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Impracticalites of ESR in repair work

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Fred Basset:
It seems everywhere there is so much information on ESR, what it is, how to measure it, how to visualize it, how important it is and so on.  Thanks to what I have read on here though, my eyes are opened now to the shear size of the variations in ESR.

So pulling a data sheet for a particular range of caps when you are designing makes a lot of sense - But I was trying to compile a chart of ESR values at which a cap should be replaced when doing repair work, but I quickly come to a halt.  The variation even for normal electrolytics kind of makes a nonsense of my chart.  So many unknown Chinese makes (rebrands?) makes obtaining data sheets impossible and even if I did, I am not very confident that the caps will be within spec. even when new.

So although I have a great little ESR meter, what use is it on anything other than well-known and respected brands?  In typical reapir works, it is still quicker and cheaper to replace all likely electrolytic caps with known good ones, or should I persevere?  What are your thoughts please?

ocset:
As you know, you  have to measure ESR at the ripple current frequency, otherwise the esr value you get is wrong.
With many ripple current shapes  being complex, and not sinusoidal, it means that there are in fact a range of frequencies in the ripple...making it even harder to assess.
With electrolytic caps, the situation is harder because an el cap doesnt "form" until  its got at least 10% of its rated voltage on it...so if your  esr measurer doesnt impose at least this voltage, then again the reading is wrong

Fred Basset:
The meters do seem to be settling now on results at a test frequency of 100KHz.  But so many tables do not state the test frequency - Usually the cheaper ones, that it just seems to add another variable.  I was trying to design a table at 100KHz test frequency, but as you rightly point out, it should be at the ripple frequency - But that also varies so much

I suppose it is not so bad if you are repairing the same boards from the same manufacturer all the time, but for me it is just a hobby now, so I work on anything.  From the day I first found out how bad the tolerances are on them, I have always felt that electrolytics have never been domesticated - They are not housebroken and would be called ferral if they had ever been domesticated enough in the first place...

I do not like blanket changing of electrolytics, so I am testing them, but using my experience to decide if the reading is acceptable or not.  I would love to see what an AI system would make of the collected results over a few years.

Thank you for your thoughts, it has given me more to consider.

kripton2035:
in standard SMPS, I usually replace caps that have an esr of more than one ohm. no need to have any chart for this.

Siwastaja:
Of course you need at least some kind of a chart. Sizes of elcaps in "typical" SMPS vary by at least two orders of magnitude, so do expected ESRs.

2 ohm ESR is just fine for a very small capacitor which can be on the control side.
0.5 ohm ESR is likely a total showstopper for a large low-voltage output capacitor, say a 4700µF 6.3V thing. Such a beast might be rated at ripple current of, say, 5A, and if, say, 4A ripple is actually applied, 0.5ohm ESR would dissipate 8W!

Fixed 1-ohm limit only works if you only consider a certain range of capacitors. This is your "hidden" knowledge.

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