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Posted on 27 April 2019

Aluminum Electrolytic Capacitors - Overview and Key Applications

Aluminum electrolytic capacitors assume a special position among the various types of capacitors since their principle of operation relies, in part, on electrochemical processes. The advantages of aluminum electrolytic capacitors that have led to their wide application range are their high volumetric efficiency (i.e. capacitance per unit volume), which enables the production of capacitors with up to one Farad capacitance, and the fact that an aluminum electrolytic capacitor provides a high ripple current capability together with a high reliability and an excellent price/performance ratio.

An aluminum electrolytic capacitor will only operate correctly if the positive pole is connected to the formed Al foil (or anode), and the negative pole to the cathode foil. If the opposite polarity were to be applied, this would cause an electrolytic process resulting in the formation of a dielectric layer on the cathode foil. In this case, strong internal heat generation and gas emission may occur and destroy the capacitor. Secondly, the cathode capacitance, which progressively decreases as the oxide layer thickness increases, and which is connected in series with the anode capacitance, would reduce the overall capacitance considerably.

An electrolytic capacitor of such a basic design is therefore only suitable for DC operation. The DC voltage may also be a ripple voltage, i.e. a DC voltage with a superimposed alternating voltage. Capacitors with this configuration are polar versions that can be used for most applications. As already pointed out, polar capacitors do not tolerate a voltage reversal. Incorrect polarities of up to 1.5 V are, however, permissible for short periods of time as the formation of a damaging oxide layer on the cathode only starts at voltages of this magnitude. (This is because the cathode foil is covered by an air-oxide layer that corresponds to an anodized dielectric layer with a breakdown voltage of approximately 1.5 V).

Aluminum electrolytic capacitors are generally divided into two basic reliability categories: capacitors for high-reliability applications and capacitors for general-purpose applications. This differentiation has also been adopted in the relevant IEC standards.

In IEC publications aluminum electrolytic capacitors for high-reliability applications are identified as "Long-Life Grade" capacitors. The abbreviation LL is stamped on the capacitors. Generally, the materials used for aluminum electrolytic capacitors must meet strict purity requirements, and those used for producing LL grade capacitors must be specially selected. The design effort required for such capacitors affects both the case size and the price.

Aluminum electrolytic capacitors are manufactured with various terminal designs, including screw terminals, snap-in, single-ended and axial lead capacitors.

Aluminum electrolytic capacitors for general applications are called "General-Purpose Grade" in IEC publications.

 

For further information on aluminum electrolytic capacitors please read the following articles:

Design of Aluminum Electrolytic Capacitors and General Features

Heat Dissipation and Cooling for Aluminum Capacitors

Mounting Positions of Aluminum Electrolytic Capacitors with Screw Terminals

 

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