capacitor decisions

[kardenm]

How do I decide when a capacitor needs to be replaced?  For example, does a 2200uF/35v cap which tests at 1700uF need to be replaced?  How about a 220uf/400v that reads 198uF or a 680uF/35V that reads 608uF?

Is there some kind of general rule for deciding when to replace (assuming I don't want to just replace them all wether they are good or bad)?
 
Expert advice or opinions are greatly appreciated.

[lilshawn]

what you really need to test is the internal resistance of the the cap. you can buy what is called an ESR meter. it measures the resistance of the cap in dc and ac situations and gives a reading of how it is performing. if the resistance is higher , it will run hot, and begin to fail.the difference of 0.15 ohm from where it should be will determine if it is a pass or fail.

but for the time being...


about 10% is what i go for general power filtering.

for caps used as in reference voltage circuit i'm a little more critical of them as they need to be close to spec.

for the 50 cents they cost (buck each if your buying onesies) just do them all.

but...

your 2200 will need replacing (anything less than 2000 i would trash)
220uf seems about right, it's probably been replaced already so i wouldn't worry too much there
680uf.... egh... i dunno so.....replace it. it's marginal.

keep in mind, that the test provided by your meter is approximate.

MY General rule regarding electronics is, if it's questionable, Replace it. it will be one less thing to be wrong if it still doesn't work.

[MonMotha]

If you've already gone to the trouble of removing the component to test it, and you've got a replacement available, replace it.
If the measured value is outside +/- 10%, replace it.  Might as well.  If it's an especially expensive cap or something, give it +/- 20% unless you have reason to believe it at fault.

Definitely test ESR.  Excessive ESR is a failure even if the capacitance is spot on.  Of course, you'll have to figure out what it was supposed to be in the first place, and that can be a little difficult sometimes.  For bulk electrolytics, I'd say anything >10ohms is probably "dead" unless you have reason to believe it started higher than that.

[kardenm]

Good information guys.  Thank you very much!

How about voltage ratings? I found two caps on this board which had been replaced with cap of higher voltage spec (a 22uf/160v replaced with 22uf/250v and a 100/25v with a 100/35v).

[lilshawn]

higher is fine.

generally you pick a cap that is about 2x the voltage of what's there (or close to it)

so if your filtering 12 volts they will use a 25 or 35 volt cap. 5 volts you would have a 10 or 25 volt cap there. 100volts would likely be a 200v, but to be safe 250v right?

it's just the maximum voltage that can be applied without shorting through the insulator paper separating the foil layers in the cap.

higher voltage = thicker paper.

sometimes if you cheap out, it ends up running hotter and failing sooner. I'm not saying replace them all with 500 volt caps, but i mean if it's supposed to be 25 volt and all you have is a 50 or a 100 volt, and it fits... by all means use it.

[kardenm]

Thank you.  Thank you.