Determine the polarity of aluminium electrolytic capacitor

This question was inspired by this one: Determine Polaritry of Aluminum Electrolytic Capacitor.

I thought that the question would be on how to determine the polarity if there are no markings and the leads have already been cut. The question was different, and the curiosity remained unsatisfied.

I found this question which is also not that into electronics, but the accepted answer suggests that measuring the capacitor's case voltage may indicate the reverse polarity. It seems interesting.

Two questions:

1. How the polarity of an electrolytic aluminium capacitor can be determined experimentally?


2. Why would the voltage of the case be different when the capacitor is reverse biased?

Answer

In an aluminum electrolytic capacitor, the positive terminal is connected to the foil that has the oxide layer on it, and the negative terminal is connected to the one without the oxide layer. This puts the negative terminal in direct contact with the electrolyte, and the case (assuming it has no insulating liner) is also in contact with the electrolyte.

Therefore, if you measure the resistance between the case and either lead, the negative lead will have relatively lower resistance to the case in both directions, while the positive lead will show essentially infinite resistance in at least one direction.

If the case is insulated, you can try applying a small bias voltage (3-5V) to the capacitor in each direction (through a current-limiting resistor of 100K or so) and see which direction allows the least current; this will be the correct polarity of the capacitor. This works because an electrolytic capacitor has a weak diode action as well. For further details, see the Wikipedia article.