video? Have you done any modification on it?

[@rocifier]

Hi, I tried a large inductor that I have and it wouldn't drop the voltage at all. I have the same signal generator as you have and dialed all the way up to ~10Mhz to get to 3V. It only drops down to about 2.7V in the low frequencies. How can I measure it seeing as the voltage doesn't drop by half?

[@calculatingcapibara2914]

Try rearranging the Karen Orton equation for a ratio of 9/10 instead of 1/2. Measure the DC resistance in advance. Note that inductancies are complicated beasts. You can not avoid the need for theory. The method here is not fit for large inductancies. You are likely on the edge of usability here.

[@Neopan210]

The (Karen Orton) method in the explanation link doesn't work if the inductor resistance is greater than 50

[@donmichaelsen5169]

Neglecting the wire resistance of the inductor can introduce some error, I would prefer to adjust the scope time scale to look at the charge time of the inductor (or capacitor) and use that to calculate the value of the component.

[@calculatingcapibara2914]

Thanks for the video. Btw, if you search for the frequency where the voltage is down to 1/3, the equation ends in L = 1/f within 1%. Having said so, a scope plus functiongenerator is a bit out of scope for me. I use an oscillator setup with a series LC circuit. In that, the Rinductor disappears. I measure the frequency with a soundcard on the PC. For an occasional measurement, it does the job.

[@calculatingcapibara2914]

Erratum: ...down to 1/3 ... is an error, it should be 1/8.

[@larrybud]

@calculatingcapibara2914 I like this method. Much easier to set everything up. Set up sig gen for 8vpp, then change frequency until we get 1vpp

[@rZERO_game]

if u increse frequency without geving load then amplitude remains same when frequency 1khz sine wave?

[@victorrodal5192]

Nice and concise explanation. Thanks for the effort :)

[@pault6533]

Good tutorial, I was looking for a way to use my new oscilloscope and signal generator to measure inductors. You helped me do this plus also determine the output impedance of my signal generator. Does this work for capacitance too?

[@tedhayton2302]

Hi Scott A really fascinating video. I am trying to use your method to measure a large unmarked smoothing choke. The FG calc works out perfectly at just under 50 ohms but I get no p/p drop whatsoever with the choke in parallel and reducing the frequency as instructed. The DC resistance of the choke is 418 ohms. Is it possible this method only works for small inductors? I expect the choke to measure 10 -15 Henrys. Keep up the great work!

[@calculatingcapibara2914]

You will not get a drop more than roughly 12 percent and only at very low frequencies. In addition, the large Rind will cause severe inaccuracy, even when compensated for. This here is not a great method for large inductors. You might try the method by "DIY made" at its 50 Hz, provided that you take care of safety(!), proper compensation equation and significant readings from the multimeter (larger reference resistor and/or multimeter with accurate low AC range).

[@octogames6823]

Would you happen to know how many uH the chines MDS-60 metal detector kit might be?

[@calculatingcapibara2914]

You could give it a try, but it will be close to impossible with the components already soldered in place. You will need a high frequency on your fg too. Moreover, to get to know the design it is important to measure the mutual inductance, not only both self inductances.

[@hanstap6502]

Where is the 3.08 V PP coming from? I think it is 3.00 V PP everywhere?

[@calculatingcapibara2914]

I have the same question. Maybe Scott had a look on the oscilloscope, giving that value?

[@larrybud]

It was the scope's measurement of his sig gen.

[@choke173]

Sir . 2.4Volt (connected with resistor) , how about 3.06 volt ? sorry sir , poor english i have ... Your method quite useful ...

[@mdasikkhan1610]

Im guessing this method is most accurate for higher inductor values. While for lower inductances, since you need to fairly large frequency, the parasitic in the circuit might be relatively big and give a erroneous result

[@calculatingcapibara2914]

It is not the parasitic here. 20Khz is too large a starting value for a large inductor. Try say 1 Khz or even less.

[@hamradio96]

Hey, The outcome of the induction in the formula is it in uH or mH ? Thankyou

[@larrybud]

The output is in henries, so if you had 5 x 10^-6, it would be 5uH, if it was 5 x 10^ -3 it would be 5mH

[@ayhan6977]

thank you very much. so, is it possible to measure the inductance with square waves instead of sine waves?

[@calculatingcapibara2914]

Not with this method.

[@drashtipatel-il7te]

How did you derived Final “L” formula?

[@binaryparrot3352]

Formula derivation is in link in the description

[@SlowfingerJC]

As I understand it, RO is output resistance not output impedance, as impedance is always frequency dependant, which is why it is used as a constant in the formular ie not as a frequency dependant value.

[@djfisjtik]

I just wanted to double check. A 0.5 mH inductor at 9300 Hz (from the video) would have a resistance of 29.2 ohms (L * F * 6.28). I think I understand the voltage divider part of the measurement, but therefor it would make more sense to me if the found frequency would result in the same resistance of the output impedance (50 ohms). To get 50 ohms at 9300 Hz you would need 0.86 mH. I'm sure your formula is correct and I simply don't understand how it works. It would be nice if you can elaborate.

[@ajgd]

He puts the original formula in the description and works like you said, you measure the output voltage on the generator unloaded, and then you found a frequency that makes that voltage drops 1/2 and you say that you found the frequency for 50 ohm impedance. He is doing that, but the first thing that he does with the resistor is measuring that +/- 5% of the generator output, that's why he writes 50.49 ohms and not 50. but the other steps, with the inductors, are right, he is searching for a frequency that makes the voltage drops 1.5V (half the initial 3V without load)..... that formula only works if you are searching for 1/2 of Vgen unloaded. sorry if my english is not the best 😅

[@calculatingcapibara2914]

Not 29.2 ohm, but j times 29.2 ohms. J equals sqrt(-1), expressing the fase shift between current and voltage from an inductor.

[@pyrhockz]

Very clear thanks!

[@kishorebadgujar7445]

can we measure low wattage line 1/4 Watt inductance value using on DSO

[@calculatingcapibara2914]

Theoretically yes. Did you try?

[@logcom482]

Now I andestent how you do))LC filter))Nice metod)

[@rZERO_game]

my function generator metravi make is bad. its amplitude going smaller when increasing frequency other OHM tech indian make is good upto 1.4Mhz its amplitude remain constant as per set point. i have tested with multisim also, One problem shows that multisim inbuilt function generator or inbuilt agilent function generator shows 50 ohm impedence but when connecting any resistance like 132 ohm its voltage accross 132 ohm does not fall. so my test as per ur methode(finding impedence) is not functioning. then i add additional 50.49 ohm to output and measure as per ur video , it is perfectly working.thanks for ur video,

[@TYGAMatt]

Fantastic. How easy is that!!

[@buffplums]

Excellent tutorial thanks chap

[@امیرحسینفتحآبادی-ظ1س]

very helpful thanks alot

[@albertlagerman]

Could you just wire a resistor in series in front of inductor, put a probe before the resistor, and after the resistor, and adjust the frequency until the voltage on the probe after the resistor is half that of the first?

[@calculatingcapibara2914]

No, the faseshift of the L should be taken in account.

[@ftmmrbs1996]

my function generator says its 50 ohm output impedance but I found it 400 ohms based on this video. is this not strange?

[@calculatingcapibara2914]

Could be that your fg is not 50 ohms output. So connect 400 ohm (or 180+220 or 4*100 in series) and observe the downfall of voltage, and at different frequencies. If 50 %, your Rout is 400, not 50.

[@kishorebadgujar7445]

how to measure low wattage value inductance(1/4 w,1/2 W) measurement using CRO and Function generator

[@choke173]

How to get the Vo & Vl value . Thx

[@pedrova8058]

Vo is the voltage of the signal generator output UNLOADED Vl is the voltage with the inductor conected

[@SureshKumar-nk2ok]

sir which frequency will be best to use

[@ScottMarley]

It doesn't matter, just try some and see what works best. 20kHz is a good starting point but you will get the same answer regardless.

[@SureshKumar-nk2ok]

sir in formula why 0.5 used for V out by V in

[@ScottMarley]

Again, it doesn't matter. Look at the link in the description, and at the maths after the line "The question now is for what frequency (Vscope/Vgen) = 0.5:". Instead of 1/2, put 1/4 in instead, and follow it through to the end. You should end up with a formula of: L=sqrt(1/15)(R/2 pi f). So if you make the oscilloscope reading 1/4 of the original value instead of 1/2, you would use this formula to calculate the inuductance. 1/2 is just chosen for convenience.

[@SureshKumar-nk2ok]

@@ScottMarley yank u so much sir

[@alanpedder3912]

Really enjoyed using this method which was only hampered by my rusty maths. But works perfectly and simplified to 4.614/f for a sig gen impedance of 50.25 ohms

[@logcom482]

Very clever metod

[@FirstLast-om8li]

Damn, my function generator doesn't have a dial for frequency you have to input it in the digital screen and then push start every time😢

[@josepeixoto3715]

m or n?

[@mohamed_abdelrahman10]

I think to be more accurate in measuring , we should measure resistance of inductor first and do our calculations based on (resistance of function generator and resistance of inductor)

[@MichaelPechner]

triple checked. pk->pk 5.08V -> 4.32v 271.5 r so aim for 2.54v 17.490Mhz Getting 2.7 mH - INductor is a 5.6mH. Any clue what I am screwing up? signal generator is new rigol. Oscilloscope is a new 2ga sample siglent. dvm is old HP that passed certs recently. I should be measuring closer to 8Mhz. Just a software engineer trying tolearn to measure toroids he needs to wind.

[@djfisjtik]

Great tips! I tried out this method on my Voltcraft DSO-1084F (rebranded Hantek DSO4084C) scope with builtin function generator and guess the measurement worked out. I measured the output impedance of the generator to be 390 Ohms. Alternative to your calculation of the output impedance of the generator, you could also measure it with a pot, aiming to achieve half of the peak voltage compared to the unloaded state.

[@ScottMarley]

Nice work!

[@fredgenius]

Superb, thanks.

[@Yorumcu63]

I must watch again

[@andrewbarriere4643]

REAL NICE

[@CircuitCreator]

NICE ONE

[@luouns]

I have got a headache

[@ernestb.2377]

👍

[@ioscaleb]

Disappointed that you completely punted on explaining the math… I didn’t really need to watch 11 minutes of hooking stuff up to measurement devices; I can figure that out myself. The key thing to know is how to derive the inductance from the measurements, and you skipped that entirely.

[@rowifi]

I was going to say exactly the same thing.

[@johnwick7175]

He explained it perfectly well. He used the formula for inductive reactance to find a value for the inductance.

[@spelunkerd]

The link in the video description goes through that in detail. I agree, most of the satisfaction comes from the understanding.

[@dalepres1]

And, in fact, he could have skipped all the math with the 459 constant (x10 to some power multiplier dependent in exponent of values) I came to this looking for the math so I would appreciate tips, links, pointers to the math for non-engineers. I knew all that math 50 years ago but it's long since evaporated from my brain.

[@sibsbubbles]

doesn't work