Question

Describe the construction of any one type of capacitor.

Answer 1

i) Air capacitors: Air capacitors are capacitors which use air as their dielectric. The simplest air capacitors are made of two conductive plates separated by an air gap. Air capacitors can be made in a variable or fixed capacitance form. Fixed capacitance air capacitors are rarely used since there are many other types with superior characteristics. Variable air capacitors are used more often because of their simple construction. They are usually made of two sets of semicircular metal plates separated by air gaps. One set is fixed and the other is attached to a shaft which allows the user to rotate the assembly, therefore changing the capacitance as needed. The larger the overlap between the two sets of plates, the higher the capacitance. The maximum capacitance state is achieved when the overlap between the two sets of plates is highest, while the lowest capacitance state is achieved when there is no overlap. 

ii) Paper capacitors: Paper capacitor is a capacitor that uses paper as the dielectric. It consists of aluminum sheets and paper sheets. The paper sheet is covered or soaked with oil or wax to protect it from outside harmful environment. Paper capacitors are the fixed type of capacitors that means these capacitors provides fixed capacitance. In other words, the paper capacitor is a type of fixed capacitor that stores fixed amount of electric charge. Paper capacitors are classified into two types based on their construction: 

Paper sheet capacitor Metalized paper capacitor 

Paper sheet capacitor: The paper sheet capacitor is made by taking two or more aluminum sheets and placing a paper sheet between them. The paper placed between the aluminum sheets acts as dielectric and the aluminum sheets acts as electrodes. The paper sheet is poor conductor of electricity so it does not allow flow of electric current or electric charges between two aluminum sheets. However, the paper sheet allows electric field through it. Therefore, the paper sheet placed between the aluminum sheets acts barrier for the electric current.

The paper sheets and aluminum sheets are rolled in the form of cylinder and wire leads are attached to both ends of the aluminum sheets. The entire cylinder is then coated with wax or plastic resin to protect it from moisture in the air. The paper sheet capacitors are used in the high voltage and high current applications. 

Metalized paper capacitor: In metalized paper capacitor, the paper is coated with thin layer of zinc or aluminum. The paper coated with zinc or aluminum is rolled in the form of cylinder. The entire cylinder is then coated with wax or plastic resin to protect it from moisture. The zinc or aluminum coated on the paper acts as electrodes and the paper acts as dielectric. Paper capacitors that are made of zinc are easily destroyed due to the chemical action. Therefore, aluminum is widely used for the construction of paper capacitors. The size of metalized paper capacitor is very small compared to the paper sheet capacitor. In metalized paper capacitor, the aluminum is directly coated on the paper. Therefore, aluminum layer of metalized paper capacitor is very thin compared to the aluminum layer of paper sheet capacitor. 

iii) Ceramic Capacitors: Ceramic Capacitor is a capacitor that uses ceramic as the dielectric. Ceramic capacitors or Disc capacitors as they are generally called, are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For very low capacitance values a single ceramic disc of about 3- 6mm is used. Ceramic capacitors have a high dielectric constant and are available so that relatively high capacitance can be obtained in a small physical size. In order to gain higher capacitances, the capacitor can be made from multiple layers. The Multi-Layer Ceramic Capacitors (MLCC) are made with Paraelectric and Ferroelectric materials mix and alternatively layered with metal contacts. After completion of the layering process, the device is brought to a high temperature and the mixture is sintered, resulting in a ceramic material of desired properties. Finally, the resulting capacitor basically consists of many smaller capacitors connected in parallel, this leads to increase in capacitance. The MLCCs consist of more than 500 layers, with the minimum layer thickness of approximately 0.5 microns. As technology progresses, the thickness of the layer decreases and capacitance increases in the same volume. Ceramic capacitor dielectrics vary from one manufacturer to another, but common compounds include titanium dioxide, Strontium Titanate, and Barium Titanate.

iv) Electrolytic capacitors: The electrolytic capacitor uses an electrolyte (an ionic conducting liquid) as one of its plates to achieve a larger capacitance per unit volume than other types. The capacitors are able to increase the capacitance in a number of ways: increasing the dielectric constant; increasing the electrode surface area; and by decreasing the distance between the electrodes. Electrolytic capacitors use the high dielectric constant of the aluminium oxide layer on the plate of the capacitor which averages between 7 and 8. This is greater than other dielectrics such as mylar which has a dielectric constant of 3 and mica of around 6 - 8. In addition to this, the effective surface area within the capacitors is increased by a factor of up to 120 by roughening the surface of the high-purity aluminium foil. This is one of the keys to producing very high levels of capacitance. The plates of an electrolytic capacitor are constructed from conducting aluminium foil. As a result they can be made very thin and they are also flexible so that they can be packaged easily at the end of the production process. The two plates, or foils are slightly different. One is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte is placed between them. The foil insulated by the oxide layer is the anode. The second foil acts as the cathode and although this does have a naturally occurring oxide layer, this is very much thinner. The thickness of the anode oxide thin film in an aluminium electrolytic capacitor is selected by the required working withstand voltage. In order to package them the two aluminium foils with the electrolyte soaked paper are rolled together to form a cylinder, and they are placed into an aluminium can. In this way the electrolytic capacitor is compact while being robust as a result of the protection afforded by the can. There are two geometries that are used for the connection leads or tags. One is to use axial leads, one coming from each circular face of the cylinder. The other alternative is to use two radial leads or tags, both of which come from the same face of the cylinder. The lead styles give rise to the descriptions used for the overall capacitors. To increase the surface area of both anode and cathode to increase the capacitance, the surface is roughened by etching. 

v) Film Capacitor: Film capacitors are capacitors which use a thin plastic film as the dielectric. Film capacitors are made of a thin dielectric film which may or may not be metallized on one side. The film is extremely thin, with the thickness being under 1 µm. After the film is drawn to the desired thickness, the film is cut into ribbons. The width of the ribbons depends on the capacity of the capacitor being produced. Two ribbons of film are wound together into a roll, which is often pressed into an oval shape so that it can fit into a rectangular case. This is important because rectangular components save precious space on the printed circuit board. Electrodes are added by connecting each of the two electrodes to one of the films. A voltage is applied to burn out any imperfections using the self-healing property of film capacitors. The case is then sealed using silicon oil to protect the film roll against moisture, and dipped in plastic to hermetically seal the interior. This film is made extremely thin using a sophisticated film drawing process. Once the film is manufactured, it may be metallized or left untreated, depending on the needed properties of the capacitor. Electrodes are then added and the assembly is mounted into a case which protects it from environmental factors. They are used in many applications because of their stability, low inductance and low cost. There are many types of film capacitors, including polyester film, metallized film, polypropylene film, PTFE film and polystyrene film. The core difference between these capacitor types is the material used as the dielectric. PTFE film capacitors are heatresistant and used in aerospace and military technology, while metallized polyester film capacitors are used in applications that require long term stability at a relatively low. Cheaper plastics are used if cost is a bigger concern than performance.

vi) Glass capacitors: The capacitor consists of three basic elements: the glass dielectric, aluminium electrodes and the encapsulation. However the assembly of the glass capacitors is undertaken in a manner that ensures the required performance is obtained. As the capacitance between two plates is not always sufficient to provide the required level of performance, the majority of capacitors use a multi-layer construction to provide several layers of plates with interspersed dielectric to give the required capacitance. Although the glass plates are always flat, and tubular forms of construction are not applicable, the glass capacitors are usually available with leads emanating in either a radial or axial form. Essentially the leads either exit the encapsulation at the side or the end.