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CAPACITORS
1. A
capacitor or condenser consists of an insulator placed between two conductors.
2.
Capacitance is ability of a dielectric to store electric charge.
3. The unit
of capacitance farad (F).
4. The
capacitance of a capacitor is given by,
C=Q/V where Q= charge is given in Coulomb.
Types of capacitors
Capacitor Symbols
A. Fixed Capacitor
1. Electrolytic (Polar) capacitors
a) These
capacitors are marked to indicate which side must positive to maintain internal
electrolytic action.
b) These capacitors provide larger capacitance values in the smallest space with minimum cost.
i. Aluminium Electrolytic Capacitors
(wet type)
a) In this
type two aluminium electrodes are in electrolyte of phosphate, borax or
carbonate.
b)
Capacitance ranges from 5 to 2000µF, approximately.
c) The
disadvantage of this type is relatively high leakage current which about 0.1 to
0.5 mA/µF.
ii. Tantalum electrolytic capacitors
a) In this
type tantalum(Ta) or titanium(Ti) plates are used.
b) These
capacitors provide following advantages:
· Larger C in a smaller size.
· Longer shelf life
· Less leakage current
d) The
drawback of this type is that their cast is more than the aluminium type
e)
Capacitance is upto about 300µF.
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2. Non-electrolytic (non Polar)
capacitors
a) These
capacitors are classified according to the type of dielectric material used.
b) In this
type polarity is not fixed.
c) The
polarity of the charging source determines the polarity of the capacitor
voltage.
i. Mica capacitors
a) In this
type, thin mica sheets are placed between tinfoil sections which are used as
conducting plates.
b) Capacitance
of mica capacitors is upto 5000pF only.
C) A six-dot
color code system is used to indicate the capacitance, temperature coefficient
etc.
d) Breakdown
voltage is 500to 20,000 V.
ii. Paper capacitor
a) A
tissue-paper is used as dielectric.
b) Two rolls
of tinfoil conductor are separated by the tissue paper.
c) Paper and
plastic capacitors provide medium capacitance values of 0.001 to 1µF.
d) Breakdown
voltage is 200 to 1600 V.
iii. Ceramic Capacitor
a) The
ceramic dielectric materials are made from earth fired under extreme heat.
b) In the
disk type, thin layer of silver present on both sides of ceramic forms the
conductor plates.
c)
Capacitance is upto 0.1 µF.
d) In
tabular type, the hollow ceramic tube has a silver coating on the inside and
outside surfaces.
e) The
capacitance is upto 1600pF.
f) Five
color bands are used to indicate the temperature coefficient, capacitance etc.
g) Ceramic
capacitors are used for temperature compensation.
h) The
temperature coefficient is given in parts per million (ppm) per degree Celsius
with a reference of 25
iv. Air capacitor
a) These capacitors can
provide greater safety.
b) Fixed value air dielectric capacitors are used as laboratory standards for calibration and measurement.
B. Variable Capacitor
1. Air gang condenser or capacitor
a) In this type, the
fixed metal plates connected together form the stator.
b) The movable plates
connected form the rotor.
c) Capacitance is
varied by rotating the shaft.
d) Air is dielectric;
therefore, two types of plates do not touch.
e) Gang condenser are
used for tuning in radio receivers.
2. Trimmers and padders
a) These capacitor are
used for fine tuning.
b) They are sometime
constructed on the gang condenser.
Ratings of capacitor (Specification)
a) Capacitance of
capacitor in µF.
b) Voltage rating.
c) The voltage rating
specifies the maximum potential difference that can be applied across the
plates without puncturing the dielectric.
d) Other specification
of capacitors include leakage resistance, power factor, tolerance, temperature
range etc.
Application of capacitors
1. Fixed capacitors are used for –
a) Coupling ac signal.
b) Bypassing ac
signals.
c) Blocking dc signals.
d) Filtering
2. Variable capacitors are used for
tuning in radio frequency.
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