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Help Creating a Capacitor Check "checklist"

Iceberg

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In the lab I have a small bin of used capacitors which I had been building up for moments when I am in a pinch and its not critical. I am not saving caps with holes or anything :) Sometimes its large caps preservation to match the originals, stuff like that.

I was hoping to use these caps with some level of confidence beyond what the little test tool we all have does, so can the community help build a list of things to check?

If you can respond by listing out what steps you'd take before using a used capacitor in a pinch?
 

DanielT

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Maybe this would be something? Please note I am not an EE, or an engineer. Wait until more knowledgeable people give tips and advice.:)

NV_0116_Coyle_Large-1.jpg


 
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Iceberg

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The problem with measuring just ESR is that ESR rating for caps varies widely. You have to dig up datasheets on every single one and often ESR numbers are never published anywhere. I saw a cap the other day measure an ESR of 21 ohms, I thought wow that cap is zonked, but the new cap's ESR was even higher.

ESR doesn't seem like it can be a 1 stop test, though should be a step.
 

SIY

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Different kinds of caps are measured differently since their failure mechanisms are different. But the basic checks are capacitance, ESR, dissipation factor, and (importantly) leakage.

edit: if you're measuring ESRs of 21 ohms and it is consistent between caps, check your tester. Exception is RC snubber caps with a resistor built in.
 

solderdude

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And for higher voltage caps perhaps a DC voltage test.
Very old electrolytics that have not seen a voltage for decades may like the need some forming done before testing.
 
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Iceberg

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Different kinds of caps are measured differently since their failure mechanisms are different. But the basic checks are capacitance, ESR, dissipation factor, and (importantly) leakage.

edit: if you're measuring ESRs of 21 ohms and it is consistent between caps, check your tester. Exception is RC snubber caps with a resistor built in.

I thought all of that too, but the datasheet showed that ESR number. The tester can be quirky. I've seen it turn up "unknown" and then test again and it works. Sometimes the leads are not set perfectly.

And for higher voltage caps perhaps a DC voltage test.
Very old electrolytics that have not seen a voltage for decades may like the need some forming done before testing.

This means having them on a power supply on a low voltage to get the materials inside moving?
 
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solderdude

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This means having them on a power supply on a low voltage to get the materials inside moving?

Connect the capacitor to a DC voltage (about 0.7x the rating) via a resistor (to limit the current) say 10k or so and monitor the voltage.
That should go up nicely and not 'jittery' and when the voltage is stabilised and very near the applied voltage the cap is ready to be tested.
Pointless to do this with new caps even if they have been in stock for some years.
This is to get the chemical reactions inside to function as they should.
Sometimes (re-)forming takes several minutes.
 
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Iceberg

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Connect the capacitor to a DC voltage (about 0.7x the rating) via a resistor (to limit the current) say 10k or so and monitor the voltage.
That should go up nicely and not 'jittery' and when the voltage is stabilised and very near the applied voltage the cap is ready to be tested.
Pointless to do this with new caps even if they have been in stock for some years.
This is to get the chemical reactions inside to function as they should.
Sometimes (re-)forming takes several minutes.

Thank you. Whats a good method for measuring dissipation factor?

Are you sure that wasn't milliohms?

I had seen 3 or 4 ohms before, but this was the largest. I just took a moment to try and find that cap but I couldn't. As best I recall it was like 1 or 10uF and a high voltage, like 250 or 400v or something. It wasn't a cap I was used to seeing, so I didn't think much of the measurement when the datasheet confirmed it.
 

SIY

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Thank you. Whats a good method for measuring dissipation factor?



I had seen 3 or 4 ohms before, but this was the largest. I just took a moment to try and find that cap but I couldn't. As best I recall it was like 1 or 10uF and a high voltage, like 250 or 400v or something. It wasn't a cap I was used to seeing, so I didn't think much of the measurement when the datasheet confirmed it.
Do you have a link to the datasheet? I have seriously never heard of such a thing.

The best way to measure any capacitor parameters (ESR, DF, capacitance) is with a component bridge. I have a Leader LCR-740, but there are a lot of good and affordable ones kicking around the used equipment market. Brands like GenRad are a good bet.
 

restorer-john

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The EA (SC) Bob Parker ESR meter I and II are excellent units. EA was Electronics Australia Magazine and he designed a few meters, with the mk2 being an update (for silicon chip magazine, run by the former editor of EA back in the day), although the old mk1 is still an excellent unit.

Now I believe there are a few people who peddle his units under different names.


The Altronics kit is excellent if you want to build one. I have been using the mk2 and mk1 units for decades and they are truly lifesaving pieces of test gear. Caps can be tested in circuit on the PCB (discharged) and until you've had a quick, accurate unit like these on hand, you'll wonder how you ever did without it. You can even use them for finding shorts on PCBs or gear. They are accurate enough that you can estimate a short location in a cable based on a few end to end measurements. On a PCB, you can find a shorted component or track- try that with a DMM!
 
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Iceberg

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Do you have a link to the datasheet? I have seriously never heard of such a thing.

The best way to measure any capacitor parameters (ESR, DF, capacitance) is with a component bridge. I have a Leader LCR-740, but there are a lot of good and affordable ones kicking around the used equipment market. Brands like GenRad are a good bet.

I cant find it. Here, from that ESR meter post, it shows an average right on that table of 20. It must have been 1uF and 400v or something.
 

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Iceberg

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Why if the meter chart above points to it as well? Look where its circled.
 

DVDdoug

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If you have a critical application or if you are just going to be "picky" my advice is, Don't use used capacitors. ;)

I would just measure the capacitance (there are multimeters that can measure capacitance) and leakage resistance. In a power supply (or as an additional power supply filter), a meghom or so of resistance isn't going to matter so it's not that critical, but if it reads low the cap might be bad. And, there may be applications where 1M is too low.

I you haven't measured leakage resistance before... The meter will read low resistance when you first connect it and the capacitor begins to charge and the reading will then start rising and level-off at some point, or it may go beyond your meter's range and read infinity/open. (Non-electrolytics should read open.)

In general capacitors aren't critical. You can always use one with a higher voltage rating, and almost always use a higher capacitance value. A manufacturer can sometimes save money by using the same capacitor in multiple places on the board and simplifying the BOM, even though a variety of lower values would work. Often you could get-by with lower capacitance but that's not something I'd try "randomly".

Of course, the main exception is tuned filter circuits, including crossovers. Those should be within tolerance.

Electrolytic Capacitors do age, whereas most other capacitors, resistors, and other solid state electronics don't. I think capacitors have a shorter shelf life than active life, but one datasheet I just looked at just says you should use a recovery procedure if it's on the shelf at 105 degrees C for more than 1000 hours... Not very long but I can't imagine ever hitting that kind of storage temperature. (It references a technical document that in a quick-search, doesn't appear to be available free.) The datasheet doesn't mention any limitations after that so I assume it's "good as new".

So...
 

SIY

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Why if the meter chart above points to it as well? Look where its circled.
I don’t know what the chart means or the units. I do know that in about 60 years of doing electronics, I have never seen any cap with an ESR anything like that. And no datasheet indicating it.
 

tomchr

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Generic el-cheapo electrolytic caps seem to sit around a few ohm at the most for a physically small capacitor. Physically larger capacitors are commonly 10s to low 100s of mΩ. Film caps get into the single digit mΩ. OSCON gets down in the low 10s of mΩ for the ESR as well.

Tom
 
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Iceberg

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I don't doubt the life experience, but that chart goes from .03 to 20 with no units indicated. I don't have a JFK magic bullet theory to explain the chart veering into mili then micro then back to the right with ohms, but that 20 sure looks like 20 good old american John Wayne ohms to me.
 

Digital Mastering System

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