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Note:
The information below is basic electrical theory information, but the devices
referred to and their construction relates to the types of components found, for
the most part, in vacuum tube radios. Technology has changed very rapidly
and though the component structure has changed dramatically, the function and
use of a capacitor is still the same.
An electrical capacitor
(used to be known as a condenser) stores electricity by accumulating free
electrons on a metal surface, and then releases them as a current into the
circuit of which the capacitor is a part.
Capacitance is a property of
a circuit in which energy may be stored in the form of an electric field.
Although capacitors are used for many different purposes, every use entails the
storage and release of electrical energy. Capacitance is generally designated by
the letter "C"
Capacitance
is the property of an electric circuit that tends to oppose a change in voltage
The action of storing electricity in
a capacitor is called charging. The electricity stored in the
capacitor is called the charge. The "charge" refers to
the amount of electricity being stored. The action of releasing the stored
electricity is called discharging.
See the diagram above. We have
two thin sheets of aluminum 10 inches square separated by air and positioned
about 1/2 inch apart. Because air separates the two active plates
(conductors) of the capacitor, the unit is called an air-dielectric
capacitor.
At first we assume the plates are
electrically neutral ...each plate contains equal amounts of positive and
negative electricity. The capacitor is therefore in an uncharged state.
In the process of charging, one plate is made to give up free electrons and be
left with a positive charge. The other plate is made to accept many free
electrons released by the positive plate, and now has a surplus of
electrons. This electrical condition is created at the same time.
When this situation prevails, the capacitor is said to contain a charge,
or be charged.
Three Factors that determine
capacitance of a capacitor include:
1. the area of plate surfaces.
2. the distance between the plates
3. the material used as the insulation or dielectric between the plates.
All other things being equal, the
greater the area, the greater the amount of charge that can be stored in the
capacitor.
For example, all else being fixed, doubling the surface area doubles the
capacitance.
All other things being equal, the
closer the plates are to each other, the greater the capacitance.
This is an inverse proportion. Halving the area of separation doubles the
capacitance. Doubling the area of separation halves the capacitance.
The reason for this is that the closer the facing plates are to one another, the
more strongly the unlike charges on the surfaces are attracted towards each
other. This tends to concentrate the free electrons on the negatively
charged surface nearest the positively charged surface, thus allowing more
negative charges to be crowded onto a plate or plates of a given area.
All other thing being equal, the
capacitance is a function of the dielectric or insulation material between the
plates. The standard for comparison is dry air, which has a dielectric
constant (K) of 1. The Dielectric constant is the ability of a
material to permit the establishment of electric lines of force between
oppositely charged plates. Materials vary greatly from air. Mica had
a Dielectric constant of from 5 to 9 and some forms of titanium dioxide have a K
of up to 120! A dielectric (other than air) makes the positively charged
surface accept more electrons than when air is the dielectric, thus increasing
the capacitance.
The dielectric material has one important
characteristic that should be considered... that of breakdown voltage.
Even though the dielectric is an insulator, voltages across the plates of a
capacitor may be sufficiently high to "tear" electrons out of the atomic orbits
of the dielectric. When this takes place, the dielectric "breaks down,"
and arcing occurs between the plates through the dielectric. Most of the
time this destroys the capacitor, as it is now short-circuited. Therefore
it is important to be aware of the dielectric strength of a material. A
high voltage would be needed to break down a vacuum dielectric, but lower
voltages could bread down certain other substances. See the chart above.
FIXED CAPACITORS - Paper
Type
Working with antique radios, it will not be long before you will come across a
very common type of capacitor that often needs to be replaced. This is
the paper type
capacitor which basically consists of two strips of metal foil rolled up, with
strips of paper which have been impregnated with an insulating material (a.k.a.
dielectric) placed between them.
The dielectric materials generally consist of a variety of oils, waxes, and
plastics. The type used determines the voltage, temperature, and
insulation-resistance characteristics of the capacitor. When the capacitor
is to be used at high working voltages, several layers of insulating paper are
used.
After the foil and paper strips are rolled up, the protruding ends of the foil
are crimped over so that the individual layers of each strip are in electrical
contact with each other. A lead is attached to each end, and an outer
cover of insulating material is added. The cover is marked with the
capacitance and working voltage, and a black ring is usually printed around one
end to mark the terminal which is connected to the outermost layer of the foil.
In paper
capacitors, the total capacitance is predetermined by the thickness and
dielectric constant of the paper and the total surface are of the foil plates.
Capacitors are
usually marked with a d-c working voltage (DCWV) which must be observed.
Though the working voltage of any replacement capacitor should be equal to the
one being replaced, it is generally okay if it is greater...just make sure it is
not rated at less voltage. Generally, the capacitance of a
replacement capacitor should be within 10 to 20% of the capacitance of the
original.
When paper capacitors are required to have a capacitance of over 1µF ( one
microfarad), their physical size generally becomes too large for convenient
mounting. Under such conditions, the capacitor is placed in a metal case
filled with insulating material and sealed. Units of this type were known
as potted or bathtub capacitors.
NOTE: Today's capacitors are MUCH, MUCH smaller than the ones they
replace. More efficient dielectrics and better
insulating materials have led to the significant decrease in size. Some
radio restorers, in order to keep the appearance of the
radio as original as possible, will often drill or melt out the old capacitor
and insert the new (smaller) capacitor inside the casing. This is not an
easy job... and I personally do not do it... so much for not being a purest...
but I certainly appreciate the
work of those who choose to go this route.
of the old capacitor.
FIXED CAPACITORS - Mica and Ceramic
The mica capacitor consists of a number of flat strips of metal foil separated
by similarly shaped strips of mica.
The foil strips serve as the capacitor plates, and the mica acts as the
dielectric. Alternate plates are connected together. An electrode is
attached to each set of plates, and a terminal or lead wire is connected to each
electrode. The entire unit is then encased in a container of plastic
insulating material.
An alternative
"silvered" mica capacitor is produced with very thin layers of silver deposited
directly on one side of the mica,
and the plates are stacked
together so that alternate layers of silver are separated by alternate layers of
mica. The result is the equivalent of the foil construction. Mica
capacitors are available in three basic types: molded, molded-case potted, and
ceramic case potted. In addition, the "button" type mica unit was very
popular.
The basic construction of
the ceramic capacitor consists of a ceramic disc or tube with silver or copper
plates deposited on the opposite faces of the ceramic material. Electrodes
are attached to the plates, leads or terminals are fastened to the electrodes,
and a moisture proof coating of plastic or ceramic is added. The
outstanding characteristic of ceramic capacitors is the high dielectric constant
of the ceramics. Dielectric constants of 6, 16, and 1200 allow increased
capacitance without increased size. In addition ceramic capacitors have
good stability under temperature and voltage changes.
COMING NEXT... Capacitor Color Coding and Temperature Coefficient | 
