Capacitor types and their uses
- an overview, information and tutorial about the different capacitor types
and the uses these different types of electronic capacitors may be suited to as
electronic components within electronic circuits.
Electronic capacitors are one of the most widely used
electronic components. These electronic capacitors only allow alternating or
changing signals to pass through them, and as a result they find applications in
many different areas of electronic circuit design. There are a wide variety of
types of capacitor including electrolytic, ceramic, tantalum, plastic, sliver
mica, and many more. Each capacitor type has its own advantages and
disadvantages can be used in different applications.
Capacitor construction
In essence the construction of an electronic capacitor is
very simple, although in practice a lot of research and development has been put
into capacitor technology. The basic electronics components consist of two
plates that are insulated from one another. In between them there is an
insulating medium known as the dielectric. The value of the electronic capacitor
is dependent upon the area of the plates, the distance between them and the
dielectric constant of the material or dielectric between them. The greater the
area of the plates, the closer they are together and the greater the value of
the dielectric constant the greater the value of capacitance.
Today, electronic capacitors are able to provide relatively
high levels of capacitance within components that occupy a small volume. This is
achieved in a number of ways. One is to have several sets of plates, and another
is to place the plates very close to one another, having a thin layer of
dielectric placed between them. In addition to this special insulating
dielectric materials have been developed to enable high levels of capacitance to
be achieved.
The method of construction of these electronic components is
also important. In some capacitors the plates may be flat, and normally these
capacitors will have rectangular, or more exactly cuboid shapes. Some will be
tubular and in these capacitors the plates will be wound round on each other.
The reasons for these types of construction are normally dependent upon the way
in which the capacitors must be manufactured. The final stage in the
construction of an electronic capacitor is to place it in a protective casing.
In some instances it may be dipped in an insulating coating, in others it may be
contained within a metal can.
Some capacitors types are what are termed polar or polarised.
When this is the case the electronic capacitor has a positive and a negative
connection and it must be placed in circuit so that the voltage across it is in
a particular sense. If the voltage is incorrectly placed across the component
then it may be damaged. Fortunately many capacitors, and in particular low value
ones are non-polar and can be placed in circuit either way round.
Although there is a large variety that are available the most
commonly used are ceramic, plastic film types, electrolytic and tantalum. These
names refer to the type of dielectric that is used within the capacitor.
Ceramic
Ceramic capacitors are normally used for radio frequency and
some audio applications. Ceramic capacitors range in value from figures as low
as a few picofarads to around 0.1 microfarads. In view of their wide range and
suitability for RF applications they are used for coupling and decoupling
applications in particular. Here these ceramic capacitors are by far the most
commonly used type being cheap and reliable and their loss factor is
particularly low although this is dependent on the exact dielectric in use.
Their stability and tolerance is not nearly as good as silver mica types, but
their cost is much less.
There are a number of dielectrics that can be used with
ceramic capacitors. For low values a dielectric designated "COG" is normally
used. This has the lowest dielectric constant but gives the highest stability
and lowest loss. Where higher values are required in a given size, a dielectric
with a higher dielectric constant must be used. Types with designations X7R and
for higher values, Z5U are used, however their stability and loss are not as
good as the capacitors made with COG dielectric.
Silver Mica
Silver mica capacitors are not as widely used these days as
they used to be. However these electronic components can still be obtained and
are used where stability of value is of the utmost importance and where low loss
is required. In view of this one of their major uses is within the tuned
elements of circuits like oscillators, or within filters.
Values are normally in the range between a few picofarads up
to two or possibly three thousand picofarads.
For this type of capacitor the silver electrodes are plated
directly on to the mica dielectric. Again several layers are used to achieve the
required capacitance. Wires for the connections are added and then the whole
assembly is encapsulated.
Plastic film capacitors
There is a number of different types of plastic film
capacitors. Polycarbonate capacitors, polyester capacitors and polystyrene
capacitors are some of the most common. Each of these electronic components has
its own properties, allowing them to be used in specific applications. Their
values may range anywhere from several picofarads to a few microfarads dependent
upon the actual type. Normally they are non-polar. In general they are good
general-purpose capacitors that may be used for a variety of purposes, although
their high frequency performance is not usually as good as that of the ceramic
types.
Electrolytic capacitors
Electrolytic capacitors are the most popular type for values
greater than about 1 microfarad. Electrolytic capacitors are constructed using a
thin film of oxide on an aluminium foil. An electrolyte is used to make contact
with the other plate. The two plates are wound around on one another and then
placed into a can that is often aluminium.
Electrolytic capacitors are polarised, and care should be
taken to ensure they are placed in circuit the correct way round. If they are
connected incorrectly they can be damaged, and in some extreme instances they
can explode. Great care should also be taken not to exceed the rated working
voltage of the electrolytic capacitor. Normally they should be operated well
below this value. Also in power supply applications significant amounts of
current may be drawn from them. Accordingly electrolytic capacitors intended for
these applications have a ripple current rating which should also not be
exceeded. If it is, then the electronic component may become excessively hot and
fail. It is also worth noting that these components have a limited life. It is
often as little as 1000 hours at the maximum rating. This may be considerably
extended if the component is run well below its maximum rating.
Electrolytic capacitors have a wide tolerance. Typically the
value of the component may be stated with a tolerance of -50% +100%. Despite
this they are widely used in audio applications as coupling capacitors, and in
smoothing applications for power supplies.
These are normally contained in a tubular aluminium can, each
end being marked to show its polarity.
Tantalum capacitors
Ordinary aluminium electrolytic capacitors are rather large
for many uses. In applications where size is of importance tantalum capacitors
may be used. These are much smaller than the aluminium electrolytic capacitors
and instead of using a film of oxide on aluminium they us a film of oxide on
tantalum. Tantalum capacitors do not normally have high working voltages, 35V is
normally the maximum, and some even have values of only a volt or so.
Like electrolytic capacitors, tantalum capacitors are also
polarised and they are very intolerant of being reverse biased, often exploding
when placed under stress. However their small size makes them very attractive
for many applications.
Table of capacitor types and capacitor uses and applications
The most suitable way to summarise the various types of
capacitor and the applications for which these electronic capacitors are suited
is in a table.
| Application |
Suitable types |
Reasons |
| Power supply smoothing |
Aluminium electrolytic |
High capacity, high ripple
current |
| Audio frequency coupling |
Aluminium electrolytic<
Tantalum
Polyester / polycarbonate |
High capacitance
High capacitance, small size
Cheap, but values not as high as electrolytics |
| RF coupling |
Ceramic COG
Ceramic X7R
Polystyrene |
Small, cheap, low loss
Small cheap, but higher loss than COG
Very low loss, but larger than ceramic |
| RF decoupling |
Ceramic COG
Ceramic X7R |
Small, low loss. Values
limited to around 1000 pF
Small, low loss, higher values available than for COG types |
| Tuned circuits |
Silver mica
Ceramic COG |
Close tolerance, low loss
Close tolerance, low loss, although not as good as silver mica |
Summary
There is a huge number of different capacitor types and they
are one of the most widely used electronic components. While different
capacitors may have the same value, each different type of capacitor has its own
properties and this will make this particular electronic capacitor suitable for
a particular application. If the wrong type of capacitor is used, then it can
make a circuit function incorrectly. As a result, choosing an electronic
capacitor for a circuit means making more than the value calculations. Choosing
the correct capacitor type is equally important.
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