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Dipped
phenolic coating
Ceramic
dielectric
Lead wire
soldered to
electrode pickup
Metal
electrodes
(Alternately deposited layers of
ceramic dielectric material and
metal electrodes fired into a
single homogeneous block)
Solder
Electrode
pickup
FIG. 10.13
Multilayer, radial-lead ceramic capacitor.
are required. They are designed primarily for use in networks where
only dc voltages will be applied across the capacitor because they have
good insulating characteristics (high leakage current) between the
plates in one direction but take on the characteristics of a conductor in
the other direction. Electrolytic capacitors are available that can be used
in ac circuits (for starting motors) and in cases where the polarity of the
dc voltage will reverse across the capacitor for short periods of time.
The basic construction of the electrolytic capacitor consists of a roll
of aluminum foil coated on one side with an aluminum oxide, the aluminum
being the positive plate and the oxide the dielectric. A layer of
paper or gauze saturated with an electrolyte is placed over the aluminum
oxide on the positive plate. Another layer of aluminum without
the oxide coating is then placed over this layer to assume the role of
the negative plate. In most cases the negative plate is connected
directly to the aluminum container, which then serves as the negative
terminal for external connections. Because of the size of the roll of
aluminum foil, the overall area of this capacitor is large; and due to
the use of an oxide as the dielectric, the distance between the plates is
extremely small. The negative terminal of the electrolytic capacitor is
usually the one with no visible identification on the casing. The positive
is usually indicated by such designs as �, �, �, and so on. Due
to the polarity requirement, the symbol for an electrolytic capacitor
will normally appear as � .
Associated with each electrolytic capacitor are the dc working voltage
and the surge voltage. The working voltage is the voltage that can
be applied across the capacitor for long periods of time without breakdown.
The surge voltage is the maximum dc voltage that can be applied
for a short period of time. Electrolytic capacitors are characterized
as having low breakdown voltages and high leakage currents (Rleakage about 1 M�). Various types of electrolytic capacitors are shown in Fig.
10.15. They can be found in values extending from a few microfarads
to several thousand microfarads and working voltages as high as 500 V.
However, increased levels of voltage are normally associated with
lower values of available capacitance.
TYPES OF CAPACITORS ⏐⏐⏐ 385
FIG. 10.14
Monolithic chip capacitors. (Courtesy of
Vitramon, Inc.)