Film Capacitors

Film Capacitors Overview

Film capacitors are very common electronic components. They’re differentiated from other forms of capacitors by the fact that their dielectric material is a plastic film. This plastic film is connected to the electrodes on the device and the entire component can be shaped as a cylinder, as a flat square or in other ways to accommodate the needs of builders.

There are several different designs and different material options for film capacitors, allowing the devices to be used at different power levels and, in some cases, giving the capacitors unique properties that make them very durable products.

These capacitors come in a very wide range of sizes, and the largest of the designs are typically referred to as power capacitors, designed for use in high-voltage applications. Even the smaller components, however, can be selected for their ability to tolerate very trying applications. Some of them, for instance, are able to handle a sustained frequency for a very long amount of time without being damaged. Others are able to operate under very high heat conditions, which makes them excellent for applications where a great deal of stability is required.

Conversely, these capacitors are sometimes chosen because stability is not a significant concern and there are very inexpensive types of these capacitors available that are widely used in such applications.

These devices involve advanced manufacturing techniques, but their core design is really quite simple.

 

Making a Film Capacitor

There are different types of film capacitors, but the manufacturing process is similar throughout.

The film needs to be interleaved or coated with metal, a process called metallization. During this process, the metal is deposited to the plastic film within a vacuum chamber. After the film is completed, it is made into a roll, which is cut to the desired width.

The widths of dielectric material are wound very tightly. Some of these capacitors will be further treated by being treated with silicone oil. After the product is perfected and the connections for the terminals are added, the device is flattened to save space and the terminals are attached.

Film capacitors can be made in a wide variety of different designs. They can be radial, axial or be made in heavy-duty designs. Heavy-duty designs include snubber capacitors and other components used in high-power environments.

Most film capacitors have a protective coating on the outside, which also contains their specifications. The finished components are tested to determine their capacitance values and, after passing quality control, they are sent off to the market.

Different films offer different properties. Popular products for making film capacitors include polypropylene, polystyrene, polyester and poly phenylene sulfide. Different materials can, in some cases, allow inexpensive film capacitors to replace much more expensive components. For example, film capacitors are now being used to replace tantalum capacitors in some applications, the latter of which are typically quite expensive.

 

Film/Foil and Metalized

There are two varieties of film capacitors. Film/foil capacitors are made of two pieces of plastic film, with electrodes being created by covering each of the layers. This sandwiched material is wound into the desired shape and is easy to manufacture. These types of film capacitors are often employed in high current applications.

The other variation is the metallized film capacitor. In this type of a capacitor, the vacuum depositing process described above is used to create the electrodes, by depositing it on the film. These types of film capacitors are not as suitable for high current applications, but they are very reliable. If the component should short out, the device is capable of healing itself.

Of the two varieties, metallized film capacitors typically have higher capacitance overall.

 

Materials and Versatility

The various materials used as the dielectric material in film capacitors make these very versatile components.

 

Polyester

Polyester is most commonly used in these capacitors. Polyester capacitors offer a very high level of capacitance compared to the volume of the unit. These capacitors do have some drawbacks, such as parasitic inductance.

Polyester is commonly used in capacitors used in DC circuits. They’re very stable components and they are often chosen for coupling applications, bypass applications and decoupling applications.

Polyester film capacitors are generally used in low current applications.

 

Polypropelene

Polypropylene capacitors have low power dissipation and perform well at higher frequencies. They are, however, vulnerable to being damaged by heat. Polypropylene film capacitors are used in power supplies and in some audio applications. Polypropylene is used in some of the largest capacitor designs on the market.

 

Polycarbonate

These are relatively expensive film capacitors that are used in very demanding applications. They have excellent tolerances to heat and work well in timing applications. They are also utilized in filter applications.

Polycarbonate capacitors have become rarer on the market and, after the largest supplier of polycarbonate film capacitors quit producing them, they were largely replaced by polypropylene designs.

 

Polyphenylene Suflide

Polyphenylene sulfide film capacitors are metallized versions of film capacitors that have very low temperature dependence. These capacitors are also used for their dialectic absorption, which is among the best available among the common materials.

Polyphenylene sulfide capacitors are utilized quite commonly in applications where high temperatures are factors. They have been, to a great extent, employed as replacements for the much rarer polycarbonate film capacitors, having many of the same advantages but now being far more common on the market.

There are other film materials in use, as well, each of which offers advantages in certain applications.

 

Characteristics of Film Capacitors

The characteristics of film capacitors largely depend upon which type of material is used as the plastic dielectric in the component. These devices, however, do share some broader characteristics.

Plastic film capacitors can have excellent dielectric strength, with polypropylene models getting up to 650 V/µm ratings. Some of the designs are able to withstand very high temperatures and perform as expected.

The most expensive models are reliable enough to use in military applications and in very demanding industrial applications, owing to the properties of the materials. In the case of polypropylene film capacitors, the components are reliable enough to work in Class 1 applications.

Compared to other types of capacitors, film capacitors have advantages over electrolytic capacitors in aging. They also have an advantage over electrolytic capacitors where ripple current is concerned.

Film capacitors are also well known for the fact that they can handle high surge currents. This is particularly true of the film/foil designs. Despite their capabilities, many of the options are relatively inexpensive, making them good all-around choices for building products, prototyping designs and for building electronics for industrial purposes.

Film capacitors also typically exhibit low dissipation rates. Some designs, typically designated as power capacitors, are capable of working at exceptionally high voltages. These are often employed in power stations and other large-scale applications, where there properties make them ideally suited to the task.

There are some drawbacks to film capacitors compared to other options, as well.

While some film capacitors have very high temperature tolerances, others may actually pose a fire hazard if they are overloaded. This, of course, is a serious safety consideration and necessitates that the maximum and minimum operating parameters are adhered to closely.

While metallized capacitors have the ability to heal damage, film/foil capacitors do not. In such instances where one is damaged, a short-circuit in the component will be permanent and will necessitate replacement.

While film capacitors can be made in compact designs, they are typically a little bit larger than some of the other capacitor options on the market, including electrolytic capacitors, over which film capacitors still do have some advantages. In designs where a surface mount is being employed to position the components, there may be fewer options for film capacitors then there are for other types, as well.

Where design is concerned, film capacitors have significant advantages over some options, as well.

 

Where They’re Used

Film capacitors are commonly employed as snubbers in electronic circuits. Polypropylene film capacitors can handle the very high current peaks, which makes them well suited for this purpose. They are also used in welding, medical applications and, among their very advanced uses, they are utilized in x-ray technology and in laser technology.

Polypropylene capacitors are also used in fluorescent lighting applications and in applications utilizing motors.

In some instances, film capacitors are not chosen specifically for their characteristics, but because they come at a very inexpensive price when made out of common materials. In these applications, polyester film capacitors tend to be very common. For applications where stability is not a primary concern, these inexpensive options for capacitors can bring down production costs considerably.

The self-healing properties of metallized film capacitors make them excellent choices for safety equipment. They are used in fault protection equipment, for fire control equipment and for other equipment where the remarkable stability makes them particularly suitable. These find common usage in devices that are designed to prevent electrical noise as well as electrical shocks.

Some of the other uses of these capacitors include temperature compensating devices, oscillating devices, voltage droppers and peak voltage detectors.

Their low dissipation factor makes them good choices for various types of filters, including stop filters and pass filters. It also makes them suitable for use in signal smoothing.

Film capacitors are also used in TV tuners. They are utilized in these applications because they have the aforementioned high dielectric strength and because of their high pulse time rating. Their low dissipation factor also plays a role. They are utilized in TV S-correction, as well, for many of the same reasons.

 

Power Film Capacitors

Power film capacitors are those that are designed to work with high voltages. There is currently no specific voltage that separates them from other film capacitors, but they can be understood to be the types of film capacitors that would be employed in very large industrial applications, power generating facilities and other applications where very high voltages and currents need to be handled.

These types of capacitors are utilized in both AC and DC applications. The oftentimes serve the role as damping capacitors in AC circuitry, able to stop voltage spikes and protect equipment.

These types of capacitors are also used to absorb current surges in high-voltage applications. This is among the applications where they are found used with lasers.

Power film capacitors are usually easily differentiated from film capacitors by their size. The process of manufacturing film capacitors can be scaled upward economically, meaning that the designs used in high-voltage applications can be very large. These types of capacitors would typically only be seen in the most demanding industrial applications or in utility applications.

Many of the desirable qualities in power film capacitors, however, are simply scaled up versions of the same desirable qualities they have in their smaller versions.

 

Purchasing

Film capacitors range significantly in price and availability, depending upon the materials of which they are manufactured. They are manufactured by major component suppliers such as RS, MFT Capacitors, Hitachi, Illinois Capacitor and many others. In some cases, one manufacturer may be the principal supplier for a given type of film capacitor, a situation that has led to some types of capacitors being be replaced by others over time.

Film capacitors have enough variety among their capabilities that they can be chosen for great precision – in which case they are likely to be higher in price – or can be purchased because they offer cost savings, such as in the case of polyester film capacitors. Replacing expensive types of capacitors such as tantalum capacitors with film capacitors can bring down the cost of electronics a great deal.

Supply houses will have many different designs, specifications and types of film capacitors available. The maximum and minimum operating temperature, the mounting type, the construction of the device and other specifications will provide guidance in choosing the right model. Some of these capacitors, however, are sold based on the specific application for which they are typically used, which makes it easier for donors to narrow down the correct choice for their particular application.

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