WHAT IS A DIELECTRIC? A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization.insulatorpolarizedelectric fieldconductor

Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field which reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field.

While the term "insulator" implies low electrical conduction, "dielectric" is typically used to describe materials with a high polarizability. The latter is expressed by a number called the dielectric constant. The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of the material (by means of polarization)electrical conductionpolarizabilitydielectric constant

A common, yet notable, example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarization of the dielectric by the applied electric field increases the capacitor's surface charge.capacitor

The study of dielectric properties is concerned with the storage and dissipation of electric and magnetic energy in materials. It is important to explain various phenomena in electronics, optics, and solid- state physics.electronicsopticssolid- state physics

The term "dielectric" was coined by William Whewell (from "dia- electric") in response to a request from Michael Faraday.dielectricWilliam WhewelldiaMichael Faraday

CAPACITORS  PAPER CAPACITORS  MICA CAPACITORS  CERAMIC CAPACITORS  PLASTIC FILM CAPACITORS  ELECTROLYTIC CAPACITORS  SC CAPACITORS  VARIABLE CAPACITORS  INTERELECTRODE CAPACITORS

PAPER CAPACITOR A capacitor whose dielectric material consists of oiled paper sandwiched between two layers of metallic foil.

A paper capacitor is made of flat thin strips of metal foil conductors that are separated by waxed paper (the dielectric material). Paper capacitors usually range in value from about 300 picofarads to about 4 microfarads.

The working voltage of a paper capacitor rarely exceeds 600 volts. Paper capacitors are sealed with wax to prevent the harmful effects of moisture and to prevent corrosion and leakage.

MICA CAPACITORS A mica capacitor is made of metal foil plates that are separated by sheets of mica (the dielectric). The whole assembly is encased in molded plastic. The figure (A) shows a cut-away view of a mica capacitor. Because the capacitor parts are molded into a plastic case, corrosion and damage to the plates and dielectric are prevented.

In addition, the molded plastic case makes the capacitor mechanically stronger. Various types of terminals are used on mica capacitors to connect them into circuits. These terminals are also molded into the plastic case.

Mica is an excellent dielectric and can withstand a higher voltage than can a paper dielectric of the same thickness. Common values of mica capacitors range from approximately 50 picofarads to 0.02 microfarad. Some different shapes of mica capacitors are shown in thefigure (B).

CERAMIC CAPACITORS A ceramic capacitor is so named because it contains a ceramic dielectric. One type of ceramic capacitor uses a hollow ceramic cylinder as both the form on which to construct the capacitor and as the dielectric material. The plates consist of thin films of metal deposited on the ceramic cylinder.

A second type of ceramic capacitor is manufactured in the shape of a disk. After leads are attached to each side of the capacitor, the capacitor is completely covered with an insulating moisture-proof coating.

Ceramic capacitors usually range in value from 1 picofarad to 0.01 microfarad and may be used with voltages as high as 30,000 volts. Some different shapes of ceramic capacitors are shown in the figure.

POLYESTER FILM (MYLAR) (Plastic Film Capacitors) This capacitor uses a thin polyester film as a dielectric. Not as high a tolerance as polypropylene, but cheap, temperature stable, readily available, widely used.

Tolerance is approximately 5%to 10%. Can be quite large depending on capacity or rated voltage and so may be suitable for all applications.

ELECTROLYTIC CAPACITORS An electrolytic capacitor is used where a large amount of capacitance is required. As the name implies, an electrolytic capacitor contains an electrolyte. This electrolyte can be in the form of a liquid (wet electrolytic capacitor). The wet electrolytic capacitor is no longer in popular use due to the care needed to prevent spilling of the electrolyte.

A dry electrolytic capacitor consists essentially of two metal plates separated by the electrolyte. In most cases the capacitor is housed in a cylindrical aluminum container which acts as the negative terminal of the capacitor (see figure).

The positive terminal (or terminals if the capacitor is of the multisection type) is a lug (or lugs) on the bottom end of the container. The capacitance value(s) and the voltage rating of the capacitor are generally printed on the side of the aluminum case.

SC CAPACITORS (Super Capacitors) The Electric Double Layer capacitor is a real miracle piece of work. Capacitance is 0.47 Farad (470,000 µF). Despite the large capacitance value, its physical dimensions are relatively small. It ha s a diameter of 21mm (almost an inch) and a height of 11mm (1/2 inch).

Like other electrolytic, the super capacitor is also polarized so exercise caution in regards to the breakdown voltage. Care must be taken in using this capacitor.

It has such large capacitance taht, without precautions, it would destroy part of a power supply such a sthe bridge rectifier, volt regulators, or whatever because of the huge inrush current at charge. For a brief moment, this capacitor acts like a short circuit when the capacitor is charged. Protection circuitry is a must for this type.

VARIABLE CAPACITORS A variable capacitor is constructed in such manner that its value of capacitance can be varied. A typical variable capacitor (adjustable capacitor) is the rotor-stator type. It consists of two sets of metal plates arranged so that the rotor plates move between the stator plates. Air is the dielectric.

As the position of the rotor is changed, the capacitance value is likewise changed. This type of capacitor is used for tuning most radio receivers. Its physical appearance and its symbol are shown in the figure.

Another type of variable capacitor (trimmer capacitor) and its symbol are shown in the figure. This capacitor consists of two plates separated by a sheet of mica. A screw adjustment is used to vary the distance between the plates, thereby changing the capacitance.