Capacitors:

        Capacitance plays an important part in the electrical and 
electronic fields. In its simplest form, a capacitor consists of two
conductive plates separated by a dielectric. The capacitance of a 
capacitor varies directly with the size of the plates and the dielectric
constant, and inversely with the spacing between the plates. This
means that increasing the plate area or using a dielectric with a higher
dielectric constant increases the capacitance.

        
        The electrical characteristics of a capacitor can be described
in terms of capacitance, tolerance, voltage rating and temperature
coeffecient. Capacitors come in many different shapes and sizes due to
the different types of construction, characteristics and use.


        Practically every electronic device, no matter how small or 
simple it may be, has at least one of each of the following components: 
Inductors, Resistors, and Capacitors. The early experimenters called these 
units condensers, a term which is still sometimes used. However, the name 
capacitor more accurately describes the electric action, and it is 
now the more commonly used name of the two.

        One definition for capacitance is that it is the ability of a 
circuit (or device) to store electric energy in an electric field.

        Just as the inductor exhibits the property of inductance and the 
resistor has resistance, the Capacitor is the electrical device 
that has the property of Capacitance.

        In general, a capacitor consists of two conducting surfaces
separated by an insulating material. The conducting surfaces are
called the Plates of the capacitor, and the insulating material
is called the Dielectric. This dielectric material can be any
good insulator, including air. The structure and composition may
vary quite a bit from one type of capacitor to the next, but it
is the purpose of all capacitors to store electric energy supplied
by some external source.

Charging or storing action:

        When voltage is applied to a capacitor, electrons collect on the
plates. This collection of electrons continues until the capacitor plates
are at the same potentials as the power supplied. In other words the
voltage reaches maximum, at this time there is little or no movement
of electrons, as long as the voltage stays at the same polarity and
value. When the voltage is disconnected, (if the circuit is opened), 
there is no circuit for the electrons to move through, the capacitor is
said to be charged. The electrons stay on the plates at whatever the
polarity they were applied. The capacitor will hold its charge until
a suitable discharge path is provided. In order for discharge to take
place, there must be a complete circuit between the plates. When the
path is provided, the electrons move from the more negative plate to 
the positive plate until the difference in potential is equal or zero.
The capacitor is said to be discharged. There is no difference of
potential between the plates, and current flow stops.

        When the applied voltage increases, the capacitor charges to a
higher value, when the applied voltage decreases, the capacitor discharges
keeping the voltage up for a period of time.

        The cycles of storing and discharging the displacement current
electrons are the practical demonstration of the capacitor's ability
to oppose any change in circuit voltage. This ability is defined as
Capacitance. As with resistance and inductance, capacitance also
has a unit of measure. The unit of measure for the capacitance is the
Farad, abbreviated (F) or fd. Named after Michael Faraday,
The Farad Represents the Capacitance of a Capacitor which accepts a
charge of one Coulomb when a voltage of one volt is applied.
The common unit of electric current, the ampere, is equal to one 
coulomb per second. Since a coulomb represents about six times 10 to
the eighteenth power, or 6,000,000,000,000,000,000 electrons, a farad
is far too large for most practical purposes. For most ordinary needs,
it is more convenient in expressing capacitance values to use a smaller
unit, the Microfarad (mfd), which is equal to one-millionth of
a farad.

        Many capacitors used in electronic circuits have capacitances
much less than even a microfarad. To avoid fractions or decimals, a
smaller unit is commonly used. This unit is the micro-microfarad (mmfd)
also called the Picofarad. Currently the Picofarad is used more
often than the micromicrofarad. You will more often find Pf on capacitor
values than mmfd. One picofarad is equal to one-millionth of a microfarad,
or one-millionth of a millionth of a farad. To change farads to microfarads
or microfarads to picofarads, move the decimal point six places to the 
right.

        When a capacitor is connected across a battery or other source
of voltage, work is done in charging the capacitor; this work is stored
as electric energy, when it dischages, the capacitor returns this stored
energy to the circuit. The Charge of the capacitor is a measure
of the energy it can store, which corresponds to the quantity of electrons
transferred when a given amount of voltage is applied. Since capacitance
of one farad represents one coulomb per volt, the expression may be 
written as an equation:


                              Q
                        C = -----
                              E

                        C is the capacitance in farads

                        Q is the charge in coulombs

                        E is the applied voltage in volts

        Like ohm's law, the capacitor charge expression contains three
quantities, and may be used to determine the unknown quantity when the
other two are known:

                             Q
                        E = ----
                             C



                        Q = CE


Capacitor Voltage Ratings:

        If too much voltage is applied to a capacitor, it will distort
the dielectric molecules until electrons are torn from the atoms. These
free electrons flow through the dielectric from one plate to the other,
destroying the action of the capacitor. In some cases, this current may
create enough heat to actually burn a hole through the dielectric. The
amount of voltage that will produce this effect is called the Breakdown
voltage of the dielectric or capacitor. The ability of a dielectric
material of a given thickness to withstand a voltage breakdown is called
its Dielectric Strength. To safegaurd against this type of breakdown
capacitors must be operated at voltages below the dielectric breakdown.
The maximum voltage that can be applied steadily to a capacitor without
damage is called the Working Voltage of the capacitor. This working
voltage is determined by the dielectric strength and thickness of the
dielectric material.

        No known materials are perfect insulators. Therefore, after a
capacitor is charged, there will be a small Leakage current
through the dielectric, causing the charge to leak-off in time. Some
capacitors hold a charge for several minutes, while others retain a 
charge for several hours, or even months, depending on the degree of
leakage.

        Often, the dielectric characteristics are altered due to aging,
heat, and humidity. Leakage and the dielectric constant also tend to
change with time. Since capacitance depends upon the dielectric constant,
it also changes with time.


Capacitors in Parallel:

        Capacitors are often combined in series or parallel groups to
produce various values of capacitance:

        In a Parallel Capacitor arrangement, the Total Capacitance
        is Equal to the Sum of the Individual Capacitance Values.

                Ct = C1 + C2 + C3 +. . . . .

Capacitors in Series:

        The Formula for finding the Capacitance of Series connected
        Capacitors is The Same As Parallel Resistors. The Reciprocal of
        the total equals the sum of the Reciprocals of the Individual
        Capacitors:


                   1         1      1      1      1
                ------   = ---- + ---- + ---- + ---- ...........
                 Ctotal     C1     C2     C3     C4


Fixed Capacitors:

        To fulfill the different requirements in electrical and electronic
equipment, many types of capacitors are manufactured. These types differ
mainly in construction of the plates and the nature of the dielectric
material. Capacitors having a definite fixed value , with no provision for
adjustment, are known as fixed capacitors.

        The basic fixed capacitor consists of two conducting plates separated
by a suitable dielectric. One of these capacitor types is the paper type
capacitor, so named because the basic form includes one or more strips of
waxed paper for the dielectric. Using strips of foil as the conducting 
plates and rolling the entire unit into a compact cylinder. Capacitors
of this type are economical to manufacture, and they are available in a
wide range of values.

        When a rolled paper type capacitor is enclosed in a container
one lead is usually designated as the outside foil. This type of capacitor
will not be damaged it its leads are reversed in a circuit, but it is
customary to connect the outside lead to the grounded side of the circuit
whenever possible. As the term implies, this is the lead connected to the
outer foil of the roll, and there is a shielding effect when this foil 
rather than the inside is grounded. The lead of the outer foil may be
designated by a marking band, dot, or by the word ground printed near 
one lead of the capacitor.
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