ESR of a cap in parallel with ceramic one

[ChildOfVision]

Hi! I'm sorry if I ask a question that has already been answered, but I couldn't find anything with search.
I have one question, and there is no better forum than "BadCaps" for this!
Namely, I am building an ESR meter and I did read somewhere (I really can't remember where) for problem with small ceramic capacitor in parallel with C.U.T.
Allegedly, certain ESR meters (that uses 100kHz test-signal or something like that) can not measure in-circuit ESR of an capacitor that is connected in parallel with cer. capacitor, as I said. I'm not a beginner in electronics, but I really can't figure out how such a small capacitor with kiloohms of reactance can affect (or even prevent) in-circuit ESR measurements with ANY kind of meters?
However, I remember of very resolute claims that such a combination of capacitors is utterly hopeless for many ESR meters.
So if somebody knows something more, please let me know!

TIA, and sorry for my bad English!

[brethin]

Re: ESR of a cap in parallel with ceramic one

The only TRUE measurement of ESR on a capacitor is out of circuit.

[ChildOfVision]

Re: ESR of a cap in parallel with ceramic one

Of course that "The only TRUE measurement of ESR on a capacitor is out of circuit". But, did you ever tried to remove capacitor from PC motherboard, or some VGA card without damaging both capacitor and MB? Well, if you didn't I can tell you it is very, very hard (if not impossible)!

[b700029]

Re: ESR of a cap in parallel with ceramic one

It's not hard, many of the members here regularly recap mobos and GPUs. You just need a powerful soldering iron and plenty of flux.

[PCBONEZ]

Re: ESR of a cap in parallel with ceramic one

Of course that "The only TRUE measurement of ESR on a capacitor is out of circuit". But, did you ever tried to remove capacitor from PC motherboard, or some VGA card without damaging both capacitor and MB? Well, if you didn't I can tell you it is very, very hard (if not impossible)!

Most people here do that all the time.
It's not that hard with the right tools and some practice.

A good share of mobo caps are in parallel with other caps on the board.
Removing the caps is REQUIRED if they are in parallel and you want to check ESR or capacitance.
It's not the only 'true' way, it's the ONLY way.

The total ESR of caps in parallel works exactly like the total resistance of resistors in parallel.
The total will always be smaller than the smallest value.

And, how do you know that ""small capacitor with kiloohms of reactance"" isn't degraded and/or partially shorted?
.

[ChildOfVision]

Re: ESR of a cap in parallel with ceramic one

Yes, you are all right, of course, if I have two or more capacitors (especially elecrolytics) in parallel there is no way to measure ESR of only one cap!
But, what drives me crazy is what theory is behind that issue with "small ceramic cap" in parallel, I know I read somewhere detailed explanation for this, but I just can't remember where! It is not related to reactance or faulty (e.g. shorted) cap. I found something on EEV-Blog-forum: one member of this forum wrote this: "Unfortunately it's common practise to connect capacitors in parallel, for example on a linear PSU you might have a monster 4700uF capacitor in parallel with a 1uF tantalum and a 100nF ceramic capacitor, the latter both having a very low EAR which will interfere with the result". Does anybody know what EAR stands for??? Is there some kind of "ringing", noise or some kind of uncontrollable oscillations that disturb measurement? Furthermore, many of ESR-meter constructors/sellers "very vigorously praise" their instruments with ability to cope with this problem!
Such a instruments generally don't use continuous, say, 100kHz sine-wave signal but "just a very short pulse" for testing. So, if someone knows more... please let me know!

TIA!

[PCBONEZ]

Re: ESR of a cap in parallel with ceramic one

What I think is that on an English keyboard the "A" key is right next to the "S" key and the poster meant to type ESR, not EAR.
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There is a condition where MOSFETs will 'ring' [at a high frequency] and they use small uF caps as a bypass to prevent it but I don't know what that has to do measurements other than the typical caps in parallel problem.
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It doesn't make sense that a short pulse [one shot] measurement would have ringing issues -in the measurement- because 'one shot' is an instantaneous thing and ringing is a continuous thing.
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It could be that what they mean is caps put there to prevent ringing mess up the one shot measurement,, but that is just a caps in parallel problem and it doesn't mean there is ringing occurring during the measurement.
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[ChildOfVision]

Re: ESR of a cap in parallel with ceramic one

What I think is that on an English keyboard the "A" key is right next to the "S" key and the poster meant to type ESR, not EAR.

Good noticed! I believe this is the case (btw. I never saw keyboard with different character layout, except, in some cases, "Z" and "Y" swap their places).

It doesn't make sense that a short pulse [one shot] measurement would have ringing issues -in the measurement- because 'one shot' is an instantaneous thing and ringing is a continuous thing.

No, you misunderstood me: I meant just the opposite - instruments that works with short pulses (better to say: fast series of pulses) actually can do measurements in such a situations.
For example, see this meter: note 9th "point" - there is explicitly stated "small ceramic capacitor...etc.:

An Hungarian LCFesR meter

I already contacted that person, and he is aware of that problem, but he could not explain what is the source of it.

[PCBONEZ]

Re: ESR of a cap in parallel with ceramic one

I doubt Dr. Le Hung [the name on the actual schematic] is Hungarian.
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That meter is a clone of Bob Parker's meter modified to use a different processor.
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I think the person you contacted stole the design and dosen't fully understand it.
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[ChildOfVision]

Re: ESR of a cap in parallel with ceramic one

You are COMPLETELY right! In everything that you said. Dr.Hung lives in Hungary, indeed, but obviously he is not Hungarian. Btw. he asks $$$ for "his" project, so, if you are interested...

[Evil Lurker]

Re: ESR of a cap in parallel with ceramic one

EAR = Equivalent Average Resistance?

[ttn328]

Re: ESR of a cap in parallel with ceramic one

Hi All,

The name " Le Hung " is a Vietnamese name. I recognized it just because the language in this name is my native language. I do not have any other thoughts.

[hutale]

Re: ESR of a cap in parallel with ceramic one

Someone notified me about this thread... I think I can explain a little: it is true that Le Hung is a Vietnamese name but I had my studies and degrees in Hungary. I live in Hungary and have a Hungarian citizenship. But from the view of LCFesR meter, all these information are not important. (If you can read in Hungarian then you can read it here: LCFesR in Hungarian).
I examined the caps in parallel problem with LCFesR meter: I measured ESR of 1000 uF, 25V cap, its ESR = 45 mOhm. Then I measured ESR of this cap parallel to an 100 nF cap and then parallel to an 2.2 WIMA uF cap, I got the same ESR results. So these small caps are not affecting the result.
As to Bob Parker's design, it is referenced in my doc as the theory of ESR measurement but if we examine the 2 designs, the implementation differs a lot, not only because of the difference of processors.

[Kiriakos GR]

Re: ESR of a cap in parallel with ceramic one

You made a page about a product that you are trying to sale and so what ?
Why some one to explore your theory when what you made does not have any certification or be tested regarding accuracy from a recognizable source.