Author Topic: Testing bad capacitor in live circuit with oscilloscope?  (Read 7951 times)

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Offline dds

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Testing bad capacitor in live circuit with oscilloscope?
« on: October 17, 2010, 12:16:25 AM »
How can I do that? I have analogue PM3082 Fluke oscilloscope. :)
Thanks!

Online Simon

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #1 on: October 17, 2010, 12:17:35 AM »
it will depend on the capacitor function, which only you know and can interpret the information to make a decision on. remove the capacitor and test it on it's own to make sure

Offline saturation

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #2 on: October 17, 2010, 12:26:33 AM »
I agree with Simon.  In many old schematics, a test point will indicate what the waveforms and voltages should be, using either a scope or DMM as recommended.

If you are winging it without such a schematic, its not easy since the test points include what should be seen give the impedance you put across the test point and ground, using the test device.

Its not safe live testing capacitors in power supplies or high voltage areas, i.e., near 50Vdc and up.  In amplifier designs of different types, the schematic usually has test points.

Best Wishes,


Saturation

Offline dds

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #3 on: October 17, 2010, 01:45:40 AM »
Is it possible to cest caps with help of function generator too? If yes, how?

Thanks!

Online Simon

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #4 on: October 17, 2010, 01:51:13 AM »
well if your looking to evaluate their capacitance you can make a filter with the capacitor and from the filters response determine the value and compare it to the value marked on it. an easier way of measuring the capacitance without a capacitance meter might be to make a timing circuit with it, if a large capacitor you could time how long it takes to reach 2/3 the supply voltage (perhaps with the help of a comparator to signal the event), if small make it part of a 555 astable timer circuit and check the frequency

Offline just

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #5 on: October 17, 2010, 02:24:11 AM »
Is it possible to cest caps with help of function generator too? If yes, how?

Thanks!


You can test the capacitor in circuit with an oscilloscope and a function generator.
ESR Testing here: http://octopus.freeyellow.com/esr.html
But I'm using an ESR meter with very good results.
« Last Edit: October 17, 2010, 03:12:42 AM by Simon »

Offline Mechatrommer

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  • reassessing directives...
« Last Edit: October 17, 2010, 03:40:53 AM by shafri »

Online Simon

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #7 on: October 17, 2010, 03:13:19 AM »
I have corrected the link so both will work now

Offline Rhythmtech

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #8 on: October 17, 2010, 01:09:04 PM »
If you have a known AC source within the limits of the capacitor you apply the voltage to the capacitor, take your measurements and use:

            -   1
X =  ----------------   
            2(pi)fC 

Measure current and you have v/i = X since there should be no other reactance or resistance in the circuit that is notable. f would be known since the source is known. Measure V if you want to, but it is most likely known to you already. Solve for C.
« Last Edit: November 03, 2010, 07:01:10 AM by Rhythmtech »

Offline qno

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #9 on: October 27, 2010, 08:23:09 PM »
You probbably will not find a "bad" capacitor with this measurement.

Look for Capacitor ESR meter for a lively discussion.
Why spend money I don't have on things I don't need to impress people I don't like?

Offline Rhythmtech

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #10 on: October 30, 2010, 04:54:18 AM »
You probbably will not find a "bad" capacitor with this measurement.

Look for Capacitor ESR meter for a lively discussion.

Not sure why not? I've done it with line voltage on motor capacitors...

alm

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #11 on: October 30, 2010, 05:54:36 AM »
Not sure why not? I've done it with line voltage on motor capacitors...
Because it only measures capacitance (like the capacitance test on most DMMs), and it's not usually the capacitance changes, unless it's shorted/blown. It's the ESR that rises, which increases the impedance at the frequencies of interest and decreases the smoothing/bypass capacity (RC filter with extra R in series with the C). Most capacitors in SMPS are specifically low-ESR, so an increased ESR is a problem.

Motor capacitors may be different, no experience with those.

Offline qno

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #12 on: October 31, 2010, 05:07:15 AM »
OK Motor capacitors can be tested by measuring the capacitance.
At 50 or 60 Hz the ESR of the cap is not important.

The capacitance can get lower in starter caps. This is easy to find when the motor does not startup anymore.
Having a spare capacitor available and replacing it will give an easy anser if the cap is good or not.
A spare cap is often cheaper than a meter.
Why spend money I don't have on things I don't need to impress people I don't like?

Offline Rhythmtech

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Re: Testing bad capacitor in live circuit with oscilloscope?
« Reply #13 on: November 03, 2010, 07:10:30 AM »
Not sure why not? I've done it with line voltage on motor capacitors...
Because it only measures capacitance (like the capacitance test on most DMMs), and it's not usually the capacitance changes, unless it's shorted/blown.

I agree it is somewhat more complicated than just measuring the ESR and caps are cheap so replacing them based on ESR is a good enough practice, but may not indicate why the cap failed or if a particular brand cap has issues in your application. 

Capacitance is not measured in this test, it is calculated. So if the series resistance is wrong you will have the wrong capacitance since the equation depends on V and I to be measured. I noticed I screwed up in my original post, X=VI is wrong should be X = V/I.

For argument's sake - if you could sweep the AC power supply's frequency you could actually get a much better idea of the cap's performance across several frequencies and profile the performance of good caps and bad? caps.

« Last Edit: November 03, 2010, 07:13:26 AM by Rhythmtech »


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