How Film Capacitors Function and how they are Used

A capacitor is a passive electronic component that is capable of storing an electric charge within an electric field. The film capacitor is a common capacitor type, and they utilize insulating plastic film as their dielectric and may act as a carrier of electrodes when paired with paper. Regularly found within X-rays, phase shifters, safety capacitors, and fluorescent light ballasts, such capacitor types are very useful for a number of electronic devices. To understand how a film capacitor may be used, it is important to first learn about their working principles and functionalities.

The plastic film that is used as the dielectric is designed to be extreme thin, requiring the use of a sophisticated film drawing process. Based on the application’s requirement, the plastic film may either be metallized or left untreated. Once electrodes have been implemented, the capacitor is shielded from the environment and contaminates with the use of a case in which the capacitor is mounted within. To further protect the capacitor from detrimental substances such as moisture, an insulating fluid such as silicone oil is added to the windings of the component. Film capacitors can come in a variety of types depending on the need, and common variations include polyester film, polypropylene film, metallized film, polystyrene film, and PTFE film. Generally, the difference between variations is due to the choice of dielectric based on the requirements of the application in question.

The various plastic film capacitors can be categorized into two primary types, those of which are film/foil and metal foil capacitors as well as metallized film capacitors. Film/foil and metal foil capacitors are all constructed with two plastic films serving as the dielectric. The dielectric is then layed with electrodes coming in the form of a thin metal foil such as aluminum. With this type of film capacitor, electrical connections to the electrodes can be established with ease and such components excel in taking on high current surges.

The metallized film capacitor, on the other hand, features a plastic film as the dielectric as well as two metallized films. For the electrodes of the component, thin aluminum metallization is vacuum-deposited onto each side. Due to the style of construction, such film capacitors are not highly at risk when faced with short-circuiting. Additionally, metallized film capacitors also can be zero defect or be designed to have very high capacitance values. Despite their various advantages, it is important to be aware that the metallized construction of such film capacitors does limit their current surge rating.

Due to the fact that the film capacitor is not polarized, it is very useful for AC signal and power applications. They also excel in storing high precision capacitance values and have great retention capabilities that allow them to age slower than other types. Altogether, their high performance qualities allow them to provide a long, reliable service life with low chance of failure. Film capacitors are also designed to function optimally in high voltage settings, and they can optimally operate within the kilovolt range with high surge current pulses. For more intensive applications that may reach over 200 volt-amperes, one may use a special film capacitor type that is known as a power film capacitor.


April 29, 2022
December 6, 2021

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