How to measure capacitor's ESL?

[Mecanix]

I'm literally failing to find ecap vendors who publishes their ESL and SRF characteristics, and highly suspect I'll have to buy half-their-internet to find out which caps I need performance-wise.

Given the award circumstance; how do you go about measuring ESL & SRF using maker benchtop equipment? a.k.a. 100meg scope, 50meg function gen, handheld LCR, and coffee...

Thanks

[TimFox]

It depends on the capacitance values you are using.  Are you only measuring electrolytic capacitors?
For large capacitors (maybe above 100 nF) and a good LCR meter, measure the series-mode capacitance over the frequency range (hopefully, up to at least 100 kHz).  If the SRF is less than the highest test frequency, this is easy.  If not, you can estimate the SRF from the apparent decrease in capacitance at high frequency.
For smaller capacitors in RF applications, I built a simple jig with two good 50 ohm resistors in series between BNC connectors on single-sided FR4.  I soldered the capacitor in question between the joint and ground and swept the frequency range on a spectrum analyzer.  With leaded parts, I found (to a good approximation) that the ESL was a function of the total length of wire (including the body) between the 50/50 node and ground over a reasonable range of capacitance for SRF below 200 MHz.
I'm not set up for SMT capacitors, but at my former employer we had good coaxial test jigs for the usual -hp- impedance meters and network analyzers.

[harerod]

Side note: If you have trouble finding information about ESR directly, check the datasheet for the frequency dependent loss angle δ or the corresponding loss tangent tan δ. https://en.wikipedia.org/wiki/Dielectric_loss

[Mecanix]

It depends on the capacitance values you are using.  Are you only measuring electrolytic capacitors?

Correct. Investigating techniques to determine series inductance and thus Self-Resonance characteristics of large electrolytic capacitors in the range of 1uF all up to 1000uF.
Essentially to avoid engineering harmonic generators... I'm not into musical stuff when it comes to let's say; switch mode power supplies.

Is there a reason why vendors keeps this ESL and SRF data somehow a grand mystery? Perhaps ECaps are ideal components?!
Or do I really have to buy Jack's, Ruby, Nichoo and all other glorious ecap collection on that Internet and sweep through freq just to find a set that actual works?

Last question; are we in 2021?! /rant_over

[RoGeorge]

Investigating techniques to determine series inductance and thus Self-Resonance characteristics of large electrolytic capacitors in the range of 1uF all up to 1000uF.

Seen a few good links posted by _Wim_ in another thread:
https://www.eevblog.com/forum/testgear/device-impedance-using-dso/msg3681559/#msg3681559

[Mecanix]

Good pointers, RoGeorge. Thanks.

Probably wrong wording used (techniques), my bad. I'm more or less after The Formula to calc ESL, preferably one that goes along with the testing equipment that I already have (maker-grade that is!).

I'll see if I can download some of those Corporate million $ testers' User Manuals and see if they share the formula for the test(s), and hopefully find a way to apply that to what's already on my bench. In a meantime if anyone of you done ESL and SRF characterizing of ECaps before or knows of a fancy workaround please do share - totally clueless and ambitious beginner here would greatly appreciate.

Thanks

[Mecanix]

So... with a 330uF 25V ECap on the LCR, I'm getting the following:

100Hz L: -7.942mH
120Hz L: -5.527mH
1kHz L: -82.6uH
10kHz L: -946.2nH
23.7kHz L: -1.024nH <--- I can dial the freq of the LCR up to about where L reaches zero, close anyway
24kHz L: 3.76nH
100kHz L: 5.767 nH

Fair to assume the ECap becomes a dumb resistor at Self-Resonance range which seems to appear at 23.7kHz??

Am I doing this correctly?

[TimFox]

That looks right for that device.

[The Electrician]

So... with a 330uF 25V ECap on the LCR, I'm getting the following:

100Hz L: -7.942mH
120Hz L: -5.527mH
1kHz L: -82.6uH
10kHz L: -946.2nH
23.7kHz L: -1.024nH <--- I can dial the freq of the LCR up to about where L reaches zero, close anyway
24kHz L: 3.76nH
100kHz L: 5.767 nH

Fair to assume the ECap becomes a dumb resistor at Self-Resonance range which seems to appear at 23.7kHz??

Am I doing this correctly?

What make and model LCR meter do you have?  I'm curious because generally hand-held LCR meters can't dial the test frequency.  They're usually limited to the 5 standard frequencies: 100 Hz, 120 Hz, 1 kHz, 10 kHz and 100 kHz.

[Mecanix]

You are correct. Most if not all are OOTB locked to 100, 120, 1, 10, 100kHz for some strange reason. Mine's different though, can be set to any freq range between 50Hz~100kHz with a 1Hz increment. So is the sinewave voltage and of course a bias (both with 1mV increment). It's a Victor4080 which is a rebrand of the East Tester ET430 if I'm not mistaken. Firmware was an easy hack to get all the features unlocked on that device (enables DCR, ECap, 1Hz & 1mV increment, etc etc) of which I'm not going to elaborate on here.

Supports all SPCI cmd so I have it hooked up to a UWP app via serial com running freq sweeps and all that fun stuff I shouldn't have to do; still completely clueless why vendors hides the fact that their ECaps are dumb resistors at low/medium frequencies and inductors at higher ones. Clever...

ps. Recommend. Had it checked by a friend side-by-side to a IM3523 last year, accurate to 0.5~1% surprisingly. Fascinating testing equip... one I rely on quite a lot in fact.

[RoGeorge]

Each piece of wire (including component terminals) has inductance.  The longer the wire, the more inductance.

The measured inductance in the pic might go smaller if you try measuring again the same capacitor, but with the kelvin probes clamped right near the capacitor case, not at the far end of the terminals.

[Mecanix]

Good advice, and yeah I was probe'd toward the canister with the cap upside down initially (think it helps the electron flow lol). Just had it hooked up that way for a quick photoshoot... But man I've given up with this nonsense rig all together anyway. Wrote to my nephew a few hours ago asking for that ESL formula I probably need, and missed out on. If he doesn't have it he sure knows who does, I'll put this on hold until then. Tests via that high-res scope ideally... I'll report back if there is anything to learn out of all this.

Appreciate all the advice and help so far.

[TimFox]

With a little math, so long as the SRF is less than the maximum test frequency, one can get a good estimate of the SRF from measurements at the usual discrete frequencies (1k, 10k, 100kHz).
Again, for leaded parts with my 50 ohm test jig, I found experimentally that the ESL is a function of the total length between test nodes, and the SRF follows from that and the capacitance.

[The Electrician]

Good advice, and yeah I was probe'd toward the canister with the cap upside down initially (think it helps the electron flow lol). Just had it hooked up that way for a quick photoshoot... But man I've given up with this nonsense rig all together anyway. Wrote to my nephew a few hours ago asking for that ESL formula I probably need, and missed out on. If he doesn't have it he sure knows who does, I'll put this on hold until then. Tests via that high-res scope ideally... I'll report back if there is anything to learn out of all this.

Appreciate all the advice and help so far.

Why do you want to know the ESL of your capacitors?  What will you do with that info?

[The Electrician]

With a little math, so long as the SRF is less than the maximum test frequency, one can get a good estimate of the SRF from measurements at the usual discrete frequencies (1k, 10k, 100kHz).
Again, for leaded parts with my 50 ohm test jig, I found experimentally that the ESL is a function of the total length between test nodes, and the SRF follows from that and the capacitance.

This would be a good thread to explain to Mecanix the details of calculating ESL when the SRF of the capacitor is below 100 kHz.  Your procedure might be easier than what his nephew will describe.

[Mecanix]

What will you do with that info?

Interestingly enough and while not knowing how to characterize my own components I certainly won't be able to do anything, at all. I'm in it for the learning curve at this stage; having recently put up two amazing (to me) SMPS. One of them using a 115kHz VIPer06 PWM with a TL103W dual-opamp for the CC/CV and feedback circuits and the other on 60kHz. Just went through magnetism so time to move on to filtering (LC, RC, PI, ect...). And before that, well, it would be nice to understand what's an ECap first!

RE Nephew. I'd like to think I have my math right (not all lost cause) so hopefully there are several formulas and test sequence(s) we can utilize to accurately characterize those fat, ugly and dumb inductive resistors. Electrolytic caps, I meant. sorry... let's see.

[TimFox]

Actually, electrolytic capacitors are crappy diodes.

[bdunham7]

Actually, electrolytic capacitors are crappy diodes.

https://simplifier.neocities.org/rectifier.html

And not only that they can be Light Emitting Diodes as well!

http://www.sparkbangbuzz.com/els/borax-el.htm

[Mecanix]

They also have magical properties and can instantly convert from dumb resistance into confetti fireworks too. Their brutal lateral velocity at Self-Resonance is incomprehensive, until one realized he accidentally mounted a 16V 470uF onto an unfiltered 24Vdc rail with 12,766,900mV of stray ripple pulsing at 115kHz :/
(I'm still finding paper-ish debris all over, been a week...)

EE aside; the importance to being able to accurately measure ESL & SRF and understanding them badboy characteristics is beyond paramount!

Anyone knows how?

[paul@yahrprobert.com]

I used to work on high power transmitters and I'd use vacuum variables or big fixed ceramic capacitors in the matching networks.  The ESL was important in such work, and the way I'd measure it was to short the capacitors with wide pieces of copper foil so that my short didn't contribute much inductance.  Then I'd make two small coils, and drive one with a signal generator and the other would be a pickup coil going to an oscilloscope. Best to position the coils on opposite sides of the cap. I'd sweep the frequency until I found a peak.  The resonant frequency is the series L resonating against the cap's nominal capacitance.

One comment I have about ESL in large electrolytics:  an important number in any resonant circuit is the characteristic impedance eta = sqrt(L/C).  For large electrolytics this will be small number, small fractions of an ohm. Now if other resistances in series with the capacitor (a load, the capacitor's ESR, the connecting wires, or a small series resistor you purposely design in) are comparable to or larger than eta then the capacitor's self resonance will be seriously overdamped and you'll never notice it.  This is often the case, and if so measuring the ESL is not very important.

[tszaboo]

Its not in the datasheet, but it is usually in some software that you can freely download/access online.
Redexpert for Würth,  Simsurfing (I think) for Murata, K-SIM for Kemet, and so on. You will find their MLCC data there.
And the simple answer for regular electrolytic capacitor ESL: It's too much.