Forums

? on old electrolytic caps

Started by Dave June 17, 2014
I have an LCR meter which shows the ESR for electrolytic caps, but I have no 
idea how to interpret this number.  How do you know if a cap is shorted or 
boardering on that status?  I mean, if it gives me a reading of zero Ohms, 
that's obvious, but what if a small (4.7uF), medium voltage (35V)  cap comes 
up with a reading of, say, 3 Ohms?  Is that too low, like I expect, or 
should it be considered acceptable?  I just don't know what a good cap would 
likely register, and have been surprised in the past when a brand new cap 
registered a lower ESR than the one I thought was bad.  Where can I find 
info on the subject?  Any ideas are welcome.

Thanks...

Dave 


On Tue, 17 Jun 2014 16:50:40 -0500, "Dave" <db5151@hotmail.com> wrote:

>I have an LCR meter which shows the ESR for electrolytic caps, but I have no >idea how to interpret this number. How do you know if a cap is shorted or >boardering on that status? I mean, if it gives me a reading of zero Ohms, >that's obvious, but what if a small (4.7uF), medium voltage (35V) cap comes >up with a reading of, say, 3 Ohms? Is that too low, like I expect, or >should it be considered acceptable? I just don't know what a good cap would >likely register, and have been surprised in the past when a brand new cap >registered a lower ESR than the one I thought was bad. Where can I find >info on the subject? Any ideas are welcome. > >Thanks... > >Dave >
In ESR, low is good. But 0 ohms could be shorted. It's easy to check for a short other ways. 3 ohms for the 4.7u sounds perhaps a little high, but might be OK; maybe it was old and getting dry. A new one could well be lower. Check some cap data sheets for max or typical ESRs. Really big aluminum caps, or polymer aluminums, can be milliohms. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 17 Jun 2014, Dave wrote:

> I have an LCR meter which shows the ESR for electrolytic caps, but I have no > idea how to interpret this number. How do you know if a cap is shorted or > boardering on that status? I mean, if it gives me a reading of zero Ohms, > that's obvious, but what if a small (4.7uF), medium voltage (35V) cap comes > up with a reading of, say, 3 Ohms? Is that too low, like I expect, or > should it be considered acceptable? I just don't know what a good cap would > likely register, and have been surprised in the past when a brand new cap > registered a lower ESR than the one I thought was bad. Where can I find > info on the subject? Any ideas are welcome. >
Practice, practice, practice. Try all the electrolytics you have on hand, try some in-curcuit (assuming the unit tests in-circuit), buy some new ones and see. Try to get a feel. Keep bringing home junk from the garbage that will likely have bad caps, like LCD monitors. Once you find some bulging capacitors, which are bound to be bad, test those and see. This will give you a feel for what's good and what's bad. Michael
"Dave"
>I have an LCR meter which shows the ESR for electrolytic caps,
** Make and model please.
> but I have no idea how to interpret this number.
** As with any measurement, you must know what to expect or it is meaningless to you.
> How do you know if a cap is shorted or boardering on that status?
** Use an ohm meter. The reading should be tens or hundreds of thousands of ohms.
> I just don't know what a good cap would likely register, and have been > surprised in the past when a brand new cap registered a lower ESR than the > one I thought was bad.
** You are way off the game. The lower the ESR reading, the better.
> Where can I find info on the subject?
** Even in old equipment, most electros are still fine so take note of the readings you get and make a list with varying values and voltages and the corresponding ESR. Make sure to post the details of your LCR meter - cos not all work the same. .... Phil
On Wed, 18 Jun 2014 14:38:55 +1000, "Phil Allison" <phil_a@tpg.com.au> wrote:

> >"Dave" >>I have an LCR meter which shows the ESR for electrolytic caps, > >** Make and model please. > >> but I have no idea how to interpret this number. > >** As with any measurement, you must know what to expect or it is >meaningless to you. > > >> How do you know if a cap is shorted or boardering on that status? > >** Use an ohm meter. > >The reading should be tens or hundreds of thousands of ohms.
One fun thing about electrolytic caps is that they store and even generate voltages, through dielectric absorption and electrochemical effects. That can really fool a DVM on its ohms range, especially if you let it auto-range. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
"John Larkin"
> "Phil Allison" >> >>"Dave" >>>I have an LCR meter which shows the ESR for electrolytic caps, >> >>** Make and model please. >> >>> but I have no idea how to interpret this number. >> >>** As with any measurement, you must know what to expect or it is >>meaningless to you. >> >> >>> How do you know if a cap is shorted or boardering on that status? >> >>** Use an ohm meter. >> >>The reading should be tens or hundreds of thousands of ohms. > > One fun thing about electrolytic caps is that they store and even generate > voltages, through dielectric absorption and electrochemical effects. That > can > really fool a DVM on its ohms range, especially if you let it auto-range. >
** Quite so. FYI: by "ohm meter" I meant an analogue one, ie a moving coil multimeter. In any case, a shorted electro would show up on a DMM quite reliably. The OP is asking the impossible cos verifying some old electro is still OK requires a bank of tests, meters and PSUs. Where possible, after doing an ESR test, I just switch old equipment on and watch out for smoke. Anecdote: ----------- I had a new in box power amplifier for repair some years back that insisted on blowing its AC fuse at switch on. However, I found it could be brought up gradually using a Variac, pulling a large AC current if done too fast. Made in Germany it used a pair chassis mount, 30mF, 100V Siemens electros in the PSU - the kind with 5mm bolts on top. Both were getting damn hot and even after an hour the Variac was set not much beyond half mains voltage - so I stopped torturing them. Turned out, the amplifier had been left in storage for about 2 years and the electros had depolarised something fierce. ... Phil
Actually ESR should be one tenth of Xc or less.
<jurb6006@gmail.com>
> > Actually ESR should be one tenth of Xc or less. >
** Huh ???? This a candidate for "Meaningless Post of the Year" ? Or is it too dumb ........... .... Phil
On Tue, 17 Jun 2014 16:50:40 -0500, "Dave" <db5151@hotmail.com>
wrote:

>I have an LCR meter which shows the ESR for electrolytic caps, but I have no >idea how to interpret this number. How do you know if a cap is shorted or >boardering on that status? I mean, if it gives me a reading of zero Ohms, >that's obvious, but what if a small (4.7uF), medium voltage (35V) cap comes >up with a reading of, say, 3 Ohms? Is that too low, like I expect, or >should it be considered acceptable? I just don't know what a good cap would >likely register, and have been surprised in the past when a brand new cap >registered a lower ESR than the one I thought was bad. Where can I find >info on the subject? Any ideas are welcome. > >Thanks... > >Dave
--- The ESR (Equivalent Series Resistance) of a capacitor is the resistance (_not_ the reactance) it presents to charge trying to flow into or out of a capacitor, and causes a capacitor to heat up due to the power dissipated by its internal I&#2013266098;R losses - generally, if not always - in the dielectric. The resistance of a capacitor is an entirely different thing, and amounts to the leakage current measured through the dielectric with a voltage across it. In terms of goodness, the lower the ESR and the higher the resistance the better. John Fields
In article <b243q9t7gq15ragisjeqisjkn30iib91do@4ax.com>, 
jfields@austininstruments.com says...
> The resistance of a capacitor is an entirely different thing, and > amounts to the leakage current measured through the dielectric with > a voltage across it. > > In terms of goodness, the lower the ESR and the higher the > resistance the better. > > John Fields >
Could you please clarify that a bit? I am only an inbred from Maine and I did quite get that one. Jamie