In this topic, you study Types of Capacitors – Theory & Applications.
Capacitors may be classified into different categories depending upon the geometrical shape Of the conducting surfaces forming a capacitor, e.g. parallel-plate capacitors, coaxial cylindrical capacitors, concentric spherical capacitors, etc. They may also be divided into different groups according to the nature Of the dielectrics as follows:
Air Capacitors
This type of capacitor usually consists of a number of parallel and interleaved fixed and moving metallic vanes. Its capacitance can be changed by varying the position Of moving vanes with respect to the fixed vanes. Such capacitors are chiefly employed in high frequency radio work.
Paper Capacitors
In this type, a capacitor consists Of long strips Of meta foils (either Of tin or aluminium) interleaved with paraffined paper and rolled into a compact size. These capacitors are mainly used in power supplies for filtering purposes.
Mica Capacitors
A capacitor with alternate layers of mica and metal foil clamped tightly together may be used for greater accuracy and higher voltages. But use of mica makes the capacitor more expensive. Silver-mica capacitors are frequently employed in high frequency radio and electronic circuits where a compact size is necessary. A thin wafer of mica with silver deposited on both sides forms such a capacitor.
Ceramic Capacitors
A ceramic capacitor has metallic coatings (usually of silver) on the opposite faces of a thin disc or plate of ceramic material like barium titanate, hydrous silicate of magnesia or talc. Applications of these capacitors are simiar to those of mica capacitors.
Polycarbonate Capacitors
This type is of recent origin and consists of a film of polycarbonate metallised with aluminium rolled into a compact size.
Electrolytic Capacitors
Electrolytic capacitor in its simplest form consists Of two aluminium foils separated with a material such as paper saturated with a suitable electrolyte like ammonium borate. The aluminium oxide film is formed on one Of the foils by passing it through an electrolytic bath. This oxide film acts as the dielectric. As the film is very thin, electrolytic capacitors can provide a large capacitance in a very small space. Commercial forms Of this type Of capacitors are Of semi-dry type, the electrolyte being in the form of a jelly. Solid types Of electrolytic capacitors have also been developed using suitable electrolyte like manganese dioxide. Electrolytic capacitors can be used only for d.c. Their main field of application is electronic circuits and filter circuits. However, the reversible or non-polarised type electrolytic capacitances have also been developed for the use in a-c. circuits and are mainly used with small single-phase induction motors.
Film capacitors
A film capacitor or plastic film capacitor consists of an insulating plastic film used as the dielectric.In one type of construction, this film is metallized mostly with aluminium and rolled into a compact size. On the other hand, in another type Of construction, the capacitor consists Of two metal (mostly aluminium) foils separated by an insulating plastic film, rolled into a compact size. The plastic fims used as dielectric for film capacitors are generally of the materials such as polypropylene, polyester or teflon. These capacitors are available in a very wide range from few picofarads to10-20 microfarads with a voltage rating as high as few thousand volts. In comparison with paper capacitors, film capacitors operate well under the conditions of high temperature, have longer service life and are more reliable. They are preferred when cost plays an important role. These capacitors are commonly used for both d.c. and low-frequency a.c. electronic networks, for various applications in power systems and in electric motor installations.
Glass Capacitors
The glass dielectric, aluminium electrodes and encapsulation are the three basic elements of this type of capacitor. The majority of the capacitors use a multi layer construction providing several layers of plates with interspersed dielectric to give the required capacitance. These capacitors are very expensive and are used for the applications in which highly accurate, stable and reliable operation in harsh environmental conditions is a must. They are resistant to nuclear radiations and capable Of operation over —750C to +2000C. These capacitors are available in the range from 10 PF to 1000 pF. They are used when ultimate performance is required for RF circuits, remote sensors, oscillators, filters, etc.
