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What are the Different Types of Capacitor? What is the Use and Type of Capacitor?
Nowadays, different types of capacitors are used for practical purposes. Some of them are discussed below.
Mica capacitor or block capacitor: This is actually a parallel combination of a few parallel plate capacitors having a fixed capacitance. It consists of a number of metal sheets with mica as dielectric between them. The alternate sheets are connected to A while the others to B [Fig.]. If a potential difference is applied between A and B, the capacitor becomes charged.
If α be the area of each metal plate, d the distance between two plates, k the dielectric constant of mica and n the number of metal plates, then the capacitance of such a capacitor in SI is given by,
C = \(\frac{(n-1) \kappa \epsilon_0 \alpha}{d}\)
This capacitor looks like a block. So it is often called a block capacitor. This type of capacitors is mainly used in wireless receiving sets.
Paper capacitor: Paper coated with paraffin is a suitable dielectric. In between two aluminium or tin foils, paraffin-waxed paper sheets are inserted. The foils are then rolled into a cylindrical shape for economy of space [Fig.].
This type is essentially a single parallel plate capacitor. It is cheaper than a paper- mica capacitor. Of course one of its disadvantages is that it cannot work at high potential differ-ence.
In low voltage ac circuits, having semiconductor diodes and transitors in particular, paper capacitors are widely used.
Variable air capacitor: It is also one type of parallel plate capacitor whose capacitance can be varied at will. It consists of two sets (F and M) of parallel plates made of aluminium or brass [Fig.].
The set F is fixed while the other set M can be turned with the help of a knob K. As the knob turns, the set M either moves into the spacings of the set F or comes out of them, thereby changing the area of overlap of the plates. Since capacitance of a capacitor depends on the area of overlap of the plates, the capacitance also changes
It is used in radio-sets or wireless receiving sets and other elec-tronic instruments where variation of capacitance is required.
Electrolytic capacitor: It consists of a pair of aluminium plates partly immersed into a solution of ammonium borate [NH4B5O6] . The two aluminium plates are connected to the positive and negative terminals of a source of steady current [Fig.], Due to electrolysis a fine layer ( ≈ 10-6 cm) of alumin-ium oxide [Al2O3] is formed on the positive plate.
This layer acts as a dielectric medium. The solution of ammonium borate acts as an extension of the other plate. Since the capacitance of the capacitor is inversely proportional to the thickness of the dielectric and the thickness of the aluminium oxide layer is very small, the capacitor has a large capacitance, often a few thou-sand microfarads.
The plate on which aluminium oxide is deposited must be con-nected to the positive potential. Otherwise the capacitor would be damaged. For this reason the anode plate is marked with + sign or a red dot. Further, the maximum voltage applicable is restricted by the thickness of the oxide layer. This is also marked on the capacitor. Higher voltage would break the layer and destroy the device.
Electrolytic capacitors are largely used as filters in rectifier cir-cuits.