What Are Film Capacitors?

A capacitor is a common passive electronic component that features two terminals, capable of retaining electrical energy within an electric field. When implemented within a circuit, a capacitor serves to add capacitance to the system. Film capacitors are one of the most widely used types, taking advantage of direct electrical connections that maintain a short path of current to the electrode. With their various capabilities, the film capacitor is known for its low ESR and ESL, making them fairly advantageous for alternating current applications.

To construct a typical film capacitor, two pieces of plastic film are covered with metallic electrodes before they are formed into a cylindrical shaped winding. After, terminals are attached to the device and the assembly is encapsulated. Film capacitors may come in a variety of types, those of which are categorized into film and foil capacitors and metalized film capacitors.

Film or foil capacitors are typically constructed with two plastic films serving as the dielectric. With a thin metal foil as the electrode, such capacitors are capable of taking on high current surges and can establish electrical connections to the metal foil electrodes with ease. Metallized film capacitors, meanwhile, take advantage of two metallized films with plastic film that act as the dielectric. For the electrodes, aluminum metallization may be added to the sides of the component through vacuum deposition. This can allow for the component to avoid destruction when short-circuiting occurs. Additionally, a large capacitance value may be achieved.

Similar to many other capacitor components, the film capacitor will store energy and charges on their electrode. Due to their non-polarity, such components may be attached to the circuit in any way and still receive a supply for charging. Generally, any AC type charge for the positive part of the cycle will be retained, while the negative part of the load will be discharged. While this is the same as other capacitor types, the film capacitor differs in its dielectric.

Film capacitors serve as an important component for many systems, often being found within electronic circuits, EMI/RFI suppression applications, lighting ballasts, and more. When placed within an electronic circuit, the film capacitor may feature low electrical losses and almost linear behavior across a wide frequency and temperature range. Generally, such capacitor types will be found within oscillators, resonant circuits, electronic filter applications, and much more.

When a system needs to have electrical noise suppression implemented, specialized film capacitors known as AC line filter safety capacitors or safety capacitors are used. Such capacitors increase frequency while decreasing electrical impedance, meaning that any noise is short circuited during high frequency operation. This allows such film capacitors to serve a variety of equipment such as motors, inverters, spark quenchers, solid-state relay snubbers, and more. In general, most EMI/RFI suppression film capacitors come in the form of polyester or metallized polypropylene film capacitors, though some may be metallized paper capacitors if flame resistance is desirable.

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