Plastic-film capacitors

Chapter Plastic-film capacitors

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • For low values of capacitance, just one layer of ceramic is needed, and two metalplates can be glued to the disk-shaped porcelain, one on each side. This type of compo-nent is known as a disk-ceramic capacitor. Alternatively, a tube or cylinder of ceramiccan be employed, and metal ink applied to the inside and outside of the tube. Such unitsare called tubular capacitors.Ceramic capacitors have values ranging from a few picofarads to about 0.5 µF.Their voltage ratings are comparable to those of paper capacitors.Plastic-film capacitorsVarious different plastics make good dielectrics for the manufacture of capacitors. Poly-ester, polyethylene, and polystyrene are commonly used. The substance called mylarthat you might have seen used to tint windows makes a good dielectric for capacitors.The method of manufacture is similar to that for paper capacitors when the plasticis flexible. Stacking methods can be used if the plastic is more rigid. The geometries canvary, and these capacitors are therefore found in several different shapes.Capacitance values for plastic-film units range from about 50 pF to several tens ofmicrofarads. Most often they are in the range of 0.001 µF to 10 µF. Plastic capacitors areemployed at audio and radio frequencies, and at low to moderate voltages. The effi-ciency is good, although not as high as that for mica-dielectric or air-dielectric units.Electrolytic capacitorsAll of the above-mentioned types of capacitors provide relatively small values of capac-itance. They are also nonpolarized, meaning that they can be hooked up in a circuit ineither direction. An electrolytic capacitor provides considerably greater capacitancethan any of the above types, but it must be connected in the proper direction in a cir-cuit to work right. Therefore, an electrolytic capacitor is a polarized component.Electrolytic capacitors are made by rolling up aluminum foil strips, separated bypaper saturated with an electrolyte liquid. The electrolyte is a conducting solution.When dc flows through the component, the aluminum oxidizes because of the elec-trolyte. The oxide layer is nonconducting, and forms the dielectric for the capacitor.The layer is extremely thin, and this results in a high capacitance per unit volume.Electrolytic capacitors can have values up to thousands of microfarads, and someunits can handle thousands of volts. These capacitors are most often seen in audio-frequency circuits and in dc power supplies.Tantalum capacitorsAnother type of electrolytic capacitor uses tantalum rather than aluminum. The tanta-lum can be foil, as is the aluminum in a conventional electrolytic capacitor. It might alsotake the form of a porous pellet, the irregular surface of which provides a large area ina small volume. An extremely thin oxide layer forms on the tantalum.206 Capacitance