Film capacitor

A film capacitor is an electrical component used to store energy within a circuit.

These devices are also known as plastic film or polymer film capacitors. They are among the most common capacitor types used in modern electronics. They work alongside ceramic and electrolytic capacitors to manage electricity in many devices. A film capacitor uses an insulating plastic film as its dielectric. The dielectric is the material that prevents electrical current from flowing through it. To create the component, conductive electrodes are added to this insulating layer. These electrodes can be separate metallic foils or a thin layer of metal applied directly to the film.

The way a capacitor stores charge depends on its physical design. The capacitance, or the ability to hold a charge, follows a specific mathematical relationship. It increases if the dielectric is thinner or if the electrode surface area is larger. A material with higher permittivity, which describes how a material affects an electric field, also increases capacitance. In most film capacitors, two conductive layers are wound together into a cylindrical shape. This winding is often flattened into an oval shape to save space on a printed circuit board. Some capacitors use a stacked configuration instead of a winding. In this method, many single layers are layered on top of each other to form the body.

There are two primary construction types for plastic film capacitors. The first type is the film/foil capacitor, also called a metal foil capacitor. These use two plastic films as the dielectric, with each layer coated in a thin metal foil, usually aluminum. This design makes it easy to connect the electrodes electrically. It also allows the capacitor to handle high current surges effectively. The second type is the metallized film capacitor. These use a very thin layer of metal, about 0.03 micrometers thick, deposited onto the plastic film. This metal is often aluminum or zinc applied in a vacuum. While these have a lower current surge rating than foil types, they are much more efficient in size.

Manufacturing a metallized film capacitor involves several precise steps. First, the plastic film undergoes bi-axial stretching to make it extremely thin. This process can result in films as thin as 0.6 micrometers. Next, the film is metallized in a vacuum system with very few air molecules. The film is then wound onto a large "mother roll" that is about 1 meter wide. These rolls are slit into smaller strips of the required width. The two films are then wound together, often slightly offset to allow the metal edges to extend outward. These edges are then covered with a metal like tin or zinc through a process called schoopage.

After the winding is complete, the capacitor undergoes a "healing" process. A precisely calibrated voltage is applied across the electrodes to burn away any existing defects. To protect the component from moisture and the environment, it is often impregnated with silicone oil. The terminals are then soldered or welded to the metal contact layers. Finally, the body is either potted in a casing or dipped in a protective lacquer coating.

Some manufacturers even produce "naked" capacitors to keep production costs low. Every capacitor must undergo a final electrical test to check its capacitance, impedance, and dissipation factor.

A remarkable feature of metallized film capacitors is their ability to self-heal.

If a short circuit occurs due to a defect, the high temperature of the electrical arc vaporizes the thin metal. This happens in less than 10 microseconds. The resulting vapor pressure blows the arc away, clearing the defect. This process can happen many times without destroying the component. Even with thousands of these tiny "pinholes," the total capacitance usually drops by less than 1%. This self-healing ability allows for higher volumetric efficiency, meaning more capacitance can fit into a smaller space.

Engineers choose different film materials based on the needs of the circuit. Polypropylene film is used for stability because it has low electrical losses. It works well in resonant circuits. Polyester film is a low-cost option that offers excellent long-term stability for high-frequency filters.

Capacitors vary greatly in size and capability. Small electronic capacitors might start at 100 picofarads and reach up to microfarads. However, very large film capacitors are used in power installations. These can withstand extremely high voltages, sometimes reaching into the four-digit voltage range. These large versions are essential for managing power in electrical plants and systems.