stevez said:
I probably wasn't as clear as I could have been in my first post. I am not sure that by measuring the value of the capacitor alone, that it will tell me the whole story about the capacitor. Certainly if it has shorted the test will (or should) reveal that. I was thinking more of the catastophic failures that I and others have experienced. The capacitor appeared to be ok but blew up when stressed at the normal operating voltage of the circuit.
A friend, with more knowledge of this stuff recalls using a curve tracer to see at what point breakdowns begin to occur.
I had wondered if applying the rated DC voltage across the terminals while monitoring what I'll call leakage current wouldn't tell part of the story. I'd use a very high value resistor to limit current to a trickle which might require a brief wait for the capacitor to charge. Not sure I'd know how to recognize a good one or bad one just yet. My thought process applies to the kinds of stuff that I use - generally 50 working volts DC or less - on occasion 100 volts.
Pulling parts out of a junk box, two concerns come to mind:
1) The part was in use and possibly overstressed (it was removed from a circuit as a suspect part and later found to be ok and thrown in the junk box) but it could have been over stressed (this is key because it could be at the end of its useful life)
2) The part was never overstressed and perhaps not even used at all but it may have been in there for a very long time.
One would like to test these with some confidence that they are still ok (or not) for general purpose use.
In the realm of electrolytics, I believe that in the long term, the electrolyte evaporates or somehow chemically degenerates and the properties of that capacitor will change. One cannot rely on only the capacitance value (as it may have had a wide tolerance to begin with)
Although, if one knows the tolerance, and the part is found to be outside, why guess about it? I would just throw it out (could save lots of time later on during testing when you've long forgotten about that "shady" part you used.)
Some of the properties I would expect to change would be:
1) leakage current
2) Dielectric absorption
3) Breakdown voltage
4) ESR
5) Nominal capacitance
A voltage breakdown test could also be part of a D.A. test, because you have applied a known voltage for a known time and this is exactly the test used to characterize D.A.
Leakage current could be used with a DC voltage.
ESR could be done by applying a step input with the capacitor in series with a known inductance. From the overshoot / ringing phenomenon, one can ascertain the ESR and the capacitance as well. A current pulse could be used as well. You would have to provide a scope hookup however to see the time domain reaction.
For quick and dirty, some of these tests are impractical I think. I also think with an unkown capacitor under test, the best you will be able to do is compare the result with a nominal one for that type. You may want to consider having some charts handy that show what a nominal parameter might look like for a given type so you can make a very quick assessment and move on.
thats my .02