Post on 08-Aug-2018
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Capacitor
A capacitor is composed of two conductors separated by an insulating material called a DIELECTRIC.
The dielectric can be paper, plastic film, ceramic, air or a vacuum.
The plates can be aluminium discs, aluminium foil or a thin film of metal applied to opposite sides of a solid
dielectric.
The CONDUCTOR - DIELECTRIC - CONDUCTOR sandwich can be rolled into a cylinder or left flat
Capacitance is a measure of a capacitors ability to store charge on its plates.
A capacitor has a capacitance of 1 farad (F) if 1 coulomb (C) of charge is deposited on the plates by a
potential difference of 1 volt across its plates.
The farad is named after Michael Faraday, a nineteenth century English chemist and physicist.
The farad is generally too large a measure of capacitance for most practical applications, so the microfarad
(10-6 ) or picofarad (10-12 ) is more commonly used.
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The capacitance of a parallel-plate capacitor constructed of two parallel plates both of
area A separated by a distance d is approximately equal to the following:
where
C is the capacitance;
A is the area of overlap of the two plates;
ris the relative permittivity (for a vacuum, r= 1);
0 is the electric constant (0 8.8541012 F m1); and
d is the separation between the plates.
The factors to be considered for choosing a capacitor for any desired application are: capacitance value,
operating voltage, power loss, power factor, dielectric strength, leakage current, frequency effects
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Voltage Current relation in Capacitor
The charge stored in a capacitor is given by
Q = CV
Where
Q - charge stored by the capacitor
C - Capacitance value of the capacitor
V - Voltage applied across the capacitor
current, I = dQ/dt
Taking the derivative with respect to time,
dQ/dt = d(CV)/dt
From the above statement, we can express the equation as
I = C (dV/dt)
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Classification of Capacitors
There are different types of capacitors, they can be fixed or variable. They are categorized into two groups,
polarized or non-polarized.. Most of the low value capacitors are non-polarized.
The symbol of capacitors from each group is shown below:
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Types of fixed capacitors
(i) Paper capacitor:
A paper capacitor is made of flat thin strips of metal (Al)foil conductors that are separated by waxed paper
(the dielectric material). The dielectric (paper )is rolled between the metal .
Thickness of Al foil: 22 micron
Thickness of paper:7.5 to 25 micron
Two papers are used to avoid short-circuit.
Many different kinds of outer covering are used on paper capacitors, the simplest being a tubular cardboard
covering. Some types of paper capacitors are encased in very hard plastic. These types are very rugged and
can be used over a much wider temperature range than can the tubular cardboard type.
working voltage:400 to 2000V
Range:1000PF to I microfarad
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(ii) Mica Capacitor
Consists of a stacked layer of mica and metal .
A mica capacitor is made of metal foil plates that are separated by sheets of mica (the
dielectric). The whole assembly is encased in moulded plastic. Because the capacitor
parts are moulded into a plastic case, corrosion and damage to the plates and dielectric
are prevented. In addition, the moulded plastic case makes the capacitor mechanically
stronger..
Operating voltage:500V
Range: 5 PF to 10000PF
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(iii) Ceramic(disc) Capacitor
A ceramic capacitor is so named because it contains a ceramic dielectric
A thin metallic film is attached to ceramic at a very high temperature.
The terminal leads are attached to the metal by soldering.
Disc capacitors are encapsulated to provide insulation and protection from environmental effects.
Range: 3 picofarad to 2 micro farad
Operating voltage: 3-6000V
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(iv) Polystyrene Capacitor (plastic dielectric capacitor)
Dielectric: polystyrene film, a thermoplastic material with melting point >90C
Two metal strips are alternately placed between two plastic film.
Range:10 microfarads
Operating voltage: 500V
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(v) Electrolytic Capacitor
Aluminum electrolytic capacitors are constructed from two conducting aluminium foils, one of
which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The
foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil
acts as the cathode. This stack is then rolled up, fitted with pin connectors and placed in a
cylindrical aluminium casing.
The electrolyte reacts chemically with the aluminium foil to form a layer of aluminium oxide
on the surface of aluminium foil and becomes the dielectric medium.. The electrochemical
reaction occurs at the positive electrode. This insulating layer( Aluminium oxide) formed 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 is very small.
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Variable Capacitor
A variable capacitor is a capacitor whose capacitance can be varied mechanically or
electronically. Variable capacitors are often used in L/C circuits to set the resonance
frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors),
or as a variable reactance, e.g. for impedance matching in antenna tuners.
Types
(i) General purpose variable capacitor
(ii) Precision variable capacitor
(iii) Trimmer capacitors
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Construction of Variable capacitor
A typical variable capacitor consists of two sets of plates.
One set is called the rotor and the other the stator. The rotor is connected to the adjustment knob outside the
capacitor.
The two sets of plates are close together but not touching.
Air is the dielectric in a variable capacitor.
As the capacitor is adjusted, the sets of plates become more or less meshed, increasing or decreasing the area
of overlap between the plates.
As the plates become more meshed, capacitance increases.
As the plates become less meshed, capacitance decreases.
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(i) General purpose Variable capacitor
The parts of varicap are: rotor, stator, spindle with bearing and frame.
Type of frame: built-up type, bent type and die-cast type.
Built type frame has a rigid construction with front and back plates held together by tie bars.
Vanes : stator and rotor vanes are aluminium , semicircular in shape are assembled in the slots of
stator by mounting brackets and rotor spindles..
Capacitance can be varied by vane spacing.
BUILT UP TYPE BENT FRAME TYPE
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(ii) Precision variable Capacitor
Rotor and stator made of aluminium plates.
Rotor plates are mounted to a duralium shaft driven by a worm wheel.
The setup is mounted onto a silica base.
Quartz pillars are used for isolation of stator end plates.
Range:40Pico farad to 1000 picofarad
Maximum operating voltage:1000V
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(iii) Trimmer capacitor(padder)
Sometimes , one setting of capacitor is sufficient for all normal applications, for such applications a
variable capacitor called trimmer is used. It has two plates , one is fixed another can be adjusted with
help of a screw and hence capacitance can be varied. The dielectric used is ceramic.
Padders are similar to trimmers but are of larger values of capacitance.
Stator made of silver and balancing plate are fixed to ceramic dielectric.
Rotor is made of high permitivity material is silvered and soldered to a centre pivot.
Range: 2 to 120 picofarads.
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Dissipation factor
It is defined as the ratio of power wasted per cycle to power stored per cycle.
The losses in the capacitor are due to current leakage and dielectric absorption.
The losses depends directly upon frequency of operation.
Applications of Capacitors
Paper capacitor: blocking,coupling,bypassing,filtering,timing circuits, motor starting.
Mica capacitor: Coupling, bypassing and filtering in AF circuits.
Ceramic capacitor: Coupling, bypassing ,filtering, temperature compensation.
Polystyrene capacitor: RF tuned circuits, digital equipments
Electrolytic capacitor: decoupling, filtering, radio circuits.
General purpose variable capacitor: Tuning capacitors in broadcast recievers
Precision variable capacitor: measuring instrument as laboratory standards
Trimmer: to balance the stray capacitance in TV tuners