The capacitor is a device
that is used to store energy in electrostatic fields. In a practical capacitor, the
device consists of two parallel metallic plates separated by an insulating
material called the dielectric. The metal plates can be made of any conducting
material like copper. The dielectric material may be ceramic, mica, etc.
Classification of capacitors
The
capacitors can be classified into two major types :
Fixed
capacitors
Variable
capacitors
Fixed
capacitors
The fixed capacitors
can be classified under physical construction, dielectric usage, voltage
rating, and polarization. Fixed capacitors are those whose capacitance values
cannot be varied mechanically. The fixed capacitors are classified depending on
their dielectric materials.
Paper
capacitors
Paper capacitors make
use of tissue or cellulose paper called Kraft paper. They are usually high-voltage
capacitors and are mechanically very strong. The capacitance value of paper
capacitors is usually between 0.002 µF and 0.05µF. Paper capacitors are very
cheap but they have poor high-frequency characteristics.
Ceramic
capacitors
Ceramic capacitors make
use of ceramic as the dielectric material. Basic ceramic materials are formed
from titanium dioxide. Some of the dielectric materials used are barium
titanate and zirconium titanate. The range of ceramic capacitance between pF
and nF. The ceramic capacitors are light in weight and they are suitable for
use in hybrid integrated circuits. However, very high-voltage ceramic capacitors are
not available. They are used in hybrid ICs and coupling capacitors in low-voltage
circuits.
Mica
capacitors
Mica is a complex
silicate of aluminium with K, Mg, Fe etc. Mica capacitors have good
mechanical strength, they can be operated to temperatures as high as 9000C.
But mica capacitors are silver migration occurs at high DC voltages, high
temperatures, and high humidity.
Plastic
capacitors
The plastic capacitors are
made of polycarbonate, polyethene and polyvinyl chloride. Plastic capacitors
have high dielectric strength and large permittivity. They can withstand
temperatures up to 2500C. Plastic capacitors have high capacitance in
small volumes. Plastic capacitors have comparatively high capacity with good
high voltage and low leakage properties. They have low leakage but capacitance of more than a few micro-farads is not available at present.
Electrolytic
capacitors
Due to an electrolysis
chemical reaction, a thin oxide film is formed around one of the electrodes,
and it is this film that acts as the dielectric material for the electrolytic
capacitor. These types of capacitors are available with 1µF to thousands of
micro-farads.
Variable capacitors
For some applications,
we need the capacitance used in the circuit to be variable. For instance radio and
television tuning sets. We also have air-cored and plastic-cored capacitors. Air-cored capacitors were used in the olden days. Plastic capacitors are widely used.
Plastic-cored capacitors are made of polyvinyl chloride. Most of the modern radios
use transistors and ICs; we use the PVC type of variable capacitors.
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