Metallized-Dielectric & Standard Capacitors
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The term "metallized dielectric" at first might seem like an oxymoron. After all, if you add a conductive substance to the dielectric of a capacitor, then you have compromised the integrity of the dielectric as a non-conducting substance. Here's the deal: The dielectric material between the conductive plates does not receive the metallization; a non-conductive (dielectric) material like paper is impregnated with metal and that is used for the plate material. The advantage of this scheme is that the plate material can be thinner, thereby reducing the size of the capacitor package for a given capacitance value. It comes with a price, of course, and that is a lower operational and maximum applied voltage rating. With proper design in circuits like power supplies, metallized-dielectric capacitors can facilitate great space savings. A major benefit of metallized-dielectric capacitors is an ability to self-heal when the plate material develops a short circuit through the dielectric. Metallized-Dielectric & Standard Capacitors
Capacitor Value Change over Temperature The use of metallized paper, film, and dual dielectrics normally results in a capacitor much smaller than foil-type capacitors of comparable rating. In some cases, a volume savings of up to 75% may be realized by the selection of a metallized dielectric capacitor. The small size and self-healing properties of metallized dielectric capacitors make them an ideal choice in many applications. The advantages of metallized capacitors, however, cannot be utilized in all situations. Metallized capacitors are not recommended for coupling applications, logic circuits, or other applications where occasional momentary break-downs and periods of low insulation resistance (sparking and subsequent self-healing) cannot be tolerated. The maximum value of the a.c. component that may be applied to the capacitor is limited by both the test voltage rating and by the limited heat conductivity of the thin metallized surfaces. Instantaneous surge voltages must not exceed the maximum, or sparking, voltage rating of the capacitor. Metallized capacitors are widely used in power-supply filter circuits, bypass applications, and decoupling and smoothing applications where low impedance is required. The graph illustrates the capacitance-temperature characteristics of paper, polyester, polycarbonate, and paper-polyester metallized dielectrics and metallized combinations. Standard Capacitors A properly designed air capacitor approaches the ideal standard reactance in that it has very low loss and very small changes with time, frequency, and environment. Capacitance changes with atmospheric pressure and relative humidity can be eliminated by hermetic sealing. Changes with temperature can be eliminated by the use of low temperature coefficient materials in the capacitor. For values above 1000 pf., solid dielectrics are used. The preferred dielectric is high-quality mica because of its dimensional stability, low loss, and high dielectric strength. At d.c. or extremely low frequencies, the mica dielectric has the disadvantage of relatively large change of capacitance with frequency as a result of dielectric absorption caused by interfacial polarization in the dielectric. Polystyrene, on the other hand, has a dielectric constant and dissipation factor very nearly constant with frequency, so that capacitance change from d.c. to 1 kc. is a small fraction of a percent instead of the 3% drop at 1 kc. typical of mica. However, the temperature coefficient of a polystyrene capacitor is on the order of -140 PPM°C. Uncertainties in the calibrated value of a two-terminal capacitor can be on the order of tenths of a pf. if the geometry, not only of the capacitor plates, but also of the environment and of the connections is not defined and specified with sufficient precision. For capacitors of 100 pf. and more, the capacitance is usually adequately defined for an accuracy of a few hundredths percent if the terminals and method of connection are specified. For smaller capacitances, or higher accuracy, a two-terminal is seldom practical and a three-terminal unit is preferred. A three-terminal capacitor has a shield that completely surrounds at least one of its terminals, its connecting wires, and its plates, except for the area that produces the desired direct capacitance to the other terminals. Changes in the environment and connections can vary the terminal capacitance but the direct capacitance is determined only by the internal geometry. The direct capacitance can be made as small as desired since the shield between the terminals can be complete except for a suitably small aperture. The losses in the direct capacitance can also be made very low because the dielectric losses in the insulating materials can be made a part of the terminal impedances. When a three-terminal capacitor is connected as a two-terminal device, the two-terminal capacitance will exceed the calibrated three-terminal value by at least the terminal capacitance. Although the characteristics of the high-quality capacitors used as standards closely approach those of the ideal capacitor, any deviations from ideal performance must be evaluated to obtain the necessary high accuracy.
Posted September 28, 2022 |
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