Resistors, Capacitors, and Inductors 257material (dielectric) in ribbon form rolled and
encapsulated.10.2.4.2 Paper Foil-Filled CapacitorsPaper foil-filled capacitors consist of alternate layers of
aluminum foil and paper rolled together. The paper may
be saturated with oil and the assembly mounted in an
oil-filled, hermetically sealed metal case. These capaci-
tors are often used as motor capacitors and are rated at
60 Hz.10.2.4.3 Mica CapacitorsTwo types of mica capacitors are in use. In one type,
alternate layers of metal foil and mica insulation, are
stacked together and encapsulated. In the silvered-mica
type, a silver electrode is screened on the mica insula-
tors that are then assembled and encapsulated. Mica
capacitors have small capacitance values and are
usually used in high frequency circuits.
10.2.4.4 Ceramic Capacitors
Ceramic capacitors are the most popular capacitors for
bypass and coupling applications because of their
variety of sizes, shapes, and ratings.
Ceramic capacitors also come with a variety of K
values or dielectric constant. The higher the K value, the
smaller the size of the capacitor. However, high K-value
capacitors are less stable. High-K capacitors have a
dielectric constant over 3000, are very small, and have
values between 0.001μF to several microfarads.
When temperature stability is important, capacitors
with a K in the 10–200 region are required. If a high Q
capacitor is also required, the capacitor will be physi-
cally larger. Ceramic capacitors can be made with a zero
capacitance/temperature change. These are called nega-
tive-positive-zero (NPO). They come in a capacitance
range of 1.0 pF–0.033μF.
A temperature-compensated capacitor with a desig-
nation of N750 is used when temperature compensation
is required. The 750 indicates that the capacitance will
decrease at a rate of 750 ppm/°C with a temperature
rise or the capacitance value will decrease 1.5% for a
20°C (68°F) temperature increase. N750 capacitors
come in values between 4.0 pF and 680 pF.
10.2.4.5 Electrolytic CapacitorsThe first electrolytic capacitor^ was made in Germany in
about 1895 although its principle was discovered some
25 years earlier. It was not until the late 1920s when
power supplies replaced batteries in radio receivers, that
aluminum electrolytics were used in any quantities. The
first electrolytics contained liquid electrolytes. These
wet units disappeared during the late 1930s when the
dry gel types took over.
Electrolytic capacitors are still not perfect. Low
temperatures reduce performance and can even freeze
electrolytes, while high temperatures can dry them out
and the electrolytes themselves can leak and corrode the
equipment. Also, repeated surges over the rated
working voltage, excessive ripple currents, and high
operating temperature reduce performance and shorten
capacitor life. Even with their faults, electrolytic capaci-
tors account for one-third of the total dollars spent on
capacitors, probably because they provide high capaci-
tance in small volume at a relatively low cost per micro-
farad-volt.
During the past few years, many new and important
developments have occurred. Process controls have
improved performance. Better seals have assured longer
life, improved etching has given a tenfold increase in
volume efficiencies, and leakage characteristics have
improved one hundredfold.
Basic to the construction of electrolytic capacitors is
the electrochemical formation of an oxide film on a metal
surface. Intimate contact is made with this oxide film by
means of another electrically conductive material. The
metal on which the oxide film is formed serves as the
anode or positive terminal of the capacitor; the oxide film
is the dielectric, and the cathode or negative terminal is
either a conducting liquid or a gel. The most commonly
used basic materials are aluminum and tantalum.Aluminum Electrolytic Capacitors. Aluminum elec-
trolytic capacitors use aluminum as the base material.
The surface is often etched to increase the surface area
as much as 100 times that of unetched foil, resulting in
higher capacitance in the same volume.
The type of etch pattern and the degree to which the
surface area is increased involve many carefully
controlled variables. If a fine etch pattern is desired to
achieve high capacitance per unit area of foil for low
voltage devices, the level of current density and time the
foil is exposed to the etching solution will be far
different from that required for a coarse etch pattern.
The foil is then electrochemically treated to form a layer
of aluminum oxide on its surface. Time and current
density determine the amount of power consumed in the