Saturday, March 17, 2012

Building Discrete Capacitors

Although electrical current can only flow through an intact circuit, if a circuit is broken or opened up a small electrical field will exist between the two ends. By altering the opened circuit to expand the area of near-contact between the ends, a structure for holding electrical charges, known as a capacitor, can be built. Applying a fixed voltage across two metal plates that are held close together but are not in direct contact and have wires extending to attach them to a circuit, causes electrons to move between positive and negative plates. If the battery providing the voltage is disconnected, the electric charge remains between the two plates. The amount of charge that can be held depends on the area of the two plates and the distance between them. One of the capacitor’s key properties is its ability to resist changes in voltage, which allows it to be useful in a wide range of voltage storage and current control applications.

Practical capacitors that are used as discrete components in electronic circuits employ conductive metal plates separated by a dielectric material, but they are not limited to a two plate configuration. These capacitors can have multiple parallel plates separated by a variety of insulating materials. Certain types of insulators can significantly increase the device’s capacitance, which is the measure of its capacity for storing an electric charge. In addition, the plates do not have to be flat, as cylindrical and curved line forms can be useful for handling stray signals that may interfere with circuit operations. Capacitor manufacturing methods are generally able to achieve accurate design values and precise characteristics for a range of discrete capacitor performance categories.

For more information on the principles of capacitance, see HyperPhysics.

Discrete Capacitor Design Types

The majority of discrete capacitors are designed in one of three basic configurations, including:

Low Loss: Low loss devices have a high level of capacitance stability and rely on dielectric materials made of glass, mica, ceramics, and low loss plastic films, such as polystyrene and polypropylene. These capacitors are generally used in precision circuitry applications like those for telecommunications filters.

Medium Loss: The second category includes capacitors that feature medium loss and medium capacitance stability, and have dielectric layers made from oil or wax reinforced paper, plastic films, and ceramics. They can function across a broad range of AC and DC voltages for applications such as coupling, decoupling, power separation filters, interference suppression, and power line control.

Electrolytic: Both aluminum and tantalum-based electrolytic capacitors balance high level capacitance with relatively small device size, although tantalum capacitors usually have significantly higher capacitance than aluminum models, as well as longer service lives. Aluminum electrolytic capacitors are more commonly found in radio and television applications, while tantalum devices are used for military or other high-durability projects.

Film Capacitors

In some types of capacitor manufacturing, a plate can be formed using the metalized side of a flexible dielectric film, while a section of foil serves as the other plate. The non-metalized side of the film faces the foil, and the unit is rolled over a plastic core with offset alternating layers. The rolled film is hardened through heat treatment, and the disks at its ends are soldered onto the extended portion of the foil. Wires are then welded to these disks and the capacitor is sealed. After sealing, the assembled capacitor can be immersed in a liquid dielectric solution that eventually dries, covered in a plastic film, or encased in molded plastic. Modified versions of this process may be used depending on the type of dielectric film. Some methods involve two foil layers separated by plain film or reinforced paper, a pair of metalized offset films, or a different sealing technique.

Electrolytic Capacitors

An electrolytic capacitor is any capacitor with a dielectric layer produced through electrolytic processes, and in dry foil form it resembles the design of paper film capacitors. To build an electrolytic device, paper infused with an electrolyte is used to separate two foil layers, which are then rolled together. The electrolyte’s oxygen creates a thin layer of metal oxide on the foil metal, and this oxide serves as the dielectric. The capacitor can then be sealed in aluminum or plastic. Alternatively, the metal oxide can be formed in a separate process and etched to enlarge its surface, thus raising capacitance. The wet format of a tantalum electrolytic capacitor has an increased surface area due to its use of a porous anode slug with a coating of tantalum pentoxide that serves as the dielectric. In contrast, a solid tantalum electrolytic capacitor uses the semiconductor manganese dioxide, followed by layers of graphite and silver, to form the dielectric surface.

Solid Dielectric Capacitors

Solid dielectric capacitors rely on hard or brittle materials, such as glass, porcelain, mica, and certain types of ceramics, that are arranged into stacked layers. The dielectric layer is usually metalized along one side and segmented into rectangular sections that are then stacked with alternating layers offset. A conductive paste or press fitting can be used to connect the end caps. Afterward, leads are attached and the capacitor can undergo a sealing process. Mica capacitors typically employ a silver electrode that is screened onto the mica insulators in the dielectric forming process, while ceramic capacitors rely on a barium titanate paste rolled into a flexible film and coated with conductive ink to form the dielectric.

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