What is a Film Capacitor : Construction, Types & Characteristics

Film capacitors are still the most frequently used in electrical & electronic devices. These capacitors are also known as plastic film, polymer film or film dielectric. Generally, these capacitors are also called film caps & power film capacitors. At present, these capacitors have high performance when compared with the new film or foil wound elements. These capacitors have several advantages like approximately limitless shelf life, these are designed to shut tolerances, and ultimately its characteristics will stay highly constant, the capacity of absorbing power surges without harm, self-inductance is low. This article discusses an overview of what is film capacitor, types, and applications.

What is Film Capacitor?

Definition: A capacitor that uses a slight plastic film like a dielectric is known as a film capacitor. These capacitors are fairly inexpensive, constant overtime, include equivalent series inductance (ESR) and low self-inductance, while some film capacitors can withstand large reactive power values. The film of this capacitor is made with a very thin film drawing process. When the film is designed, then it may be metalized based on the capacitor properties. After that, electrodes are added to it and it can be arranged into a case. So that it can be protected from environmental factors. These are used in several applications due to their properties like stability, low-cost and low inductance.

Construction and Working

The film capacitor working with construction is shown below. This capacitor is designed with a thin dielectric film where one side of the capacitor is metalized. The film of this capacitor is very thin and the thickness of this is below 1 µm.

Once the film of the capacitor is drawn to the desired thickness, then the film can be slash into bands. The thickness of the bands mainly depends on the ability of the capacitor being generated.

Two bands of the film are connected in the form of a roll, and they pushed into an oval shape to arrange in a rectangular box. This is significant as rectangular components stores valuable space on the PCB. Electrodes are used to connect one of the films with each of the two electrodes.

Whenever the voltage is applied to a film capacitor with the self-healing property, then the defects will burn out. After that, the rectangular box is sealed using silicon oil to defend the film’s roll from wetness, and placed within plastic to hermetically close the inner. The capacitance range of these capacitors will be ranged from under 1nF toward 30µF.

The voltage ratings of this capacitor are ranges from 50V to above 2kV. These are designed to use in different applications like automotive environments with high-vibration, temperature high-power & environment applications. These capacitors will provide low losses & high efficiency while awaiting a long service life.

Film Capacitor Types

The classification of film capacitors can be done based on the application like metalized film, polyester film, PTFE film, polystyrene film, and polypropylene film. The main difference between these types of capacitors is the dielectric material used & based on the application the proper material must be selected.

Styles of Film Capacitors

There are different styles of film capacitors are used in the industries which include the following.

• Axial style capacitors are used for point-to-point & mounting through-hole
• Radial style is for solder mounting through a hole on PCBs
• Radial style using heavy-duty solder terminals are used for high surge pulse loads & snubber applications
• Heavy-duty snubber capacitor using screw terminals
• SMD style capacitors are used for surface mounting of PCB with metalized contacts on top of two reverse edges.

When these capacitors are used in electronic equipment then they can be enclosed in the usual industry methods like radial, axial & SMD. At present, traditional axial kind packages are used less, however, they are used for some usual through-hole PCBs and point to point construction. The radial type is the most frequent form factor where both terminals of the capacitor are available on one side.

To make easily automated inclusion, radial plastic-type film capacitors are usually designed using terminal spacings at standard distances. Radial capacitors are sealed in plastic boxes to defend the body of the capacitor from the environment.

Characteristics

Film capacitors are extensively used in different applications due to their superior characteristics. This type of capacitor is not polarized, so it can be apt for AC signal as well as power use. These capacitors can be designed with extremely high accuracy capacitance values to maintain the value longer when we compare with other types of capacitors. This means the lifespan of these capacitors is slower than other capacitors like electrolytic capacitors. So the service life of these capacitors have very long, reliable and the failure rate is below average.

These capacitors have low equivalent series resistance (ESR), low self-inductance (ESL), and also extremely low dissipation factors. They can be made to tolerate voltages in the range of kilovolt and they provide extremely higher surge current pulses.

Film Capacitor Markings and Codes

In capacitors, markings and codes play a key role, as they specify various properties of capacitors. So understanding this is very important while selecting the required capacitor. At present most of the capacitors have marked by alphanumeric codes but in older capacitors, they have color codes. In previous years, the color codes of these capacitors are less common however some of them still exist.

The capacitor codes may change based on the format of whether the capacitor is a surface mount type or led & also the dielectric of the capacitor. The capacitor size plays the main role in verifying how this capacitor is marked.

Applications

The applications of the film capacitor include the following.

Power film capacitor is used in power electronics like pulsed lasers, phase shifters & X-ray flashes whereas the low power alternatives are used like decoupling capacitors, in A/D converters & filters. Other prominent applications are electromagnetic interference suppression, safety capacitors, snubber capacitors & fluorescent light ballasts.

Lighting ballasts are mainly used for the proper operating of fluorescent lights. As the ballast is defective, then the light will flash or fall short to start correctly. Older ballasts use an inductor but provide a poor PF (power factor). Current designs employ a switched power supply which depends on film capacitors for improving the power factor.

Snubber type capacitors protect devices from voltage spikes. These capacitors are frequently used in many circuits due to their factors like high peak current, low ESR and low self-inductance. These factors are critical factors within a snubber design. Snubbers are employed in many areas of electronics, particularly power electronics like flyback DC to DC converters & others.

FAQs

1). Do film capacitors have polarity?

They do not have polarity because they are non-polarized

2). Can a capacitor be wired backward?

Yes, the electrolytic capacitor can be wired backward.

3). What’s the difference between a start capacitor and a run capacitor?

The start capacitor makes a current to voltage lag within the motor’s start windings whereas run capacitor utilizes the charge within the dielectric to enhance the current to supply the power to the electric motor

4). What side of the capacitor is positive?

The longer leg of the capacitor is positive.

5). What is a non-polarized capacitor?

A capacitor that has positive or negative polarity is known as a non-polarized capacitor. These capacitors are randomly used in the circuits like feedback, coupling, compensation, decoupling & oscillation.

Thus, this is all about the film capacitor which can be used in straightway like voltage smoothing capacitors, audio crossovers, in filters. These are used for storing energy & releases high current pulse when required. These pulses are mainly used or providing power to pulsed lasers otherwise to produce lighting discharges.