Capacitors are components that have the ability to store electrical energy. This energy is stored in an electrostatic field which is created by electrical charges accumulating on conducting plates placed across an electrical potential and separated by an insulating medium. Capacitance as a value is dependent upon the distance between the conducting plates, the dielectric constant of the insulating medium, and the common area of the conducting plates.
A film type capacitor made from this simple approach would create a large component. To prevent the waste of space the plates are “rolled” together, with an extra layer of dielectric to prevent the plates from touching, which results in a higher capacitance and smaller size. There are two basic types of “rolled” construction film capacitors. In the extended foil type the connection to the plate is made by extending the foil beyond the dielectric. In the tab type the connection is made by inserting a tab between the dielectric and the foil to provide the connection to the foil.
Final termination is made by soldering or welding a lead onto the tab or foil. The distance that the electrode foil is indented from the edge of the dielectric is the margin. This margin provides some protection against arc-over and short circuit. Metalized film construction is different from film-foil in that the foil is replaced with a very thin metallic film which is vacuum deposited on the dielectric. The thickness of the deposited layer is negligible to the foil it replaced, thereby saving more size.
Final encasement types of film capacitors include molded, hermetic seal, dipped epoxy, tape wrapped, and epoxy endfilled. Each encasement affects the environmental capability of the finished part so it needs to be taken into consideration when choosing a case.
There are two categories of capacitors, Electrolytic and Electrostatic. Electrolytic capacitors are polarized units which use tantalum oxide or aluminum oxide as the dielectric. The oxides are very thin which result in very high capacitance values. The negatives are low insulation resistance, sensitivity to changes in temperature and frequency, and a high dielectric absorption.
Electrostatic capacitors have sub-categories that they are divided into such as Film, Air, Mica, Ceramic, and Vacuum. Mica and Vacuum capacitors are usually found in high frequency, high voltage applications because of their dielectric characteristics. Air capacitors are low loss devices that are most commonly used for variable tuning. Ceramic capacitors offer a wide range of physical configurations and electrical characteristics and come in Low K class I ceramics and Higher K class II ceramics.
Film capacitors use either plastic film or impregnated paper for their dielectric material and are the most versatile of all capacitors. The capacitors have a reputation for outstanding electrical characteristics in severe environmental conditions, high reliability, and stability in long life applications. Being non-polar gives these capacitors a predictability in DC or AC applications as well as being stable over a broad temperature range. They have the lowest Dissipation Factor (DF) and Equivalent Series Resistance (ESR) of all capacitor types. Film capacitors are available in many designs and casings to stop severe shock and vibration problems.
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