# How does electrolytic capacitors work?

Basically I am interested about the polarity of an electrolytic capacitor.How is the polarity of an electrolytic capacitor is defined and what if I reverse the polarity in a simple circuit containing battery, resistance and the capacitor?

• Explain (in your question) what you learned in your research so far and what you didn't understand. Add links if required. Please don't ask someone to write an essay for you if you haven't done your research. – Transistor Jun 30 '17 at 19:34
• Have you even read: en.wikipedia.org/wiki/Electrolytic_capacitor ? I think that would answer at least part of your question. – Bimpelrekkie Jun 30 '17 at 19:35
• The question in your title and what you ask in the body don't match. – Solar Mike Jun 30 '17 at 20:09

Aluminum electrolytic capacitors (the most common kind) use aluminum oxide as the dielectric. The aluminum oxide is a thin layer formed by anodizing aluminum foil. The thickness of the oxide is only nanometers on low voltage caps.

Aluminum oxide is an excellent dielectric. The capacitor is made by rolling two pieces of aluminum metal together and stuffing them into a can. A conductive fluid (the electrolyte) is introduced to effectively place both sides of the anodized foil into intimate contact even though they are separated by a paper spacer. Diagram from Wikipedia.

That's the simple story. In fact the foil is etched (roughened) to increase the effective surface area, which increases the capacitance considerably. Also, one side of the foil is typically anodized a thicker degree than the other. The thicker side is anodized to have a breakdown voltage suitable for the working voltage of the capacitor.

This both increases the capacitance (because it's effectively a high capacitance capacitor in series with a lower capacitance capacitor) and somewhat limits the use since it limits the reverse voltage capability of the capacitor to around a volt for lower voltage capacitors, and typically less than 10% of the working voltage.

If both sides are anodized to the same thickness, the capacitor can be used in either polarity, however it will be almost twice the volume for the same total capacitance and voltage rating. This is called a bipolar electrolytic capacitor.

If you apply excessive reverse voltage to an electrolytic capacitor the thinner oxide layer can break down and it will typically exhibit a lot of reverse current.

Typical electrolytic capacitors are based on aluminum foil. Aluminum forms a insulating oxide if you do the right things to it. That is the dielectric.

Now the problem is how to arrange electrodes to have this dielectric between them. One side is easy. That's just the aluminum foil. A conductive liquid or gel electrolyte is used to contact the other side.

The way the chemistry works, you can only apply voltage in one direction. If you apply the voltage the wrong way, the dielectric is eaten away and you don't have a capacitor anymore.