Ceramic capacitors, Physics

As the name indicates, the dielectric of this type of capacitor is a ceramic. The basis of ceramic material is mainly barium titanate or a similar material, but other ceramic substances including hydrous silicate of magnesia or talc are also used. The electrodes are applied in the form of silver which is either sprayed or plated on to the opposite faces of a thin tube, wafer or disc made from the ceramic material. Connecting wires are then soldered to this deposit and the entire capacitor dropped in for a appropriate coating. These capacitors tend to have low values of inductance because of their small size. These capacitors can be divided into three classes:

(a)         Low loss low permittivity type capacitor

(b)         Medium permittivity type capacitor

(c)         High permittivity type capacitor

LOW LOSS LOW PERMITTIVITY TYPE: These lave low loss. Low permittivity usually made from steatite material. It is finely ground, mixed. Compressed and heated to 9000C to remove impurities. It is calcined at a temperature below melting point. The resultant mass is reground. It is then mixed with water, gum and oil and form into stiff paste. With pressure mouldings and extrusions capacitors bodies can be formed. These capacitors can be made in disc, tubular and rectangular formed. Leads are then fixed by soldering. Organic protective coating is providing. Value is measured and colour coding is provided.

Medium permittivity type capacitor (type 1): These dielectrics are based on titanium. They are hard, refractory and chemically inert. The method of manufacturing is same as low permittivity types. This change in capacitance with temperature is 800ppm/0C for TiO2. Power factor is less than 0.003 at RF. Using suitable mixture of titanium and alkali earths, temperature coefficient of capacitor can be controlled to zero or positive.

Specifications

Range of values                                           1000 to 1300pf

Power factor                                                 0.0003

Temperature coefficient                               -8000ppm/0C to + 100ppm/0C

High permittivity type capacitor (type 2): Pure barium titanate or mixed metals can be used as a dielectric. These are manufactured in same manner. These capacitors have relatively large capacitance in small volume. These capacitors are used for common purpose coupling and decoupling where there are wide variations in capacitance value because of temperature. Frequency, voltage and time can be tolerated.

Specifications

Permittivity                                     3000

Range of values                               greater than 1000pf

Operating voltage                               500 V

Posted Date: 7/9/2012 6:38:24 AM | Location : United States







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