basic electricity - capacitors

8/3/2019 Basic Electricity - Capacitors

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JJ102- Electric Technology

Capacitors


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Unit Objectives

List the three (3) factors that determine the

capacitance of a capacitor

Explain electrostatic charge

Determine the difference between polarized

and nonpolarized capacitors

Upon completion of this unit, you will be able to:


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Introduction

Capacitors are devices that oppose a change of

voltage, at their base level are a simple

electrical storage device yet the applications for

these devices are varied and wide ranging.

Capacitors can be used in timing circuits, as

electronic filters, and for power factor correction

among other uses. Capacitors are used in bothAC and DC circuitry.


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The Capacitance of a Capacitor

1. The area of the plates2. The distance between the plates

3. The type of dielectric used


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Charging a Capacitor

Electrons are removed from one plateand deposited on the opposite plate


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Current flows until voltage acrosscapacitor is the same as the source

voltage


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The capacitor remains charged afterthe source of voltage (battery) is

removed from the circuit


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Capacitor Leakage

Leakage is current flow through the dielectricProportional to the resistance of the dielectric

and the charge across the plates


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Electrostatic Charge

An electrostatic charge refers to electricalcharges that are stationary

Static electricity is an electrostatic charge Electrostatic charges are created by

removing electrons from one plate and

depositing them on the other Energy in a capacitor is stored in the form

of an electrostatic charge


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Dielectric Stress

A lack of electrons = a positive charge An excess of electrons = a negative charge


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Dielectric Stress

When the atoms of thedielectric are under stressthey are creating a potentialenergy, or stored energy

Like a drawn bow, thepotential energy is beingstored

When a charged capacitor isdischarged it can producecurrents of thousands ofamperes for short periods of

time


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Dielectric Stress

Dielectric stress is proportional to thevoltage difference between the plates

Excessive voltage can cause the dielectric tobreak down (short) and permit current flowbetween the plates

Capacitors have a voltage rating that shouldnot be exceeded

Preventing excessive supply voltage to acapacitor is critical to its life span


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Dielectric Constants

A number is assigned to different materials as

an indicator of their insulating quality in a

capacitor

This number is referred to as its dielectric

constant

Changing the dielectric material of a capacitor

changes its capacitance properties


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Dielectric Constants

Assume that a capacitor uses

air as its dielectric material

and has a total capacitance of

1 mF

Changing the dielectric

material to dry paper

would change thecapacitance of the capacitor

to 3.5 mF


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Capacitor Ratings

A farad is the basic unit of capacitance named

after Michael Faraday, a famous scientist

Symbolized by the letter F

One farad = the change of 1 volt across the

plates results in the movement of 1 coulomb of

current

A farad is such a large basic unit of capacitance

that it is seldom used for capacitor values


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Capacitor Ratings

More common designations of capacitance:

mF Microfarad = 1/1,000,000

nF Nanofarad = 1/1,000,000,000

pF Picofarad = 1/1,000,000,000,000

The Picofarad is sometimes referred to as a

micro/microfarad with the symbol mmF


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Capacitancecharacteristic of a capacitor to store electrical

energy

Capacitance, (Farad) =)(

)(

VoltBezaupaya

CoulombCas


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Elecrical Energy in Capasitor

QVE2

1

2

2

1CVE dan )(

2

12

C

QE


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Sample..

A 8pF capacitor is connected to

the 600V power supply. Calculate the

charge and energy canbe stored by the capacitor.???????


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Solution

Charge, CVQ

= )600)(108(12

x = 4.8 x 10-7 C

Elctrical Energy,

)600)(108.4(21 12xQVE 2.88 x 10-9 Joule


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Capacitors in Parallel Has the same effect of increasing the plate area of

one capacitor


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b)Voltage drop on each capacitor is equal

c) Charge in each capacitor

jCCC VVVV 321

1

1C

VQ

j

C ,

2

2C

VQ

j

C and

3

3C

VQ

j

C


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Capacitors in Series

Has the effect of increasing the distance between the

plates / reducing the total capacitance of the circuit


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b) Voltage Drop is not equal

c) Charge For each Capasitor1

1

C

QV

j

C ,

2

2

C

QV

j

C dan3

3

C

QV

j

C

jQQQQ 321


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sample

Two (2) of the capacitor has a capacitance

value of each and 10F 6F connected in

series with a 200V power supply. Get;a) Total capacitance

b) The charge of the capacitors

c) Voltage Drop across each capacitor.


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solutiona) Total Capacitance, jC

21

21

CC

CCCj

=

F75.3106

)10)(6(

b) Charge for each Capasitor ,CxxVCQQQ jj 75010750)200)(1075.3(

66

21

c) Voltage Drop for each capasitorV

x

x

C

QV

j125

106

10750

6

6

1

1

Vx

x

C

QV

j75

1010

107506

6

2

2


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The Use Of CAPASITOR

a) Increasing the power factor of electrical

circuits.

b) Reduce the spark when the switch opensthe circuit.

c) Reducing the radio repeat interruptions

in the starter circuit pendaflour lights.

d) Strengthen the electrical current.

e) To keep the electrical charge.


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The Two Basic Groups of

Capacitors

Polarized

&

Non-Polarized


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Non-Polarized Capacitors

Not sensitive to polarity

connection

May also be referred toas AC capacitors

Can be used in AC/DC

circuits


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Paper / Oil Filled Capacitors


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Polarized Capacitors

Also known as electrolytic capacitors

They are sensitive to polarity

Used only in DC circuits

Reversing polarity could cause capacitor

damage or explosion

Electrolytic capacitors can have very high

capacitance in a small case


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Variable Capacitors

Capacitance range can be changed

within a certain range

Contains a moveable set of plates

The plates are interleaved with the

stationary plates to increase ordecrease the capacitance value


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A Trimmer Capacitor

Contains one stationary plate and one moveable

plate An adjusting screw moves the moveable plate to

change the capacitance value


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Please Check Your CIDOS for AssignmentThank YOUUUUU !!!!!

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JJ102 Electrical Technology

basic electricity - capacitors

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