CAPACITORS:
A capacitor consists of two metal plates, separated by an insulating layer called the dielectric.
It has the ability of storing a quantity of electricity as an excess of electrons on one plate and a deficiency on the other.The p.d. which may be maintained across the plates of a capacitor is determined by the type and thickness of the dielectric medium. Capacitor manufacturers usually indicate the maximum safe working voltage for their products.Capacitors are classified by the type of dielectric material used in their construction. Figure shows the general construction and appearance of some capacitor types to be found in installation work.
1) Air-dielectric capacitors:
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| Fig:air dielectric capacitor |
Air-dielectric capacitors are usually constructed of multiple aluminium vanes of which one section moves to make the capacitance variable. They are often used for radio tuning circuits.Mica-dielectric capacitors Mica-dielectric capacitors are constructed of thin aluminium foils separated by a layer of mica. They are expensive, but this dielectric is very stable and has low dielectric loss. They are often used in high-frequency electronic circuits.
2) Paper-dielectric capacitors:
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| Paper-dielectric capacitors |
Paper-dielectric capacitors usually consist of thin aluminum foils separated by a layer of waxed paper. This paper–foil sandwich is rolled into a cylinder and usually contained in a metal cylinder. These capacitors are used in fluorescent lighting fittings and motor circuits.
3) Electrolytic capacitors:
The construction of these is similar to that of the paper-dielectric capacitors, but the dielectric material in this case is an oxide skin formed electrolytically by the manufacturers. Since the oxide skin is very thin, a large capacitance is achieved for a small physical size,but if a voltage of the wrong polarity is applied, the oxide skin is damaged and the gas inside the sealed
container explodes. For this reason electrolytic capacitors must be connected to the correct voltage polarity.
They are used where a large capacitance is required from a small physical size and where the terminal voltage never reverses polarity
4) Ceramic capacitors:
A ceramic capacitor is a non-polarized fixed capacitor made out of two or more alternating layers of ceramic and metal in which the ceramic material acts as the dielectric and the metal acts as the electrodes. The ceramic material is a mixture of finely ground granules of paraelectric or ferroelectric materials, modified by mixed oxides that are necessary to achieve the capacitor's desired characteristics.
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| Electrolytic capacitors photo credit:123rf |
The construction of these is similar to that of the paper-dielectric capacitors, but the dielectric material in this case is an oxide skin formed electrolytically by the manufacturers. Since the oxide skin is very thin, a large capacitance is achieved for a small physical size,but if a voltage of the wrong polarity is applied, the oxide skin is damaged and the gas inside the sealed
container explodes. For this reason electrolytic capacitors must be connected to the correct voltage polarity.
They are used where a large capacitance is required from a small physical size and where the terminal voltage never reverses polarity
4) Ceramic capacitors:
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| Ceramic capacitors |
A ceramic capacitor is a non-polarized fixed capacitor made out of two or more alternating layers of ceramic and metal in which the ceramic material acts as the dielectric and the metal acts as the electrodes. The ceramic material is a mixture of finely ground granules of paraelectric or ferroelectric materials, modified by mixed oxides that are necessary to achieve the capacitor's desired characteristics.
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