Author Topic: Using a DMM, testing a capacitor?  (Read 6168 times)

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Offline davidk

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Using a DMM, testing a capacitor?
« on: December 25, 2012, 01:37:32 pm »
I've pulled out (unsoldered) a few capacitors I saw lying around from old circuitboards. I noticed that there were some voltages (less than a volt and diminishing as I test using the DMM). The ratings are on the side of the caps. They are in the order of uFs and nFs

Using a digital multimeter like an Extech EX330 (I saw it was an okay kind of multimedia from the last video review here so I went ahead to get it), how good are these things to test whether or not the capacitors are good/bad? (I'm sure the DMM itself should read to uF and nF with ease...)

I've noticed on almost all of the elctrolytic capacitor readings on the DMM are either no reading at all, or something completely different to the label (worse, I think I got the same reading as not having connecting the pin to the reader...)

I've tried googling but it seems to point towards only having to charge it up and look at the rate of discharge, I'm concerned that will take ages to go through a whole bucket of capacitors :) Any ideas here?
 

Offline c4757p

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Re: Using a DMM, testing a capacitor?
« Reply #1 on: December 25, 2012, 01:55:35 pm »
A DMM is nearly worthless to test capacitors. Most large electrolytics are over-range for most DMMs, and they don't test ESR. You can get an ESR meter cheap, though I recommend an LCR meter as it isn't so specialized. Lots of uses.

As for the values being off, they usually are. Tolerances on big electrolytics can be up to -20% +100%.
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Offline davidk

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Re: Using a DMM, testing a capacitor?
« Reply #2 on: December 25, 2012, 02:05:23 pm »
Thanks! That just confirms I will just keep all those capacitors I thought were "completely off" :)
 

Offline c4757p

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Re: Using a DMM, testing a capacitor?
« Reply #3 on: December 25, 2012, 02:07:34 pm »
Before I got an LCR meter, I'd estimate capacitance by charging it to 6V and seeing how long it would light an LED.
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Offline AndyC_772

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Re: Using a DMM, testing a capacitor?
« Reply #4 on: December 25, 2012, 02:27:44 pm »
Good quality, new capacitors are cheap compared to buying test equipment or spending time poking around with a box of potentially faulty ones. Is it really worth the bother? How many will you actually use in projects?

When I kit for a new project I always buy a few surplus components and keep those unused on the shelf, but I gave up trying to re-use old parts a long time ago. It's just not worth the hassle, IMHO.

Offline notsob

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Re: Using a DMM, testing a capacitor?
« Reply #5 on: December 25, 2012, 02:37:03 pm »
These days, I'd probably go for a real LCR meter, but not that long ago I wanted to measure a few largish electros and had nothing to measure with, so I made Fr Tom McGahees PIC capacitance meter. I cannot attach the original project as this forum will not accept .rar files ( will accept zips tho)
The PIC he used is now obsolete, so I replaced it with a 16F88, I'll attach the general description pdf, if you want more  -->> google it
 

Offline davidk

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Re: Using a DMM, testing a capacitor?
« Reply #6 on: December 25, 2012, 03:00:55 pm »
Thanks for the insights guys. You're right, if I were building a new project I'm probably keen to buy new capacitors, they are relatively cheap :)

I'm just curious about the old things I have lying around, just felt like a shame to chuck them away without some quick basic test. Given the feedback I'm better off just chucking them in the bin :)

By the way, how do you go about testing capacitors in the field (meaning you can't really charge them on site and measure by discharge). Dave did a quick test on a LCD monitor a while back where he identified bad caps just from looking at bulges, but I've noticed from many troubleshoot videos they have been a primary source of faults. If that's the case, is the LCR meter the best way to test these caps in the field?
 

Offline Mr Smiley

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Re: Using a DMM, testing a capacitor?
« Reply #7 on: December 25, 2012, 03:50:27 pm »
These days, I'd probably go for a real LCR meter, but not that long ago I wanted to measure a few largish electros and had nothing to measure with, so I made Fr Tom McGahees PIC capacitance meter. I cannot attach the original project as this forum will not accept .rar files ( will accept zips tho)
The PIC he used is now obsolete, so I replaced it with a 16F88, I'll attach the general description pdf, if you want more  -->> google it

This is also by Tom McGahees

http://www.pupman.com/listarchives/1998/April/msg00625.html

Detailed descriptions and methods of how to measure capacitance

 :-+

 :)
There is enough on this planet to sustain mans needs. There will never be enough on this planet to sustain mans greed.
 

Offline ptricks

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Re: Using a DMM, testing a capacitor?
« Reply #8 on: December 26, 2012, 08:52:09 pm »

By the way, how do you go about testing capacitors in the field (meaning you can't really charge them on site and measure by discharge). Dave did a quick test on a LCD monitor a while back where he identified bad caps just from looking at bulges, but I've noticed from many troubleshoot videos they have been a primary source of faults. If that's the case, is the LCR meter the best way to test these caps in the field?

Capacitors are actually fairly reliable parts so most of the time when they do fail you can tell from the outside. Needing an LCR meter is for when you are doing something like a switched supply where ESR is a concern. For things like a linear supply you can get by with just seeing if the capacitor will hold a charge when power is removed.  A common practice in the field is to just replace suspected caps without testing. Labor cost way more than the parts so it is cheaper to just toss the old one and solder a new one unless it is an expensive cap , high voltage, etc.

If you have an analog scope you can use a curve tracer to test capacitors.
 

Offline davidk

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Re: Using a DMM, testing a capacitor?
« Reply #9 on: December 27, 2012, 12:00:13 pm »
Fantastic, thanks ptricks!
 

Offline PeeweePete

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Re: Using a DMM, testing a capacitor?
« Reply #10 on: December 27, 2012, 01:51:04 pm »
A DMM is nearly worthless to test capacitors. Most large electrolytics are over-range for most DMMs, and they don't test ESR. You can get an ESR meter cheap, though I recommend an LCR meter as it isn't so specialized. Lots of uses.

Is a DMM OK for testing capacitors that are in its range?
 

Offline dr_p

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Re: Using a DMM, testing a capacitor?
« Reply #11 on: December 27, 2012, 02:54:04 pm »
A multimeter will measure capacitance  :-DMM, but there is more to the quality and performance of a capacitor than just it's capacitance value. (like ESR, leakage current, insulation resistance etc)

Measuring with a DMM (with the capacitor discharged and within it's measuring range) will most always give a correct value (in pF, nF or uF) however the cap is not guaranteed to be good.

Think of DMMs like a way of reading what it says on the capacitor's label, NOT a test device for them.
« Last Edit: December 27, 2012, 02:59:43 pm by dr_p »
 

Online kripton2035

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Re: Using a DMM, testing a capacitor?
« Reply #12 on: December 27, 2012, 03:26:39 pm »
it's eazsy to build yourself a simple esr - capacitor meter
or even go for a full lcr meter !
check here for many DIY ESR meters : http://kripton2035.free.fr/esr-repository.html
check here for many DIY LCR meters : http://kripton2035.free.fr/lcr-repository.html

Offline SeanB

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Re: Using a DMM, testing a capacitor?
« Reply #13 on: December 27, 2012, 04:11:28 pm »
Tested a capacitor today, it read 97nF........ not good on a 5uF motor run cap. As I did not have one to hand ( and really needed the aircon to work) I put in a 4uF one, and went out and bought 4 5uF units along with some 50uF ones, as the 45uF next to it was not terribly healthy either ( used a hard start to get it running until I could get new ones this afternoon) and will see what it has self healed down to tomorrow. PRC caps are not reliable, the ones I removed from a 1960's Carrier were still in spec in all respects ( aside from being full of PCB oil of course) and were perfect aside from being 5 times as big as any modern variant.
 


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