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What Is an Electrolytic Capacitor?

By Jerry Morrison
Updated May 17, 2024
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An electrolytic capacitor is a type of electrical storage device that uses an electrolyte solution as one or both of its conductors. Charged chemical particles called ions conduct electricity in an electrolyte solution. Standard capacitors are composed of two metallic conductors separated by an insulator called the dielectric. Using an electrolyte solution as a conductor effectively increases its surface area, allowing for a greater charge to be stored than in standard capacitors of the same size. Electrolytic capacitors are often used in power supply filters, storing the electricity needed to mitigate fluctuations in output.

The metal conductor of an electrolytic capacitor is made from a thin foil. The dielectric is an oxide of this metal layered onto the foil by an electrochemical process called anodization. Only certain metals support this process, and aluminum and tantalum are most frequently used. The foil with a dielectric layer forms the anode, or path of entry, for an electrical current. The electrolyte solution and an uninsulated length of foil make up the cathode, or exit path, for an electrical current.

An aluminum electrolytic capacitor is made from a high-purity aluminum foil. The effective surface area of the foil is increased by etching before the aluminum oxide dielectric layer is generated. A layer of absorbent paper is placed between the anode and cathode foils, then the entirety is rolled into a coil. Pin connectors are attached and the construct is placed in an aluminum casing. The device is then submersed in an electrolyte bath, usually a solution of boric acid or sodium borate, thoroughly wetting the foil and paper layers.

Tantalum electrolytic capacitors were originally produced similar to the aluminum version, with the notable difference that the electrolyte was a sulfuric acid solution. A solid rather than wet tantalum capacitor is now produced. Since tantalum capacitors are much more expensive than aluminum, they are generally available only in small, low-voltage versions for electronic devices such as cell phones.

The thin, dielectric oxide layer is generated by an electrolytic process very similar to that employed in the capacitor's normal use. Damage to an electrolytic capacitor's dielectric can be self-repairing with regular use within its stated parameters. It may also be slowly destroyed if attention is not paid to the proper polarity, or direction of current flow.

The design of the electrolytic capacitor enables properties particularly suited to certain purposes. The use of an electrolytic solution gives them the ability to hold a greater charge in a smaller volume. This also has led to certain liabilities. In the early 2000s, an incorrect formulation of the electrolyte solution used by some manufacturers resulted in a series of failures known as capacitor plague. Even in a properly constructed electrolytic capacitor, the use of a liquid solution makes them vulnerable to failure from drying out.

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