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.
