In my quiet times I like to salvage old electronic circuits. As I sort through the parts I often take the time to check the capacitance of the electrolytics. Of course there are many that show signs of leakage and test low against their rated capacity. These get tossed. There are however a surprising number that look fine but test well above their rated values.
Here we have a typical old electrolytic with a rating of 330 uF at 16 Volts. However when I measure it with a couple different meters I get a capacitance of 460 uF. This is 40% above the rated value. What I am hoping someone can explain to me is the physical mechanism by which this capacitance has increased with age.
As a continuation of this experiment I have put 16 volts on this capacitor and charged it and discharged it several times. While I could not document this it appeared to take more current on the first charge. I understand that the dielectric layer between the metal and the electrolyte can be damaged and then repaired by the correct polarity being applied. Perhaps this explains the apparent extra leakage on the first charge. Now that I have processed the capacitor through several charge and discharge cycles and left it at full voltage for several minutes I have retested the capacitance. The capacitance has come down to 455 uF which doesn't indicate any significant change that might have been related to the healing of the dielectric layer.
It is probably a simple physical change that has occurred with the passing of time but it has gotten me curious. The other explanation, of course , is that the capacitors just had wildly bad tolerances years ago and I am just imagining this conspiracy to increase in value with time.
John
