Types of Capacitors – Capacitors have been classified into various types depending upon their construction and the nature of dielectric used in them. If the capacitor is such that its plates are immovable. It is known as a fixed capacitor. Its value does not change. Paper capacitor is an example of fixed capacitors. The paper capacitor is cylindrical in shape. Generally, an oiled or greased paper or a thin plastic sheet is used as a dielectric between two aluminum foils which is firmly rolled in the form of a cylinder and is then enclosed into a plastic case (Fig. 115.20)
Mica capacitor is another example of fixed capacitors. In these capacitors, mica is used as dielectric between two metal plates. For convenience and safety it is enclosed in a plastic case or in a case of some insulator. Wires attached with plates project out of the case for making connections. If the capacitance is to be increased, a large number of plates are piled up, one over the other with layers of dielectric in between and alternate plates are connected with each other (Fig. 15. 21)
Fig. 15. 21: mica capacitor
If in a parallel plate capacitor some arrangement is made so that the area of the plates facing each other can be charged then it is called a variable capacitor. It is usually a combination of many capacitors with air as dielectric. It consists of two sets of plates. One set remains static while the second one can rotate so the distance between the plates remains the same and they do not touch each other. The common area of the plates of the two sets which faces each other, determines the value of the capacitance. Thus the capacitance of the capacitor can be increased or decreased by turning the rotatable plates in or out of the space between the static plates (fig. 15.22).
Fig. 15.22: variable capacitor