Aluminum electrolytic capacitor is a kind of capacitor with polarity. Its positive plate is made of aluminum foil, which is immersed in electrolyte for anodizing. A layer of aluminum oxide film is formed on the surface of aluminum foil, and its thickness is generally 0.02-0.03 μ m。 This layer of oxide film is the insulating medium between the positive and negative plates. The negative electrode of capacitor is composed of electrolyte, which is generally composed of boric acid, ammonia, glycol, etc. In order to facilitate the manufacture of capacitors, the electrolyte solution is usually impregnated on a special paper, and then a strip of original aluminum is bonded with the paper impregnated with the electrolyte solution, so that the negative electrode can be easily drawn out from the original aluminum foil. The positive and negative electrodes are wound according to the central axis to form the core of the aluminum electrolytic capacitor, and then the core is put into the aluminum shell to package to form the aluminum electrolytic capacitor. In order to keep the electrolyte solution from leaking and drying, rubber plug is used to seal the opening of aluminum shell.

The two plates can be divided into positive (positive) and negative (negative) electrodes. As the anode, a specific valve metal is used, and an extremely thin oxide film with unidirectional conductivity is generated on the surface of the metal by electrochemical method as the medium. As the cathode, a liquid or solid electrolyte that can generate and repair the oxide film of the medium is usually used, The capacitor with such a special structure and special process is manufactured.

In order to obtain larger capacitance and smaller volume, a chemical etching method is used to form a rough surface on one side of the positive aluminum foil, which increases the surface area of the electrode and the capacitance. The reason why aluminum electrolytic capacitor has polarity is that the alumina film on the positive plate has single conductivity. Only when the positive electrode of the capacitor is connected to the positive electrode of the power supply and the negative electrode is connected to the negative electrode of the power supply, the alumina film can act as an insulating medium. If the polarity of the aluminum electrolytic capacitor is reversed, the aluminum oxide film will become a conductor. The electrolytic capacitor will not only fail to work, but also cause overheating and damage to the capacitor due to large current passing through. In order to prevent the accidental explosion accident of aluminum electrolytic capacitor in use, the mechanical weak link with directional groove is usually pressed on the end face of aluminum shell. Once the internal pressure of electrolytic capacitor is too high, the groove of weak link will crack and carry out world pressure explosion-proof.