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Old 11-23-2011, 07:47 AM   #1
eltrkbrd
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Default How to test bad capacitor with digital multimeter?

Hi Guys,

Just a short introduction as I'm a new member here:

I recently purchased a really nice set of 120W powered bookshelf speakers and each on has a miniature computer and power supply built into them (each on separate PCB).

Well, it appears one of the caps is bad on the power supply. It is bulging and I noticed what could be a small amount of electrolyte leakage. I swapped the broken power supply out with a known working power supply from the other speaker and it works.

I noticed when I pushed down on the bulging cap it made a fizzle sound.

MY QUESTION:

How do I test for a bad capacitor with a simple digital multimeter?

The cap in question is a 1000μF 6.3V Electrolytic, 105 deg C, Low ESR capacitor, 8x12mm.

I looked through the, Stickies/FAQs and other sections of the forum here; I even did a title search for "test capacitor" but no love.

I would think this would be a popular question but the answer eludes me.

I understand there are capacitor testers but all I have right now is a simple GB Instruments GDT-11 Multimeter.

It has continuity testing capabilities from 200 - 2000k Ω

Will this work? If so, how?

Picture of my cheapie multimeter below.

Thanks in advance!

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Old 11-23-2011, 08:00 AM   #2
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Default Re: How to test bad capacitor with digital multimeter?

That multimeter has no way of testing capacitors. The ohm range is for testing resistance.

Slightly more expensive multimeter can test the capacity but the more important property of capacitors - ESR value - can not be usually measured by regular multimeters.

See http://en.wikipedia.org/wiki/Equival...ies_resistance

When capacitors go bad, the ESR value usually goes high while often capacitors look visually OK and the capacity value is still within +/- 20% of their value, as required by the specification.

Less often, capacitors become shorted or open, so you'd be able to determine they're bad just by trying to measure the capacitance - I've had for example 1000uF capacitors looking good on a video card yet they showed only 300uF of capacity when tested.

So in conclusion, the best tool to test capacitors is specialized tool, an ESR meter... Search the forum here, you'll find plenty of threads.

A popular one is the Blue ESR meter that's about 80$ in kit form... http://www.anatekcorp.com/blueesr.htm

ESR Micro is also a known one ... go to http://translate.google.com/translat...nfo%2F&act=url (russian seller and website, so linking you through google translate)
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Old 12-27-2011, 06:28 AM   #3
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
Originally Posted by eltrkbrd View Post
Hi Guys,

MY QUESTION:

How do I test for a bad capacitor with a simple digital multimeter?







THE ANSWER

in simple words , with this digital multimeter you c a n n o t !
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Old 12-27-2011, 09:03 AM   #4
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Default Re: How to test bad capacitor with digital multimeter?

Well you can sort'a but it's not reliable or practical by today's standards and it's not complete testing.

The old old 'shade tree mechanic' way to check caps [before ESR meters at all and before digital meters] was to select a resistance scale and place the leads one way.
[To charge the cap from the meter. Is not enough charge or current to hurt the cap but it's enough to see it charge on a meter.]
-
Reverse the leads - watch the resistance go low and then go high.
Should go over 20kOhms and more is better.
[This is cap discharging then charging in the reverse direction.
-
Reverse the leads again - watch the resistance go low low and then go high again.
Should go over 20kOhms and more is better.
[This is cap discharging then charging in the other direction.]
-
If it does not charge and discharge the cap is obviously bad.
But that's only one way caps fail so if it passes you still dunno if it's actually a good cap.
.
Also that check is better done with an old analog ohm meter [Go Simpson!] because watching the needle rise and fall makes more sense than seeing random numbers flashing while the cap is charging and discharging.
.
You'll also have to mess around with which range works best on a digital and that will be different for different caps. You have to make the situation where the charging rate is slow enough to see it on the DMM. Not an issue on an analog meter because the needle can go quite fast and you still know if it's going up or down. Can't tell that on a DMM unless the rate of change is slow.
.
Once again. This method dates from the days of tubes..
That's not a complete test and it will only tell you that some bad caps are bad.
Other bad caps will pass that test just fine.
.
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Old 12-27-2011, 11:35 PM   #5
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Default Re: How to test bad capacitor with digital multimeter?

I think for the best testing experience you will want a capacitance meter and an ESR meter, and maybe a capacitor leakage tester too.

You can do open\short tests with a multimeter, but as PCBONEZ says, with a digital one it's hard to tell what's actually going on.
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Old 07-03-2012, 06:32 AM   #6
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Default Re: How to test bad capacitor with digital multimeter?

There are many ways you can test a capacitor with a digital multimeter.

One is to put the meter on the ohmmeter setting and take the resistance of the capacitor. One side should read a moderately low resistance and the other side a high resistance.

You can then put the meter on the capacitor meter setting. It should read a capacitance near the value which it is rated for, taking into account tolerance.

Another test is to supply the capacitor with DC voltage. You can then put the meter on DC voltmeter setting and make sure it reads the voltage you charged it up.

For a great resource in learning how to test capacitors with a digital multimeter, check out:

http://www.learningaboutelectronics....st-a-capacitor

It's a resource that goes through all of these steps. Best of luck.
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Old 07-03-2012, 09:32 AM   #7
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
Originally Posted by dlhylton View Post
For a great resource in learning how to test capacitors with a digital multimeter, check out:

http://www.learningaboutelectronics....st-a-capacitor

It's a resource that goes through all of these steps. Best of luck.
He will not have best of luck with advices coming from totally anonymous sources.
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Old 07-03-2012, 11:08 AM   #8
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Default Re: How to test bad capacitor with digital multimeter?

Cap meter that can read the capacitance will not be good enough unless it can at least read the ESR.
For example:
this cap is rated at 100uf, the meter reads 96uF (within +/-20% for electrolytic caps), but the ESR is more than 3 Ohms. These bad caps were from GATEWAY monitor with no backlights problem.
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Old 07-03-2012, 04:55 PM   #9
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
Originally Posted by budm View Post
Cap meter that can read the capacitance will not be good enough unless it can at least read the ESR.
For example:
this cap is rated at 100uf, the meter reads 96uF (within +/-20% for electrolytic caps), but the ESR is more than 3 Ohms. These bad caps were from GATEWAY monitor with no backlights problem.
Exactly. While basic tests from a multimeter are OK for some things, you MUST have an ESR meter to know the real condition. Most of the time it's the ESR that's gone bad.

They say that sometimes capacitance can fall drastically while ESR stays good, so having something that can measure capacitance is not a bad idea either, however this is apparently not common.

About the only thing a multimeter will tell you is if the capacitor is shorted. An ESR meter can't tell the difference between a dead short and a capacitor with really low ESR.
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Last edited by Agent24; 07-03-2012 at 04:58 PM..
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Old 07-03-2012, 06:17 PM   #10
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Default Re: How to test bad capacitor with digital multimeter?

Did any one made any chart like .. Healthy ESR vs Capacity?
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Old 07-03-2012, 06:28 PM   #11
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Default Re: How to test bad capacitor with digital multimeter?

You can only make the chart for that particular model of the cap with its own spec and find out what happen to that model when it is aged because you can have two brands with the same capacitance but they can have difference ESR spec.
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Old 07-03-2012, 07:39 PM   #12
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
Originally Posted by Kiriakos GR View Post
Did any one made any chart like .. Healthy ESR vs Capacity?
Yes, it's called the datasheet for the particular series and brand of capacitor in question.
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Old 07-05-2012, 04:41 AM   #13
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
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Yes, it's called the datasheet for the particular series and brand of capacitor in question.

Well you are correct but for general use I was hopping like many do,
for a simple rule of the thump.
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Old 07-05-2012, 05:29 AM   #14
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
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Well you are correct but for general use I was hopping like many do,
for a simple rule of the thump.
I assume you mean "Hoping for a simple rule of thumb" and not "hopping for a simple rule of thump" (whatever that is!)

There would be a general range of values I guess, for each basic type (Low ESR, Ultra Low ESR, Polymer, General Purpose etc) but I think the range is too wide when you look at all the brands, to make it of much use.

You might call two different capacitors with 0.02 Ohm ESR and 0.06 Ohm ESR respectively both "Low ESR" but note that one is three times higher impedance than the other. In critical circuits this could well be a big enough difference to matter.

That said I bet someone else here has a better answer, but I would say, if you want to do it "properly" then you should try to find a datasheet.

Or some brand new specimens of the capacitors in question and an ESR meter.
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Old 07-05-2012, 10:19 AM   #15
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
Originally Posted by dlhylton View Post
...
For a great resource in learning how to test capacitors with a digital multimeter, check out:

http://www.learningaboutelectronics....st-a-capacitor...
I was just looking at the part where it shows using a function generator to test a cap. Is this the principle of how the ESR meters work?
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Old 07-05-2012, 10:45 AM   #16
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Default Re: How to test bad capacitor with digital multimeter?

tmcw:
that test is basically look at the Xc (capacitive reactance of the cap, unit in Ohm, Xc is not ESR) of the cap which will go down (less Xc) as the frequency goes up.
From the URL: "This is a solid test to know if the reactance of a capacitor is in tune. "
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Old 07-06-2012, 04:50 AM   #17
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
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I was just looking at the part where it shows using a function generator to test a cap. Is this the principle of how the ESR meters work?
Yes this page is just hilarious, if anything it was real on it, the measurements would be shown with real pictures.
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Old 07-06-2012, 04:53 AM   #18
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Default Re: How to test bad capacitor with digital multimeter?

Quote:
Originally Posted by Agent24 View Post
I assume you mean "Hoping for a simple rule of thumb" and not "hopping for a simple rule of thump" (whatever that is!)
Some times my blind faith at the plug-in that I use as spell checker in Firefox, it does play games to me. LOL
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