HOW CAN CAPACITY RAISE IN OLD CAPS?

[aquila2010]

I read some posts about the capacity raise in old caps with ageing. How can be it explained from a physical viewpoint?

[soundminded]

The value of a capacitor depends on three variables. The area of the conductive plates (which should not change and certainly shouldn't increase.) The distance between the plates occupied by the "dielectric" or insulator and the dielectric constant which is a physical property of the dielectric material. Although dimensional stability could change reducing the distance and increasing the capacitance, it is probably more likely that the dielectric constant changes as the material undergoes slow chemical deterioration. Perhaps both play a factor but not knowing for sure, I'd put my bet on the last one. I guess that this means that newer materials such as mylar should remain relatively constant over much greater periods of time than say paper.

[johnieo]

In electrolytic capacitors the capacitance value can change if the encapsulation develops a small leak. A leak allows water to evaporate slowly over a long time interval. The electrolyte then contains the same amount of acid but less water -- a more concentrated electroyte. The thickness of an anodic oxide (the dielectric layer) is determined by a balance between oxide growth and oxide dissolution. Increased electrolyte concentration shifts this balance. In oxalic acid electrolyte this results in reduced oxide thickness e.g. increased capacitance. This is the cause of the very large capacitance shifts you may have seen posted for Callins non-polar electrolytics -- changes such as a 6-uF capacitor measuring 22 uF, a 30-uF cap measuring 50 uF or a 10-uF cap measuring 20 uF.