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TB 10th August 2011 02:20 PM

How old electrolytic capacitors can safely be used?
Please advice....
I have many electrolytic capacitors which were never used. I got them brand new from reputable makers like Panasonic, RIFA, Sanyo......
I have never had time to used them until now. All capacitors are around 6-7 years old.
Can they still be safely used?
In current economical situation it would be hard for me to buy them all over again.
I have also few OSCONs. Do they aged the same way as other type of electrolytic caps?
Thanks for the help.

DBMandrake 10th August 2011 02:32 PM

Ideally you'd want to "reform" them, which you can do by connecting them to their rated DC voltage through a current limiting resistor for some period of time. (Hours or more)

If an electrolytic needs reforming it will have unusually high leakage current, in extreme cases the current flowing from high leakage will cause it to heat up and explode. By limiting the current with a resistor you give it a chance for the oxide layer to reform but in a controlled manner that prevents it from overheating.

The value of resistor you would want to use and what leakage current would be considered acceptable for a fully formed electrolytic is going to vary depending on the value and size of the capacitor so it's hard to give generalizations, except that you'd want the current to be limited by the resistor to the tens of milliamps region.

If you monitor the current flow it will start off high and after some period of time eventually stabilise at a low current when fully formed. If they haven't been used for years it could take many hours.

Try a google search for "Electrolytic Reforming".

PS, this applies mostly to large value electrolytics that you might use for filtering DC supply rails, which are large enough to explode and are subject to a high percentage of their voltage rating all the time. For very small values or AC signal coupling capacitors its not really an issue.

Conrad Hoffman 10th August 2011 03:04 PM

I have hundred of NOS caps just as you describe and have never had a bit of trouble with them. I don't reform anything except HV caps. I also have the ability to measure every important parameter and do this whenever I use them. I've yet to see even a degradation of performance, much less any failures. Modern electrolytics are way better than in decades past. IMO there are still some completely junk parts used in cheap equipment that won't last a year, but if you stick with NOS Panasonics, Rubycon, Nichicon, OSCONs, Illinois Capacitor and a couple other high quality brands, you'll have no trouble.

TB 10th August 2011 05:15 PM

Thank you very much for responses. I will use them then.
DBMandrake, thanks for the link. This is interesting website.

! 11th August 2011 08:04 PM

I'll second what Conrad Hoffman wrote, use good brands not Chinese generic capacitors and they should still be viable for use after sitting for a few years.

Oscons do not degrade like electrolytics do. Maybe over many, many years they would degrade but not in the shorter term of 6-7 years.

HOWEVER, whether the electrolytics are reasonable to use depends on what you are making, the expected or desired service life of the finished "thing" you are building. Since we are here, taking audio amps for example, a good audio amp is (arguably, someone will always say newer electronic parts get better over time) timeless, so do you want to have to replace the capacitors in roughly 14 years instead of 20 years? That's a random generalization about # of years, each circuit and environment, and type of capacitor of course, will age differently... I just picked 20 years for a new capacitor as a nice round number and subtracted 6 years per your 6 year old capacitors.

TonyTecson 16th August 2011 10:52 AM

shorting the the cap terminals keeps DA away when storing caps or when they will not be used for a long time....

imix500 16th August 2011 12:10 PM

I've seen it both ways. Just yesterday I tested a few 10000uf/75v from capacitor technologies circa 1986 that tested like new. Then I ran into 12 bc/Phillips 8200uf/110v that were all bad after only 9 years. Granted, the latter were from Crown K2's which are notorious for eating through psu caps.

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