ELECTRICAL AND ELECTRONIC PRINCIPLES.pptx

7/22/2019 ELECTRICAL AND ELECTRONIC PRINCIPLES.pptx

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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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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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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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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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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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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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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

ELECTRICAL AND ELECTRONIC PRINCIPLES.pptx

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