Capacitors are commercially available in different values and types.
Typically, capacitors have values in the picofarad (pF) to microfarad (µF)
range. They are described by the dielectric material they are made of and
by whether they are of fixed or variable type. [Fig. 1]
shows the circuit
symbols for fixed and variable capacitors.
Fig. 1: Circuit symbols for capacitors: (a) fixed capacitor, (b) variable capacitor.
Note that according to the
passive sign convention, current is considered to flow into the positive
terminal of the capacitor when the capacitor is being charged, and out of
the positive terminal when the capacitor is discharging.
Like resistors, all capacitors can be included under either of two general
headings: fixed or variable. The curved line represents the plate that
is usually connected to the point of lower potential.
Many types of fixed capacitors are available today. Some of the most
common are the mica, ceramic, electrolytic, tantalum, and polyester film capacitors.
: The typical flat mica capacitor consists basically of
mica sheets separated by sheets of metal foil. The plates are connected
to two electrodes, as shown in [Fig. 2]
Fig. 2: Basic structure of a stacked mica capacitor
Fig. 3: Mica capacitors.
The total area is the area of 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. 3]
. 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
(1000 MΩ). Mica capacitors are typically between a few
picofarads and $0.2 \mu F$, with voltages of 100 V or more.
: The ceramic capacitor
is made in many shapes and sizes, two of
which are shown in [Fig. 4]
. 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 ( 1000 MΩ) and can be used in both dc and ac networks. They can be found in values ranging from a few picofarads to perhaps $2 \mu F$, with very high working voltages such as $5000 V$ or more.
Fig. 4: Ceramic capacitor
: The electrolytic capacitor is used most commonly in situations where capacitances of the order of one to several thousand microfarads 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.
Fig. 5: Electrolyte Capacitor
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.
There are fundamentally two types of tantalum capacitors
: the solid
and the wet-slug. In each case, tantalum powder of high purity is
pressed into a rectangular or cylindrical shape, as shown in [Fig. 6]
Next the anode (+) connection is simply pressed into the resulting
structures, as shown in the figure. The resulting unit is then sintered
(baked) in a vacuum at very high temperatures to establish a very
porous material. The result is a structure with a very large surface area
in a limited volume. Through immersion in an acid solution, a very thin
manganese dioxide ($MnO_2$) coating is established on the large, porous
surface area. An electrolyte is then added to establish contact between
the surface area and the cathode, producing a solid tantalum capacitor.
If an appropriate "wet" acid is introduced, it is called a wet-slug tantalum capacitor.
Fig. 6: Tantalum capacitors
: The last type of fixed capacitor to be introduced is the polyester-film
capacitor, the basic construction of which is shown in [Fig. 7]
consists simply of two metal foils separated by a strip of polyester
material such as Mylar. The outside layer of polyester is applied to act
as an insulating jacket. Each metal foil is connected to a lead that
extends either axially or radially from the capacitor. The rolled construction results in a large surface area, and the use of the plastic dielectric results in a very thin layer between the conducting surfaces.
Fig. 7: Polyester capacitors
Data such as capacitance and working voltage are printed on the
outer wrapping if the polyester capacitor is large enough. Color coding
is used on smaller devices. A band (usually black) is
sometimes printed near the lead that is connected to the outer metal foil.
The lead nearest this band should always be connected to the point of lower potential. This capacitor can be used for both dc and ac networks. Its leakage resistance is of the order of $100 MΩ$.
An axial lead and radial lead polyester-film capacitor appear in [Fig. 8]
. The axial lead
variety is available with capacitance levels of $0.1 \mu F$ to $18 \mu F$, with
working voltages extending to $630 V$. The radial lead variety has a
capacitance range of $0.01 \mu F$ to $10 \mu F$, with working voltages extending to $1000 V$.
The most common of the variable-type capacitors is shown in [Fig. 8]
. The dielectric for each capacitor is air. The capacitance in [Fig. 8(a)]
is changed by turning the shaft at one end to vary the common
area of the movable and fixed plates. The greater the common area, the
larger the capacitance. The capacitance of
the trimmer capacitor in [Fig. 8(b)]
is changed by turning the screw,
which will vary the distance between the plates (the common area is
fixed) and thereby the capacitance.
Fig. 8: Variable air capacitors. (a) Tuning capacitor, (b) Trimmer capacitor