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Wednesday, 6 November 2019

Plastic Capacitor Construction


Plastic Capacitors is defined as those capacitors which uses plastic film as a dielectric. The different plastics that are used are Teflon, Polystyrene, Polyethylene and Polyester. When sealed properly these plastics exhibit good mechanical strength, resistance to heat and chemical inertness. These have high insulation resistance and low dielectric absorption. The Construction of different plastic capacitor are explained below.

Teflon :

Polytetrafluoro ethylene also known as Teflon is a heat resistance material. A thin films of Teflon are produced by dispersing a thin colloidal solution of Teflon over a metal surface. The film is removed after drying and heating the metal. In another method a metal strip is made to pass through liquid polytetrafluoro ethylene. The coating is, heated and then removed.

A thin film of aluminium is coated onto this by vacuum evaporation. This metalized film with plain film of Teflon is rolled to form a capacitor. Teflon can operate at higher temperatures and has better temperature coefficient. Insulation resistance is high and power factor is low.

Polystyrene :

These capacitors are made by rolling the polystyrene film with aluminium foil. These are compactly wound and the leads are welded at the ends. The entire assembly is heat treated so that the plastic softens and makes good contact with the metal foil. The winding requires particular attention because it may accumulate electrostatic charges and attracts dust. Polystyrene capacitors have a low power factor and cannot be used at temperatures beyond 65°C. It has high insulation resistance and good stability. It has the disadvantage that the capacitance decreases with temperature.

Polyester :

These are made of polyesters like Mylar, Melinex, Terelyne etc. also called as polyethylene terephthalate. This is usually metallised to give better results. Sometimes impregnation is done with silicons or mineral oil to improve the dielectric, characteristics. A thin film of silver is deposited on this plastic by vacuum evaporation. These capacitors are capable of withstanding temperatures upto 150°C and voltage upto 400 volts. These capacitors are available in the ranges of 100 pF to 2 μF. These capacitors have the disadvantages that dielectric constant increase with temperature and decreases with frequency. Since the dielectric is prone to moisture, good encasing is needed. The capacitors are usually encased in glass or ceramic containers; resin moulded or in jackets of silicon treated polystyrene.

These capacitors are preferred to those of paper capacitors. They have excellent high frequency characteristics and can be used at frequencies of 40 MHz. Insulation resistance is high. The disadvantages are that the power factor varies with the temperature. The capacitance also increases with increase in temperature. The dielectric absorption is high and can withstand voltages upto 250 volts.

Polycarbonate :

The metallised polycarbonate film is rolled and hermetically sealed in a metal case with epoxy resin. It is insulated with a polyester sleeve. Because of its protection from moisture it provides stable capacitances with a high temperature coefficient. It has very high insulation resistance but can be operated only upto 85°C. They are not capable of withstanding high voltages. These are available in the ranges 0.1 to 10 μF.

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