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Wednesday, December 24, 2014

ABOUT ELECTROLYTIC CAPACITORS

They provide more capacitance for their size than any other type. Electrolytics all have one thing in common, instead of usual plates separated by a dielectric, the electrolytic capacitor has a metallic anode coated with an oxide film. This outer covering is the dielectric, and a liquid electrolyte acts as a cathode. A second metallic conductor serves primarily as the connection to the liquid cathode, providing an external termination.
In actual practice, porous paper is wrapped around the anode and saturated with the electrolyte to eliminate the spillage problem.

There are two common types of electrolytic capacitors: a)  Aluminum  b) Tantalum

Both employ the same basic principle. The aluminum or Tantalum anode is covered with an oxide film. A suitable liquid or solid electrolyte is the cathode. The Aluminum type is by far the most popular because of its lower cost. The aluminum –oxide film has a very high resistance to current in one direction and has very low resistance to current in opposite direction. In other words, film acts as a dielectric in first instance and as a plate in second case. Because of this electrolytic are polarized. If the designated polarity is not observed, the oxide film on anode will breakdown and migrate to cathode connection, resulting in prompt failure of capacitor.

Electrolytics are described as belonging to one of the three basic families;

Polarized type
This type has one anode, the liquid cathode and a cathode connection. Polarity must be observed.

Semi polarized type
In many energy storage applications a certain amount of current reversal is often encountered. In such cases this type is recommended. In this type, the primary anode has a relatively thick oxide surface. The cathode connection now becomes the secondary anode with a thin oxide surface. Also liquid cathode has a slightly different chemical composition.

Non-polarized

This type is used in audio cross over networks and ac motor starting applications. Here there is a complete reversal of polarity; therefore, two anodes are required. The cathode connection now becomes a second, and equal, anode. Obviously size will be affected. Infact, the third type called nonpolarized is just twice as large as a polarized type of equivalent capacitance and voltage rating.