Electrolytic capacitors use an electrolyte solution as a conductor to increase surface area and store more charge than standard capacitors. The anode is a thin foil with an oxide dielectric layer, while the cathode is an electrolyte solution and film. They are used in power filters and can self-heal with regular use, but can also fail due to incorrect polarity or drying out. The aluminum and tantalum versions differ in their electrolyte solution and cost.
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 metal conductors separated by an insulator called a dielectric. Using an electrolyte solution as a conductor effectively increases its surface area, allowing it to store more charge than standard capacitors of the same size. Electrolytic capacitors are often used in power filters, storing the electricity needed to smooth out fluctuations in output.
The metallic conductor of an electrolytic capacitor consists of a thin foil. The dielectric is an oxide of this metal layered on the foil by an electrochemical process called anodization. Only a few metals support this process and aluminum and tantalum are the most used. The foil with a dielectric layer forms the anode, or input path, for an electric current. The electrolyte solution and a piece of uninsulated film make up the cathode, or outlet path, for an electric current.
An aluminum electrolytic capacitor is made from 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 blotting paper is placed between the anode and cathode foils, then rolled into a coil. The pin connectors are connected and the structure is inserted in an aluminum casing. The device is then immersed in an electrolytic bath, usually a boric acid or sodium borate solution, thoroughly wetting the foil and paper layers.
Tantalum electrolytic capacitors were originally manufactured similar to the aluminum version, with the notable difference that the electrolyte was a sulfuric acid solution. A solid rather than a wet tantalum capacitor is now produced. Since tantalum capacitors are much more expensive than aluminum, they are usually only available 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 used in normal use of the capacitor. Damage to the dielectric of an electrolytic capacitor can self-heal with regular use within the stated parameters. It can also be slowly destroyed if you don’t pay attention to the correct polarity or direction of current flow.
The design of the electrolytic capacitor allows for properties that are particularly suited to certain purposes. Using an electrolyte solution gives them the ability to hold a larger charge in a smaller volume. This has also 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 the capacitor plague. Even in a properly constructed electrolytic capacitor, the use of a liquid solution makes them vulnerable to failure due to drying out.
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