Ceramic capacitors
Porcelain is another material that works well as a dielectric. Sheets of metal are stacked
alternately with wafers of ceramic to make these capacitors. The meshing/layering
geometry of Fig. 11-6 is used. Ceramic, like mica, has quite low loss, and therefore allows
for high efficiency.
For low values of capacitance, just one layer of ceramic is needed, and two metal
plates can be glued to the disk-shaped porcelain, one on each side. This type of component
is known as a disk-ceramic capacitor. Alternatively, a tube or cylinder of ceramic
can be employed, and metal ink applied to the inside and outside of the tube. Such units
are called tubular capacitors.
Ceramic capacitors have values ranging from a few picofarads to about 0.5 μF.
Their voltage ratings are comparable to those of paper capacitors.
Plastic-film capacitors
Various different plastics make good dielectrics for the manufacture of capacitors. Polyester,
polyethylene, and polystyrene are commonly used. The substance called mylar that you might have seen used to tint windows makes a good dielectric for capacitors.
The method of manufacture is similar to that for paper capacitors when the plastic
is flexible. Stacking methods can be used if the plastic is more rigid. The geometries can
vary, and these capacitors are therefore found in several different shapes.
Capacitance values for plastic-film units range from about 50 pF to several tens of
microfarads. Most often they are in the range of 0.001 μF to 10 μF. Plastic capacitors are
employed at audio and radio frequencies, and at low to moderate voltages. The efficiency
is good, although not as high as that for mica-dielectric or air-dielectric units.
Electrolytic capacitors
All of the above-mentioned types of capacitors provide relatively small values of capacitance. They are also nonpolarized, meaning that they can be hooked up in a circuit in
either direction. An electrolytic capacitor provides considerably greater capacitance
than any of the above types, but it must be connected in the proper direction in a circuit
to work right. Therefore, an electrolytic capacitor is a polarized component.
Electrolytic capacitors are made by rolling up aluminum foil strips, separated by
paper saturated with an electrolyte liquid. The electrolyte is a conducting solution. When dc flows through the component, the aluminum oxidizes because of the electrolyte.
The oxide layer is nonconducting, and forms the dielectric for the capacitor.The layer is extremely thin, and this results in a high capacitance per unit volume.
Electrolytic capacitors can have values up to thousands of microfarads, and some
units can handle thousands of volts. These capacitors are most often seen in audiofrequency
circuits and in dc power supplies.
Tantalum capacitors
Another type of electrolytic capacitor uses tantalum rather than aluminum. The tantalum
can be foil, as is the aluminum in a conventional electrolytic capacitor. It might also
take the form of a porous pellet, the irregular surface of which provides a large area in
a small volume. An extremely thin oxide layer forms on the tantalum.
Tantalum capacitors have high reliability and excellent efficiency. They are often
used in military applications because they do not fail often. They can be used in audio-
frequency and digital circuits in place of aluminum electrolytics.
