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384 ⏐⏐⏐ CAPACITORS
FIG. 10.11
Mica capacitors. (Courtesy of Custom
Electronics Inc.)
(a)
one sheet times the number of dielectric sheets. The entire system is
encased in a plastic insulating material as shown for the two central
units of Fig. 10.11. The mica capacitor exhibits excellent characteristics
under stress of temperature variations and high voltage applications (its
dielectric strength is 5000 V/mil). Its leakage current is also very small
(R leakage about 1000 M�). Mica capacitors are typically between a few
picofarads and 0.2 mF, with voltages of 100 V or more.
The ability to “roll” the mica to form the cylindrical shapes of Fig.
10.11 is due to a process whereby the soluble contaminants in natural
mica are removed, leaving a paperlike structure resulting from the cohesive
forces in natural mica. It is commonly referred to as reconstituted
mica, although the terminology does not mean “recycled” or “secondhand”
mica. For some of the units in the photograph, different levels of
capacitance are available between different sets of terminals.
The ceramic capacitor is made in many shapes and sizes, two of
which are shown in Fig. 10.12. The basic construction, however, is
about the same for each, as shown in Fig. 10.13. A ceramic base is
coated on two sides with a metal, such as copper or silver, to act as the
two plates. The leads are then attached through electrodes to the plates.
An insulating coating of ceramic or plastic is then applied over the
plates and dielectric. Ceramic capacitors also have a very low leakage
current (R leakage about 1000 M�) and can be used in both dc and ac networks.
They can be found in values ranging from a few picofarads to
perhaps 2 mF, with very high working voltages such as 5000 V or more.
In recent years there has been increasing interest in monolithic
(single-structure) chip capacitors such as those appearing in Fig.
10.14(a) due to their application on hybrid circuitry [networks using
both discrete and integrated circuit (IC) components]. There has also
been increasing use of microstrip (strip-line) circuitry such as the one in
Fig. 10.14(b). Note the small chips in this cutaway section. The L and
H of Fig. 10.14(a) indicate the level of capacitance. For example, if in
black ink, the letter H represents 16 units of capacitance (in picofarads),
or 16 pF. If blue ink is used, a multiplier of 100 is applied, resulting in
1600 pF. Although the size is similar, the type of ceramic material controls
the capacitance level.
The electrolytic capacitor is used most commonly in situations
where capacitances of the order of one to several thousand microfarads
Lead wire soldered
to silver electrode
Solder
Ceramic dielectric
FIG. 10.12
Ceramic disc capacitors: (a) photograph; (b) construction.
(b)
Dipped phenolic coating
Silver electrodes deposited on
top and bottom of ceramic disc