electrical and electronic principles.pptx
TRANSCRIPT
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Unit 6: Electrical and Electronic
Principles
Capacitors and Capacitance
Learning Outcome P4, P5, P6, M2, D1
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Lessons objectives
Look at the basic construction of capacitors
Explain common terminology used with capacitors
Understand the concepts of capacitance and determine capacitance
value in DC circuits
Look at Different types of capacitors
Charging and discharging of capacitors
DC capacitor network
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Recap
DC circuit theory
Ohms Law
Kirchhoff's law
Resistors in series
Resistors in Parallel
Complex networks
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Capacitors
Definition:Capacitors are components that are used to store an electrical charge
When power is supplied to a circuit that includes a capacitor - the
capacitor charges up. When power is turned off the capacitor
discharges its electrical charge slowly
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CapacitorsBasic TheoryAny arrangement of two conductors separated by an electric insulator(i.e., dielectric) is a
capacitor. An electric charge deposited on one of the conductors induces an equal charge
of opposite polarity on the other conductor. As a result, an electric field exists between the
two conductor surfaces and there is a potential difference between them.
Consider two plates each with an area A and separated by a
distance d the material between them is the Dielectric. The ChargeQ stored on the pair of the plates depends upon the voltage,
physical dimensions and properties of the plate.
Note: Charge is directly proportional to the voltage
Therefore : =
Where C, is a constant, referred to as the capacitance and ismeasured in Farads (F)
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CapacitorsBasic theory
Example1. A Capacitor stores 0.05 Coulombs with a terminal voltage of100V. Calculate the
capacitance
2. A Capacitor has a value of20 x 10 ^-3 C with a terminal voltage of3 x10^3. calculate
the capacitance?
3. Determine the voltage across a 4capacitor when charged with 5.
4. Find the charge on a50 capacitor when the voltage applied to it is 2.
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Insert worked answers
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Types and construction of
capacitors
Group activitya. List different types of capacitors and for each give their
design and intended use
b. What will be the effects of increasing the number of platesin a capacitor.
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Types and Construction of
capacitors
Capacitor
FixedCapacitors
Electrolytic
capacitors
Non-electrolyticcapacitors
VariableCapacitors
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Types and Construction of
capacitorsElectrolytic Capacitors
Used when very large capacitance values are required
The majority of electrolytic types of capacitors are Polarised,
that is the DC voltage applied to the capacitor terminals mustbe of the correct polarity.
One main disadvantage of electrolytic capacitors is theirrelatively low voltage rating and due to the polarisation ofelectrolytic capacitors
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Types and Construction of
capacitors
Ceramic Capacitors
These may be disc shaped, tubular or flat.
The dielectric is ceramic and metal films are deposited to the plates. They are
encapsulated in resin of epoxy
Ceramic types of capacitors generally have a 3-digit code printed onto their body to
identify theircapacitance value in Pico-farads.
Ceramic capacitors are divided into two application classes:
Class 1: offerhigh stability and low losses for resonant circuit applications.
Class 2: offer high volumetric efficiency for buffer, by-pass and coupling
applications.
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Types and Construction of
capacitors
Tantalum Bead
Tantalum capacitors are a new type of an electrolytic capacitor.It uses tantalum and tantalum oxide to give a further
capacitance and size advantageTantalum is used as the dielectric and produces a largecapacitance in small volume but with low working voltages.
These small capacitors are encapsulated in resin
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Types and Construction of
capacitorsMica
Mica is a naturally occurring dielectric and has a very highresistance
This gives excellent stability and allows the capacitors to beaccurate within a value of 1% of the marked value.
Due to increased accuracy, they are more expensive than plasticfilm capacitors
They are used where high stability is required, for example in tunedcircuits and filters required in radio transmission.
(Mica is a common rock
forming mineral. It is found
in rocks such as granite and
sand stones and
mudstones)
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Types and Construction of
capacitors
Variable Capacitor
An air variable capacitor consists of a rotor
and a stator. A statoris a set of plates on a
frame, a rotoris a set of plates on a shaft.
Turning shaft from one to another side you
may change a capacitance of an air variable
capacitor tuning an antenna on a desired
station. As you may see on the picture, the
capacitor contains three stators, each one is
mounted on ceramic isolators. It means we
may use it to tune three tuned circuits
simultaneously for better selectivity. The
third section of the particular capacitor
shown on the picture has lesser capacity,
because some plates has been removed
from it
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Types and Construction of
capacitors
Home work!!!!
Explain the key terms associated with capacitors listed below
Dielectric strength
Dielectric Materials
Flux density
Flux permittivity
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Recap
Looked at the basic construction of capacitors
Looked at the concepts of capacitance and determined the capacitance
value in DC circuits
Looked at Different types of capacitors
Given you homework to look common terminology used with capacitors.
The homework will be due next lesson.
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Next
Capacitor Circuits in series
Capacitor circuits in parallel
Complex circuits
Charging and discharging of capacitor