Ceramic Capacitors :-
• Ceramic
Capacitors
or Disc Capacitors are made by coating two
sides of a small porcelain or
ceramic disc with silver and are then
stacked them together .
• For
very low capacitance values a single ceramic disc of about 3-
6mm is used.
• Ceramic
capacitors have a high dielectric constant and are used
so that relatively high
capacitance can be obtained in a small
physical size. Ceramic capacitors
have values ranging from a few
picofarads to one or two microfarads ( μF ).
• They
exhibit large non-linear changes in capacitance against
temperature and hence,
used as de-coupling or by-pass
capacitors as they are also non-polarized
devices.
• Ceramic
types of capacitors generally have a 3-digit code
printed onto their body to
identify their capacitance value in
pico-farads. Generally the first two digits
indicate the capacitors
value and the third digit indicates the number of
zero’s to be
added. For example, a ceramic disc capacitor with the
markings 103 would indicate 10 and 3 zero’s in pico-farads
which is equivalent to 10,000 pF or 10nF. Letter
codes are
sometimes used to indicate their tolerance value such as: J =
5%, K = 10% or M = 20% etc.
Ceramic Capacitors :-
Electrolytic Capacitors :-
• Electrolytic
Capacitors
are generally used
when very large capacitance values are
required.
• Here
instead of using a very thin metallic
film layer for one of the electrodes, a
semi-
liquid electrolyte solution in the form of a
jelly or paste is used which
serves as the
second electrode (usually the cathode).
• The
dielectric is a very thin layer of oxide
which is grown electro-chemically in
production with the thickness of the film
being less than ten microns. This
insulating
layer is so thin that it is possible to make
capacitors with a large
value of capacitance
for a small physical size as the distance
between the
plates, d is very small.
• Most
of the electrolytic types of capacitors are Polarised ,
which means the DC voltage applied to the
capacitor terminals must be of the
correct
polarity, i.e. positive to the positive
terminal and negative to the
negative
terminal as an incorrect polarisation
will
break down the insulating oxide
layer and permanently damage the
capacitor.
• Hence,
all polarised
electrolytic
capacitors have their polarity clearly
marked with a negative sign
to indicate
the negative terminal and this polarity
must be followed.
• Electrolytic Capacitors are generally
used in DC power
supply circuits due to their large capacitance’s and small
size to help reduce the ripple voltage or for coupling and
decoupling
applications.
• One
main disadvantage of electrolytic capacitors is their
relatively low voltage
rating and due to the polarisation of
electrolytic capacitors, which means that they must not be
used on AC supplies.
• The
other drawbacks of electrolytic capacitors are large
leakage currents,
value tolerances, equivalent series
resistance and a limited lifetime.
• Electrolytic
capacitors can be either wet-electrolyte or
solid polymer.
• They
are commonly made of tantalum or aluminum,
although other materials may be
used.
• Supercapacitors are
a special subtype of electrolytic
capacitors, also called double-layer
electrolytic
capacitors, with capacitances of hundreds and
thousands of farads.
• Electrolytic
capacitors generally come
in two basic forms ;
• Aluminium
Electrolytic Capacitors.
• Tantalum
Electrolytic Capacitors.
1. Aluminium Electrolytic Capacitors :-
•There
are basically two types of Aluminium
Electrolytic Capacitor, the plain foil
type and the
etched foil type.
• The
thickness of the aluminium
oxide film and
high breakdown voltage give these capacitors very
high
capacitance values for their size.
• The
foil plates of the capacitor are anodized with
a DC current. This anodizing
process sets up the
polarity of the plate material and determines
which side of
the plate is positive and which side
is negative.
•The
etched foil type differs from the plain foil
type in that the aluminium
oxide on the anode
and cathode foils has been chemically etched to
increase its
surface area and permittivity.
•This
gives a smaller sized capacitor than a plain
foil type of equivalent value but
has the
disadvantage of not being able to withstand high
DC currents compared
to the plain type. Also
their tolerance range is quite large at up to 20%.
•Typical
values of capacitance for an aluminium
electrolytic capacitor range from 1uF up
to
47,000uF.
•Etched
foil electrolytic’s are
best used in
coupling, DC blocking and by-pass circuits while
plain foil types
are better suited as smoothing
capacitors in power supplies.
2. Tantalum Electrolytic Capacitors :-
• Tantalum
Electrolytic Capacitors are available in
both wet (foil) and dry (solid)
electrolytic types
with the dry or solid tantalum being the most
common.
• Solid
tantalum capacitors use manganese
dioxide as their second terminal and are
physically smaller than the equivalent aluminium
capacitors.
• The
dielectric properties of tantalum oxide is
also much better than those of aluminium
oxide giving a lower leakage currents and
better capacitance stability which
makes them
suitable for use in blocking, by-passing,
decoupling, filtering and
timing applications.
• Also,
Tantalum Capacitors although
polarised, can tolerate being connected to a
reverse voltage much more easily than the
aluminium types but are rated at much lower
working voltages.
• Solid
tantalum capacitors are usually used in
circuits where the AC voltage is small
compared to the DC voltage.
•However,
some tantalum capacitor types
contain two capacitors in-one, connected
negative-to-negative to form a “non-
polarised” capacitor for use in low voltage AC
circuits as a non-polarised
device.
• Generally,
the positive lead is identified on
the capacitor body by a polarity mark, with
the body of a tantalum bead capacitor being
an oval geometrical shape.
• Typical
values of capacitance range from
47nF to 470uF.
•The
etched foil type differs from the plain foil
type in that the aluminium
oxide on the anode
and cathode foils has been chemically etched to
increase its
surface area and permittivity.
•This
gives a smaller sized capacitor than a plain
foil type of equivalent value but
has the
disadvantage of not being able to withstand high
DC currents compared
to the plain type. Also
their tolerance range is quite large at up to 20%.
•Typical
values of capacitance for an aluminium
electrolytic capacitor range from 1uF up
to
47,000uF.
•Etched
foil electrolytic’s are
best used in
coupling, DC blocking and by-pass circuits while
plain foil types
are better suited as smoothing
capacitors in power supplies.
2. Tantalum Electrolytic Capacitors :-
• Tantalum
Electrolytic Capacitors are available in
both wet (foil) and dry (solid)
electrolytic types
with the dry or solid tantalum being the most
common.
• Solid
tantalum capacitors use manganese
dioxide as their second terminal and are
physically smaller than the equivalent aluminium
capacitors.
• The
dielectric properties of tantalum oxide is
also much better than those of aluminium
oxide giving a lower leakage currents and
better capacitance stability which
makes them
suitable for use in blocking, by-passing,
decoupling, filtering and
timing applications.
• Also,
Tantalum Capacitors although
polarised, can tolerate being connected to a
reverse voltage much more easily than the
aluminium types but are rated at much lower
working voltages.
• Solid
tantalum capacitors are usually used in
circuits where the AC voltage is small
compared to the DC voltage.
•However,
some tantalum capacitor types
contain two capacitors in-one, connected
negative-to-negative to form a “non-
polarised” capacitor for use in low voltage AC
circuits as a non-polarised
device.
• Generally,
the positive lead is identified on
the capacitor body by a polarity mark, with
the body of a tantalum bead capacitor being
an oval geometrical shape.
• Typical
values of capacitance range from
47nF to 470uF.
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