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Capacitors, Part 6 Electrolytic Capacitors [1]

■Electrolytic capacitor types

An electrolytic capacitor is a capacitor that uses an oxide film made of aluminum, tantalum or other oxidizable metal as a dielectric. Because of its potential for large capacitance, this type of capacitor is used extensively in power supply circuits and similar applications. There are two major categories, using either a water-based (wet) electrolyte or solid electrolyte. These categories can be further subdivided as shown below. Electrolytic capacitors normally have polarity, but there are also nonpolar types.

Electrolytic capacitor types

■Basic construction principle of an electrolytic capacitor

The capacitance of a capacitor is calculated using the formula shown below. As can be seen from the formula, the larger the electrode surface area, and the smaller the electrode gap distance, the higher the capacitance of the capacitor.

Basic construction principle of an electrolytic capacitor

For multilayer ceramic capacitors and film capacitors, a sheet type dielectric is used. By contrast, the dielectric in an aluminum electrolytic capacitor is an oxide layer formed on a metallic surface by means of electrochemical surface treatment. The electrode foil surface is roughened through an etching process to increase the effective surface area and thereby the capacitance.

Conventional capacitor / Electrolytic capacitor

■Structure of an aluminum electrolytic capacitor

An aluminum electrolytic capacitor is a capacitor that uses an aluminum oxide film as dielectric. A method for forming an oxide layer by electrochemical surface treatment was developed near the end of the 19th century, and the precursor of today's aluminum electrolytic capacitor appeared as a product at the beginning of the 20th century. Featuring high capacitance, such capacitors are now widely used for smoothing and decoupling applications. The wet type using an electrolyte is the most common, but there are also products using organic semiconductor material and similar. In terms of construction principles, there are capacitors with radial and axial leads, SMD (surface mount device) types, screw terminal types and others.

Principle of aluminum electrolytic capacitor
Structure of aluminum electrolytic capacitor (radial lead type)
Cross-section of etched aluminum foil (anode) [photographed with 400 times magnification]

■Manufacturing process of aluminum electrolytic capacitor

Manufacturing process of aluminum electrolytic capacitor
Electronics ABC