Polarized capacitors are manufactured from a variety of materials. The two most popular types are aluminium electrolytic and tantalum capacitors. As with all capacitors they are constructed of two conducting surfaces separated by a dielectric. The capacity is determined entirely by the surface area and the thickness of the dielectric. An electrolytic capacitor however has only one conducting surface being the metal plate while the other surface is a chemical compound called an "electrolyte". In this case the dielectric is a thin film of the "oxide" of the metal used in the metallic plate. The "oxide" dielectric is in fact an insulator.

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POLARIZATION OF ELECTROLYTIC CAPACITORS


 

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Why are some capacitors polarized?

To understand polarization of capacitors it is necessary to understand the construction of these types of capacitors. Polarized capacitors are manufactured from a variety of materials. The two most popular types are aluminium electrolytic and tantalum capacitors.

As with all capacitors they are constructed of two conducting surfaces separated by a dielectric. The capacity is determined entirely by the surface area and the thickness of the dielectric. An electrolytic capacitor however has only one conducting surface being the metal plate while the other surface is a chemical compound called an "electrolyte". In this case the dielectric is a thin film of the "oxide" of the metal used in the metallic plate. The "oxide" dielectric is in fact an insulator.

There are two types of electrolytic capacitors, wet electrolytic and dry electrolytic.

Metals, such as tantalum, aluminum, magnesium, titanium, niobium, zirconium and zinc, can be coated with an oxide film by electrochemical means. Tantalum oxide is unfortunately very expensive and while it is ideal, its use is limited by pure economics. Aluminium oxide on the other hand is plentiful and relatively cheap.

Using a process of electrolysis, the surface of the narrow but long aluminium sheet is oxidized and becomes the anode or positive pole. An electrolyte is then added and the whole is "rolled" into a cylinder with two electrodes attached. The electrode attached to the aluminium sheet is the positive or anode and the electrode attached to the electrolyte is the negative.

What causes this polarization in capactors?

From the topic electron theory we know that metals emit electrons easily and semi-conductors and electrolytes emit them with great difficulty. The electrons in the electrolyte are in fact not free but are bound in ions. The electrolytic capacitor must always be connected in such a manner that the electrolyte is the negative electrode, for then only will a small current flow through the capacitor.

If the electrolytic capacitor were connected in reverse then large currents would flow - quite spectacularly actually. Looking at the topic of power supplies could you imagine the effect if capacitor C1 in figure 1 were reversed?

Further recommended reading

What I have given is of necessity an overview of polarization of capacitors. A very in-depth discussion on electrolytic capacitors can be found in the FREE on-line book Electrolytic Capacitors - theory, construction, characteristics and applications.

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