PLASTIC-FILM CAPACITORS

A plastic-film capacitor, as shown in Fig. 1, is typically made by rolling a thin film of plastic dielectric with metal foil or a metallized dielectric film into a cylindrical form and attaching leads. The dielectrics include polyester, polypropylene, polystyrene, and polycarbonate. Film thickness can range from 0.06 mil (1.5 μm) to over 0.8 mil (20 μm). The most popular film capacitors have capacitance values of 0.001 to 10 μF, although values from 50 pF to 500 μF are available as standard products. Working voltages range from 50 to 1600 VDC, and capacitance tolerance is from +-1 to +-20 percent.



Fig. 1

In film-and-foil construction, tin or aluminum foil about 0.00025 in (0.00635 mm) thick is wound with the dielectric film, but in metallized-film construction, aluminum or zinc is vacuum deposited to thicknesses of 200 to 500 Å (20 to 50 nm) on the film. Film capacitors can also be made by cutting and stacking metallized foil with attached leads. A capacitor with metallized film is smaller and weighs less than a comparably rated film-and-foil unit. Moreover, metallized-film capacitors are self-healing; that is, if the capacitor dielectric is pierced by a transient overvoltage, the metal film around the hole will evaporate, effectively lining the hole with molten plastic dielectric. This prevents short-circuits between adjacent metal layers and preserves the capacitor.

After rolling or stacking is complete, the capacitor is dipped in or conformally coated with an insulating plastic jacket. Some units are also hermetically sealed in tubular or rectangular metal cases for added environmental protection. Both film-and-foil and metallizedfilm capacitors are available with axial or radial leads in a wide variety of case styles.