Bypassing large electro's with smaller caps - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: September 22, 2012, 01:29:43 pm
content: Hello all.This weekend I've been making high frequency measurements to show exactly how effective it is (or not) to place smaller caps in parallel with large electrolytic capacitors. In "theory" parallel resonant effects are always a potential danger, but thus far (having measured with electros in the range of 100uF to 1000uF) it seems that the ESR of the electrolytic capacitors is still large enough to dampen them out.Here is the data:http://www.users.on.net/~glenk/capacitor/capacitor.htmThere will be more to come, but probably no much more this weekend. I'm sweeping for resonances and recording and entering all of the points on the plots manually, which is a bit tedious and time consuming. I will be adding a write up of my method. A 2200uF cap will be next, then 4700uF, than 10,000uF, etc. I also plan to do surface mount stuff like 1206 tants in parallel with 1206 chip ceramic, etc. Perhaps some might find the data useful for perspective.Cheers,Glen

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2012, 01:48:06 pm
content: I have seen some caps specified for GSM phones where the resonant frequeny of the caps are optimised so that there is a dip at the GSM frequency bands, the most impressive having 3 dips to cover the world GSM bands in a single component in a tiny SM package. Intended for PA decoupling, and having an impressive current rating for the size.

author: Rerouter
author_rank: Super Contributor
timestamp: September 22, 2012, 01:49:51 pm
content: would also love to see with cermics smd packages if 4 smaller values that equal close to the original outperform it, e.g. 4x 0.22uF vs 1 x 1uF while in theory the lower esr should mean that it does, i am still left curious,

author: kg4arn
author_rank: Unknown
timestamp: September 22, 2012, 02:10:51 pm
content: Very interested in the results of your tests. Thanks for sharing them.

author: free_electron
author_rank: Super Contributor
timestamp: September 22, 2012, 02:21:27 pm
content: There is a flaw in your test....The big electrolytics are used as bulk capacitance and sit far away from the point of load. The small ceramics sit close to the point of load. I between you have the wiring/pcb trace network and the power distribution grid.For high edge rates, at the point of load, you don't 'see" the electrolytic part anymore. They are dampened in the inductance of the wiring between bulk and decoupling caps.Take a circuit board , depopulate it, except for the elco and the decoupling caps. Then measure across the decoupling cap only. The elco needs to be 3 to 4 inch away from the decoupling cap.Second element is that the capacitance values you use are waaaay too large. The load imposed on the tracking generator is so large the effects are lost. The analyser doesn't have enough dynamic range to show it.Try this with a 10uF electrolytic and a 10nF ceramic . Then repeat with a 10uf ceramic and 10nF ceramic. They need to be on a circuit board that has a power grid , inches apart.Tdk has an appnote on this kind of testing i believe.You either sweep a cap by itself to get the impedance curve. Or you do it on a real board. You cannot do this on a 'perfect' setup where the caps are in parallel.

author: nctnico
author_rank: Super Contributor
timestamp: September 22, 2012, 03:40:31 pm
content: Quote from: Rerouter on September 22, 2012, 01:49:51 pmIts all about the size of the package. The smaller the package the lower the impedance. With 0402 it doesn't matter much if you use 10nf, 100nf or 1uf. The impedance versus frequency graph is pretty much the same.

author: GK
author_rank: Super Contributor
timestamp: September 22, 2012, 05:22:43 pm
content: Quote from: free_electron on September 22, 2012, 02:21:27 pmEh?Small value capacitors are often put directly in parallel with large value electrolytic capacitors to improve HF bypassing in a variety of scenarios - it is the performance/effectiveness of thisspecificallythat I am measuring (the effects of the smaller cap resonating with the inductance of the electrolytic). For example I might have a base bias regulator for an bipolar HF RF power amplifier - I don't wan't mains ripple getting in and 30MHz getting out.I really cant make much sense of the rests of you post. Tracking generator? Analyser? I'm using neither. Dynamic range? The lower limit (maximum attenuation) on the plots is governed by the ESR of the capacitors; it's not a measurement limitation, if that is what you are thinking. In fact the ESR of the electrolytics can be calculated from the 1k-1M frequency plots (the flat before the the inductance kicks in). The series resistor is 50 ohms and the drive voltage for each point of measurement was 2.5V. The graphs show the output voltage versus frequency.

author: GK
author_rank: Super Contributor
timestamp: September 22, 2012, 05:42:46 pm
content: Quote from: nctnico on September 22, 2012, 03:40:31 pmWith electrolytic capacitors, the ESR goes down as the capacitance value goes up. Suppose you have a "chip amp" audio power amplifier. You bypass the supply pins right at the chip with a 220uF electro directly in parallel with a 100nF chip capacitor. You want to be sure that the ESR of the electrolytic chosen is high enough so that the parallel resonance between the 100nF and the electros ESL is properly damped, otherwise the 100nF could be doing more harm than good by introducing a resonant peak.

author: GK
author_rank: Super Contributor
timestamp: September 28, 2012, 10:53:19 am
content: Hello all.I have just updated my web page with a whole lot more measurements - on a range of low ESR capacitors. I have also re-formatted the previous data. I have yet to write up the measurement method, but it is pretty simple. The capacitor being tested is driven by a signal generator via a 50 ohm resistor. I measure both the input voltage and the voltage developed across the capacitor ("Vout") at each frequency measurement point. All of the graphs have been normalised to 1V input. The impedance of the capacitor at any point can be computed with ohms law. For example, if Vout at 10MHz is 0.5V, than the capacitor impedance is simply 50 ohms.As can be seen, bypassing the large, low-ESR electros isn't really a good idea. This is important stuff to know! Suppose you are designing a low voltage, high current switch mode power supply, with a large value, low-ESR electrolytic on the input side. You might reason that sticking a 100nF MKT or ceramic in parallel with it to improve bypassing at high frequencies would be a good idea - Well, not necessarily! It may very well make things quite a bit worse over a frequency range of importance.http://www.users.on.net/~glenk/capacitor/capacitor.htmCheers,Glen

author: Unknown
author_rank: Unknown
timestamp: September 28, 2012, 11:27:15 am
content: Very interesting stuff Glenn,I guess you prove the old adage that "Unless you measure it it's wrong!"But then again maybe there is some other type/value cap that can work with low ESR caps.Interesting site btw.

author: GK
author_rank: Super Contributor
timestamp: September 28, 2012, 11:32:47 am
content: Quote from: HAL-42b on September 28, 2012, 11:27:15 amYou can fix the resonant peaking problem simply by putting a low value (well under an ohm) damping resistor in series with the small value capacitor.That will be the next series of tests - but not this weekend.

author: robrenz
author_rank: Super Contributor
timestamp: September 28, 2012, 11:35:56 am
content: Nice work, keep it up.

author: free_electron
author_rank: Super Contributor
timestamp: September 28, 2012, 01:06:14 pm
content: Quote from: GK on September 28, 2012, 10:53:19 amAnd what do you do with phase shift ?Some reading material :http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdfhttp://si-list.net/files/published/sun/ectc_2001.pdf

author: poptones
author_rank: Frequent Contributor
timestamp: September 28, 2012, 07:07:46 pm
content: Actually, many of your measurements bear out one of the tenets given through the work of Walt Jung and richard Marsh - that is, larger value electros should be bypassed with more than a single value. Once you get to several hundred uF, there should be more than a single film cap. For example, 330uF || 5uF || .5uF.http://waltjung.org/PDFs/Picking_Capacitors_2.pdf

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 12:07:44 am
content: Quote from: poptones on September 28, 2012, 07:07:46 pmSomething like a 5uF film cap will be physically huge with probably more inductance than the electro; in otherwords likely mostly useless. Putting multiple, progressively smaller values in parallel (with large value, low-ESR electros) runs the danger of introducing multiple resonant peaks and ruining the bypassing capacity of the combo over an even greater range of frequencies. That is something that I should also measure and demonstrate.

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 12:12:19 am
content: Quote from: free_electron on September 28, 2012, 01:06:14 pmFor an approximation it can be ignored for the most part. There is a reason I graphed the vertical axis directly in measured volts rather than computed impedance.

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 01:46:16 am
content: Something like a 5uF film cap will be physically huge with probably more inductance than the electro; in otherwords likely mostly useless.Itissomething you should measure first, because this is not new technology. I'm guessing you didn't bother with the link? Real world examples are presented, and they're thirty years old. Think of what better it could be today. A 5uF X7R ceramic isn't very big at all.Anyway, a properly constructed film cap will have no more self inductance than a piece of wire the same length.Of course, in some applications it also depends on what dielectric you're using.http://www.edn.com/design/components-and-packaging/4314822/Choose-capacitor-types-to-optimize-PC-sound-qualitySure to piss of some, but Bob Pease recognizes the truth in it, and if it's good enough for Bob it's good enough, as they say, for me.

author: T4P
author_rank: Super Contributor
timestamp: September 29, 2012, 02:00:58 am
content: Wellhttp://diyaudioprojects.com/mirror/members.aol.com/sbench102/caps.htmlCapacitor SoundI don't believe in "capacitor sound" at low frequencies (<100KHz)

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 02:04:15 am
content: Then you got it all wrong. Because it's at the lowest frequencies where it is proven again and again to make a difference. Even freakingMicrosoftspecifies certain materials for "Vista" certification (whatever that is).http://www.edn.com/file/11593-Figure_9.pdf

author: T4P
author_rank: Super Contributor
timestamp: September 29, 2012, 02:17:06 am
content: Quote from: poptones on September 29, 2012, 02:04:15 amOnly a audiophool like you will defend capacitor sound like a roman knight

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 02:23:09 am
content: Quote from: poptones on September 29, 2012, 01:46:16 amWTF?A X7R ceramic isn't a film cap. The rest of your post is just trolling. If this thread is going to be hijacked by audiophools with associated BS then I'm out of here.

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 02:33:34 am
content: You know, there's a certain point at which steadfastness and objectivity turns into stubborness and stupidity. The fuckingdata- real, measured data - is right in front of your face.Do you know Bob Pease?

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 02:50:56 am
content: WTF#2The "data" you have posted has nothing to do with my previous comments. A 5uF "film" capacitor (with its intrinsic, high inductance along with that of its long leads and the PCB tracks required) in parallel with alarge value, low-ESRelectrolytic (such as those measured on my website) is would be largely pointless. You'd be much better off paralleling the electro with another identical one. It would be cheaper, take up less board space and perform much better. You haven't provided any data to the contrary and I am already familiar with the links you have posted (including just about everything on WJ's site).Do I "know Bob Pease"? What a daft and irrelevant question.

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 05:01:01 am
content: QuoteI just reacquainted myself with that WJ/RM article – the link again:http://waltjung.org/PDFs/Picking_Capacitors_2.pdfFirstly, the article is, for the most part, a great example of the quasi-technical gobbledegook typically published in audio journals.As for the “fucking data”, when the paper gets to “Figure 17” (page 8 of 9), which is supposedly some transcendent parallel capacitor combination, there isn’tANY“fucking data” and neither a single measurement – just several paragraphs of subjectivist drivel and philosophising about how it supposedly sounded and how it works. And secondly, the “Figure 17” brilliance is all about the AC coupling of small signal audio. Hardly something I have been concerned with in my measurements.

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 05:16:48 am
content: OK. Well, I'll certainly be sure to avoid any stereo equipment you may have had your hands in.It's funny how stubborn some folks can be over well established ideas that challenge their desire to be right. Even Bob Pease presented a paper at the AES about how he was forced to accept the fact different types of caps couldsounddifferent. He also presented data on how equipment thatmeasuredthe same could, under AB conditions, sound different. This guy was an analog design engineer since the fifties, designed countless integrated circuits for National Semiconductor, did a monthly article usuallydebunkingsome idiocy... and yet, he was still willing to learn.

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 05:40:39 am
content: You are just making crap up and blowing things out of proportion. Bob Pease just made some embarrassing, unsupported comments in acquiescence to some ardent audiophile types who were let loose in NS’s audio components applications division at the time. There was even leaked marketing/application talk of metal can-packaged LME series op-amps “sounding better” than the SOIC versions! I’ve had personal correspondence with more than one person in the industry who was thoroughly unimpressed with Bob’s retrenchment and treatment by management following NS’s restructure, but, contrastingly, not in the least with the swift turfing of some of the audio applications crew.

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 05:58:15 am
content: ROTFL. Yeah, because Bob Pease was just like that...Now who's making up nonsense?

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 06:05:42 am
content: Quote from: poptones on September 29, 2012, 05:58:15 amWell, you are.Where is the "real, measured data"? Found it yet? Is there an on-topic point to any of your posts? Did I start a thread entitled "capacitor sound - dumb trolls invited to comment"?

author: T4P
author_rank: Super Contributor
timestamp: September 29, 2012, 06:10:10 am
content: *Grabs popcorn* There has been real, measured data that capacitors have not much influences on low frequencies.I don't care what he says about caps sound because i know he's been pushed off the ledge by some weird audio sector in NS. Why would you want Bob Pease be angry from the grave?

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 07:17:43 am
content: All capacitor types are nonlinear to some degree and can definitely introduce levels of distortion well above (orders of magnitude) measurable thresholds – and especially so if the wrong type is misapplied.In an audio circuit, such an audio power amplifier, an electrolytic capacitor coupling the input signal can become a dominant source of distortion at the lower few octaves of the audio frequency spectrum. This is because the reactance rises as the frequency drops and a significant level of signal voltage begins to develop across the capacitor. This is well covered in the literature (eg D. Self.). The demonstrated cure for electrolytic capacitor distortion is to simply select a value much higher in capacitance than what would otherwise be required for the desired lower (high-pass) cut-off frequency.However high frequency bypassing of electrolytic capacitors by paralleling with smaller caps (in particular boutique film caps) for demonstrated benefits in the audio frequency spectrum is mostly complete bunkum. And keep in mind that demonstrated measurable differences do not necessarily equate to demonstrated audible differences.

author: jahonen
author_rank: Super Contributor
timestamp: September 29, 2012, 07:22:09 am
content: I have in fact came into pretty much same conclusion when it comes to bypass applications, where you want just lowest possible impedance for all frequencies. Paralleling same sized capacitors often yield best overall response with no unexpected surprises. Apart from very small regions of frequency, having multiple sizes paralleled seems to be inferior overall.That is not in fact purely my own conclusion, but Dr. Bruce Archambeault seemed to arrive to same conclusion in his EMC book. For most bypass applications, it is best to use same kind of capacitors to avoid parallel resonances. Just use largest practical capacitance in the smallest package (lowest inductance). But then, this lowered inductance is often difficult to utilize because of the PCB involved. For two sided PCB, it is almost impossible and requires care on a multilayer board. Any extra mm^2 in interconnect 3D loop area tends to skyrocket the inductance.I think that in the signal path, the situation is somewhat different (possibly no DC bias etc). Thus I tend to avoid any capacitors there if at all possibleRegards,Janne

author: GK
author_rank: Super Contributor
timestamp: September 29, 2012, 07:46:39 am
content: Quote from: jahonen on September 29, 2012, 07:22:09 amThanks for contributing this info - real data!A have Otts book book, which covers the same ground. However I don't think that the technique (paralleling with much smaller values) is always a bad idea. In fact in many cases it can offer quite an improvement, as I think my measurements with the "standard" electrolytics show. It all depends on the ESR of the electrolytic. In some cases I don't think there would ever be a probem - say, just for example, putting a 1206 100nF in parallel with a 22uF electrolytic at the pins of an op-amp. A 22uF will have such a high ESR that any parallel resonance will be thoroughly damped out.However, even in the case of large value (a few thousand uF), low ESR electros, I think the scheme can still be made to work very well simply by putting a small value damping resistor in series with the small value (>100nF) bypass cap. This is something I hope to be able to demonstrate in my next round of measurements.

author: free_electron
author_rank: Super Contributor
timestamp: September 29, 2012, 03:31:32 pm
content: When paralleling caps stick to one value only. All you are doing is 'pulling the dip' lower. Look at the curve that janne posted. It starts capacitive, dips to the resonance poi t and then goes inducctive and rises again.Paralleling identical values pulls that 'dip' lower and changes the slope and baseline of the overall curve.Paralleling different values creates multiple dips , but at the risk of creating 'bumps' as well ( parallel resonance).Now here is a scenario where you can't avoid paralelling different values : EMI. If the system radiates and fails certain frequencies you will need to dampen these by adding different values. And that's where the misery begins. There is the interplay between caps but also the interplay with the pcb ! A multilayer board with large planes is essentially a capacitor as well. And that thing can resonate as well! Merely placing one capacitor on it is enough to create parallel resonance. SUN microsystems has done extensive research on that ( see link i posted before the flamewars started in this topic ). They even have a piece of software where you can plug the parts you are using and it will show you the result. Many capacitor vendors like TDK Murata and others now have software avaialble that contains the 'augmented' spice models with this data in it. I have posted links to this software already on the forum a couple of months ago.Second: capacitor value is one thong. How you mount them is another thing. One via , stub no stub , 2 via , close , far ... There is very interesting reading material in the works of Bruce Archambault , Mark Montrose and Lee Ritchey (the speeding edge)One notable effect ( experienced myself) is that the planes in the board itself can resonate and emit crap at very high amplitudes, very effectively in one particular direction...Lets take a big board , half a meter by 1/3 ( standard size for big telecoms racks ) we slap some circuitry on there that doesn't quite 'fill' the board, but hey, those are the dimensions given. Turns out we use a six layer stackup. The parts are placed in close proximity to one another since this stuff is all relatively high speed. So we end up with a clump of circuitry about 10cm by 10cm. Where do we position this on the board and what do we do with the empty space.It turns out that , no matter where you place it , you are damned. The copper planes sticking out around you clump of material become effectively a dipole antenna that radiates sideways. The whole damn plane structure becomes a tuned network. If that catches any signal from you circuitry that it likes, all hell breaks loose and it becomes a very efficient transmitter.Ideally the planes need to be cut and all copper removed so that no plane goes beyoun half an inch outside the 'clump'. But, you cant do that for multiple reasons. There are comnectors on the back and on the front of this massive board....The solution is to place additional decoupling capacitors close to the edge of the board at a fixed interval ( every few inches ). These capacitors effectively 'short' the planes for RF at the edge so the 'antenna' is destroyed.We had a diagnostic board in a big rack. Rack fails EM compliance. Pull out diagnostic card. Pass.A stupid board with a single 8051, an extra uart and some led's.Put a few caps on the edge of the pcb , literally half a meter away ( this board was huge ). And problem solved. We had many people look at the board and wonder why we put four decoupling capacitors ' in the wild' where there was no circuitry. One even wondered if we knew that decoupling caps were supposed to be close to the chip....Very interesting experience.

author: GK
author_rank: Super Contributor
timestamp: September 30, 2012, 10:10:53 am
content: Quote from: free_electron on September 29, 2012, 03:31:32 pmIt is clear to me what the graph shows and can we not make blanket assertions?In cases where wide-band bypassing is required (from audio frequencies out to tens of MHz) large values of (electrolytic) capacitance are required and it is not always desirable (board space / cost) to connect multiple large units in parallel. If the electro has sufficent ESR (as I think I have demonstrated) there isn't any harm at all in bypassing it with a physically small 100nF (or 470nF or so), and the HF performance is significantly improved. In such a case it wouldn't be out of the question either to parallel the electrolytic with two, three or even four 100nF chip resistors in parallel, to improve the bypassing at high frequencies even further. Even in the scenario where the electro ESR isn't sufficently high enough to dampen out the parallel resonance(s) a small series damping resistance(s) will fix the problem. Measurements to this effect are currently under way........................

author: jahonen
author_rank: Super Contributor
timestamp: September 30, 2012, 10:47:58 am
content: I should probably point out that my interest was primary on paralleling different valued ceramic capacitors, which tend to have quite low ESR thus parallel resonances tend to spoil the final result. Here is one result with same sized capacitors in different quantities paralleled compared (graph title seems to be incorrect but you'll get the idea):The measurement is not valid for lowest frequencies and above 700-800 MHz, due to spectrum analyzer dynamics and measurement jig resonances, but it still shows consistent reduction of the impedance in the whole frequency spectrum.My current understanding is that ideal component for paralleling would be a a capacitor with a quite high ESR, then the resulting parallel impedance would be resistive and thus non-resonant. I'm not sure that adding an external resistor will work as well since the inductance spoils the idea somewhat. The resistance would be best embedded into a capacitor, in such a way that it will not affect the inductance.Regards,Janne

author: GK
author_rank: Super Contributor
timestamp: September 30, 2012, 11:08:49 am
content: Quote from: jahonen on September 30, 2012, 10:47:58 amI agree that the resistor does add some more counter-productive inductance, but in my specific tests/application it is still relatively small in comparison to the series sum of the PCB track and capacitor inductances. Two or more SMD series R-C's can be put in parallel to cut the net inductance down further. I'm not looking at bypassing solutions for frequencies anywhere near several hundred MHz. I do appreciate your measured data, but it's a bit apples and oranges at the moment. My primary interest is switch mode power supplies and audio. As a matter of fact, a SMPS powering an audio power amplifier is a great example of where wide-band bypassing, from the lowest audio frequencies to many MHz is required.

author: GK
author_rank: Super Contributor
timestamp: September 30, 2012, 12:31:12 pm
content: .......also putting multiple large value electro's in parallel to reduce the net inductance and improve HF bypassing isn't very effective due to the physical size of the capacitors and the inductance of the PCB track lengths required.

author: kg4arn
author_rank: Unknown
timestamp: September 30, 2012, 01:41:40 pm
content: GK, so if I understand you correctly, you are studying:- decoupling (not distortion in the signal path)- low audio to mid HF (e.g. 1 Hz to 10 MHz)Is that correct?This is an interesting topic for me and I am interested in your results.

author: GK
author_rank: Super Contributor
timestamp: October 02, 2012, 12:14:25 am
content: Hi,Yes, decoupling effectiveness from audio frequencies out to a few 10's of MHz. I'm particularly interested in demonstrating the parallel resonant effects of the smaller "HF" bypass capacitors with the large electros. The low omic SMD resistors I need to finish the next round of measurements are on back-order though, so I might not get any further measurements up the coming weekend.I've now got a test jig PCB made up in which the large electros are connected directly in parallel with surface-mount series R-C's soldered onto the PCB underside.

author: GK
author_rank: Super Contributor
timestamp: October 03, 2012, 12:44:01 pm
content: Hello all,I’ve just updated my webpage:http://www.users.on.net/~glenk/capacitor/capacitor.htmI’ve redone all of the measurements for the “Standard ESR” electrolytic capacitors. As it turns out, when I did these measurements the first time round, my probe earthing wasn’t adequate. As a consequence the plots actually significantly underemphasised both the effectiveness of the bypassing at high frequencies, above the parallel resonance, as well as the amplitude of the parallel resonant peaks themselves.From the measured capacitor parameters and some reasonable estimation of PCB parasitic inductance and resistances, I’ve also replicated/cross-correlated/verified/duplicated all measured results in SPICE. I’ve posted up the SPICE plot and simulation circuits for the 4700uF capacitor.My 1206 low-ohmic resistors will arrive in the post tomorrow morning. Tomorrow evening I will post up details of how to very effectively HF bypass even that 4700uF low-ESR electro without incurring any parallel resonances at all (with the aid of multiple, parallel R-C "snubbers")

author: GK
author_rank: Super Contributor
timestamp: October 04, 2012, 09:48:50 am
content: Quote from: GK on October 03, 2012, 12:44:01 pmDone. Don't have time to complement it with much of a write up this evening, but a picture says....................

author: robrenz
author_rank: Super Contributor
timestamp: October 04, 2012, 12:27:04 pm
content: Nice work, Thanks fornotletting some critique discourage your continued posting

author: GK
author_rank: Super Contributor
timestamp: October 05, 2012, 12:00:05 am
content: Hey, now worries, thanks!I’ve come up with a rule of thumb for selecting the snubbing resistance. You want to dampen the Q of the equivalent tank circuit down to 1 or less. The simplest textbook parallel resonant tank circuit equations are adequate enough.For example, 660nF resonates with 15nH at 1.6MHz (f =1/(2pi(SQRT(LC))). The Q of the basic/simplified tank circuit is equal to the (equal value) reactance of the L and C at resonance divided by the series resistance.Xl = 2*pi*f*LXc=1/(2*pi*f*C)At 1.6MHz 15nH = 0.15R Xl and 660nF = 0.15R Xc.Therefore the total effective series resistance (the sum of the deliberately added damping resistance plus the capacitor ESR’s) required for a Q of 1 is 0.15R.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 02, 2013, 11:09:55 am
content: Continued from another thread.Quote from: GK on March 01, 2013, 08:23:48 amWhy do you say this? Have you seen an instance where the term was not used for the real part of the impedance?Quote from: GK on March 01, 2013, 08:23:48 amWhat do you mean when you say "...the ESR can dominate over a wide frequency range. "?

author: GK
author_rank: Super Contributor
timestamp: March 02, 2013, 11:20:43 am
content: Quote from: The Electrician on March 02, 2013, 11:09:55 amI thought that would be obvious. Over a wide frequency range Xc and Xl are negligibly small in comparison to the capacitors resistive losses; the capacitor is essentially resistive.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 02, 2013, 11:24:39 am
content: I didn't think so or I wouldn't have asked.How wide is a "wide frequency range", for example. It's not obvious to me.What does it mean for ESR to "dominate"?

author: GK
author_rank: Super Contributor
timestamp: March 02, 2013, 11:35:40 am
content: Consider a 4700uF 16V capacitor. Assume it has an inductance of 8nH.At 10 kHz Xc = 3.4mR. Xl = 502uR.Those values are negligible in comparison to the capacitors ESR, which may be in the order of 0.1 ohms or more. As frequency increases the ESR will "dominate" the capacitors total impedance until Xl become significant.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 02, 2013, 11:38:42 am
content: On your web site with all the measured curves, the 4700 uF low ESR cap shows an effect that puzzles me. At around 30 kHz there is a hump in the curve. This hump is present with the lower valued caps also, but to a lesser extent. Do you know what causes it?

author: GK
author_rank: Super Contributor
timestamp: March 02, 2013, 11:48:06 am
content: The weird effect occurs at the the series resonant frequency between Xc and Xl. Why it should cause an impedance peak as well as a dip, and why the effect isn't completely swamped out by the capacitors resistive losses, I really dunno.

author: GK
author_rank: Super Contributor
timestamp: March 02, 2013, 12:33:26 pm
content: On the correct use of the term "Equivalent SeriesResistance":When specifying the ESR for a series of electrolytic capacitors, a manufacturer usually picks a frequency at which ESR is specified; typically 100kHz.100 kHz is low enough a frequency so that Xl will generally be negligible, and for most values of capacitance, Xc will be negligible as well. For example, Xc for a 100uF capacitor at 100kHz is 0.016 ohms. That's negligible next to the capacitors resistive component, which, for a run of the mill 100uF capacitor may be a few hundred milli-ohms. If the manufacturer specifies ESR=0.2 ohms @ 100kHz for 100uF, that 0.2 ohms is essentially just the resistive component.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 02, 2013, 08:48:16 pm
content: What I really wanted to know is why you are bringing it up. Where have you seen the term misused on the forum?I'm guessing you may be thinking of the so-called "ESR meters" that work by applying 100 kHz pulses to the capacitor (rather than a sine wave) and looking at the waveform of the voltage across the capacitor, which is an instantaneous rise followed by a ramp up, inferring the resistive part of the cap impedance from that.But, other than that, is there another example of misuse?

author: GK
author_rank: Super Contributor
timestamp: March 03, 2013, 03:05:35 am
content: Quote from: The Electrician on March 02, 2013, 08:48:16 pmBecause at the time I just felt like it.QuoteYes, but I really couldn't care to argue the point either way.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 07, 2013, 02:47:02 am
content: Quote from: GK on March 03, 2013, 03:05:35 amI asked "Where have you seen the term misused on the forum?" any your answer to "where..." is "Yes"?I'm not asking you to argue or not. I'm just asking for a link to a post where the term ESR was misused.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 07, 2013, 03:03:54 am
content: A comment about your plotted data.Your measurements show the effect of paralleling different size caps, but I find it difficult to compare to other similar measurements because you have plotted your data on a linear vertical scale rather than a log scale which is more common.The use of a linear vertical scale makes it hard to see just how a wide a frequency range is dominated by the ESR of the cap.To show what I mean, have a look at these two images. They show plots of impedance magnitude and ESR from 1 kHz to 30 MHz for a 4700 uF, very low ESR electrolytic in parallel with a 100 nF film cap. The first image uses a linear vertical scale and the second image uses a logarithmic vertical scale.The image with the linear vertical scale appears to show ESR dominating the impedance magnitude over a much wider frequency range than the logarithmic plot. I consider the ESR to be dominant only when I can't distinguish the impedance curve from the ESR curve.On the log plot we can clearly see the impedance curve separate from the ESR curve, with coincidence occurring over less than a decade of frequency. On the linear plot, everything is squashed down at the bottom of the screen, and it's hard to see the frequency range of dominance.Besides which, you haven't shown ESR at all, so it's just speculation as to the range of dominance.The curves Janne posted in this thread are logarithmic, as are the typical curves given by manufacturers:http://lowesr.com/esrfreqperfcurves.aspYou could, of course, re-plot the data with a log vertical scale. A few more data points wouldn't hurt, either.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 07, 2013, 03:07:27 am
content: I posted in another thread an impedance sweep of 4 different valued caps in parallel that clearly shows the result is not what one might think:https://www.eevblog.com/forum/projects/capacitor-measurements-on-an-impedance-analyzer/msg180419/#msg180419

author: GK
author_rank: Super Contributor
timestamp: March 07, 2013, 04:09:27 am
content: Quote from: The Electrician on March 07, 2013, 03:03:54 amNonsense.