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ENGR. VIKRAM KUMAR B.E (ELECTRONICS) M.E (ELECTRONICS SYSTEM ENGG:) MUET JAMSHORO

CAPACITOR

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Capacitor

• A capacitor is a device for storing charge and electrical potential energy.

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Capacitor• All capacitors consists of two metal plates

separated by an insulator. The insulator is called dielectric. (e.g. polystyrene, oil or air)

• Circuit symbol:

+

_

Dielectric

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Structure of capacitor• Capacitor

= conductor (metal plate)+

insulator (dielectric)+

conductor (metal plate)

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Capacitors• Basic capacitor construction

Dielectric material

Plate 1

Plate 2The dielectric material is an insulator therefore no current flows through the capacitor

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Formation of a Capacitor• Capacitors are formed all of

the time in everyday situations: – when a charged

thunderstorm cloud induces an opposite charge in the ground below,

– when you put your hand near the monitor screen of this computer.

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CapacitorStoring a charge

between the plates

• Electrons on the left plate are attracted toward the positive terminal of the voltage source

• This leaves an excess of positively charged holes

• The electrons are pushed toward the right plate

• Excess electrons leave a negative charge

+ -

+_

+ _

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Charged Capacitor• A capacitor is said to be charged when there

are more electrons on one conductor plate than on the other.

When a capacitor is charged, energy is stored in the dielectric material in the form of an electrostatic field.

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Capacitance • Consider any isolated pair of conductors with charge Q

Capacitance is defined asVQC

Where Q = charge on one conductor V = potential difference between two conductors

Unit : farad (F)

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Capacitance

• The capacitance of a conductor is the charge required to cause unit change in the potential of the conductor.

• A one-farad capacitor stores one coulomb of charge when a potential of 1 volt is applied across the terminals of the capacitor.

• The smaller the change in potential of the conductor when a certain charge is transferred to it, the more charge it can store before breakdown occurs.

• In electronics, the microfarad (μF) and the picofarad (pF) are usually used to measure capacitance.

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Capacitance of a Capacitor

• Note that Q is not the net charge on the capacitor, which is zero.

• Capacitance is a measure of a capacitor's ability to store charge.

• The more charge a capacitor can hold at a given potential difference, the larger is the capacitance.

• Capacitance is also a measure of the energy storage capability of a capacitor.

VQC

Capacitance

Fringing – At the edge of the capacitor plates the flux lines extend outside the common surface area of the plates.

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Capacitance

Dielectric – Insulator of the capacitor The purpose of the dielectric is to create an electric

field to oppose the electric field setup by free charges on the parallel plates.

Di for “opposing” and electric for “electric field”Dipoles – Formed within the insulator of a capacitor

when the electrons of the insulating material are unable to leave the parent atom and travel to the positive plate of the capacitor

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Capacitance

With different dielectric materials between the same two parallel plates, different amounts of charge will deposit on the plates.

Permittivity – The ratio of the flux density to the electric field intensity in the dielectric. A measure of how easily the dielectric will “permit” the establishment of flux lines within the dielectric.

Relative permittivity – Often called the dielectric constant, it is the ratio of the permittivity of any dielectric to that of a vacuum.

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Capacitors

Working voltage – the voltage that can be applied across a capacitor for long periods of time with out breakdown

Surge voltage – The maximum dc voltage that can be applied for a short period of time

Leakage current – The current that results in the total discharge of a capacitor as the capacitor is disconnected from the charging network for a sufficient length of time.

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Voltage Rating of Capacitors

• If the voltage applied across the capacitor is too great, the dielectric will break down and arcing will occur between the capacitor plates.

• The voltage rating of the capacitor is the maximum voltage that can be steadily applied without danger of breaking down the dielectric.

Energy Stored by a Capacitor The ideal capacitor does not dissipate any energy supplied to it. It

stores the energy in the form of an electric field between the conducting surfaces.

Charging of a capacitor always involves some expenditure of energy by the charging agency.

This energy is stored up in the electrostatic field set up in the dielectric medium.

• On discharging the capacitor, the field collapses and the stored energy is released.

when the capacitor is uncharged,• little work is done in transferring charge from

one plate to another.• Let us find the energy spent in charging a capacitor

of capacitance C to a voltage V.

QVE21

2

21CV

CQ2

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Schematic of simple circuit with capacitor

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Capacitor Charging

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Capacitor Charging

Current stops when capacitor is fully charged

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Capacitor Charging

Capacitor remains charged when power source is removed

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Charging of Capacitors • As a capacitor becomes charged, the current flow

decreases because the voltage developed by the capacitor increases over time and opposes the source voltage.

R

R

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Transients in Capacitive Networks: Charging Phase

The placement of charge on the plates of a capacitor does not occur instantaneously.

Transient Period – A period of time where the voltage or current changes from one steady-state level to another.

The current ( ic ) through a capacitive network is essentially zero after five time constants of the capacitor charging phase.

Initial Conditions The voltage across a capacitor at the instant of the start of

the charging phase is called the initial value. Once the voltage is applied the transient phase will commence until a leveling off occurs after five time constants called steady-state as shown in the figure.

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Charging a Capacitor

• Voltage-charge characteristics

• Current flow

I

t

Vc or Q

t

)1(0 RCt

C eVV

RCt

oeII

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Capacitor Discharging

Capacitor discharges when a current path is applied

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Capacitor Discharging

Capacitor discharges when a current path is applied

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Capacitor Discharging

Capacitor discharges when a current path is applied

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Capacitor Discharging

Capacitor discharges when a current path is applied

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Capacitor Discharging

Capacitor discharges when a current path is applied

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Capacitor Discharging

Capacitor discharges when a current path is applied

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Capacitor Discharging

Capacitor is fully discharged when plates are neutral

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Discharging of Capacitors

• The charged capacitor is the source of voltage for the current flow. The current will cease flowing when the charges of the two plates are again equal, meaning that the capacitor is completely discharged.

R

R

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Discharging a Capacitor

• Voltage-charge characteristics

• Current flow

VC or Q

tI

t

RCt

eQQ

0

RCt

oeII

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Time Constant• Resistance is unavoidable in circuits, through the wire itself or

through components such as resistors.• Resistance introduces the element of time into charging and

discharging a capacitor• The voltage across a capacitor cannot change instantaneously

because it takes finite time to move charge from one point to another.

• The rate at which a capacitor charges or discharges is determined by the time constant t = RC (seconds)

• During one time constant interval, the charge on a capacitor changes approx. 63%

• Five time constant intervals is accepted as the time to fully charge or discharge a capacitor and is called the transient time.

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Charging Capacitor Voltage

Time

appliedvoltage

chargevoltage

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Charging Capacitor Voltage

Time

appliedvoltage

chargevoltage

resistance added

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Discharging Capacitor Voltage

Time

appliedvoltage

chargevoltage

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Discharging Capacitor Voltage

Time

appliedvoltage

chargevoltage

resistance added

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ApplicationsCapacitors find applications in:

Electronic flash lamps for cameras

Line conditionersTiming circuitsElectronic power supplies

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Applications of Capacitors

• Press the key on a computer keyboard reduce the capacitor spacing thus increasing the capacitance which can be detected electronically.

• The capacitance is varied by altering the overlap between a fixed set of metal plates and a moving set. These are used to tune radio receiver.

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Applications of Capacitors

• Capacitive touch-screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact.

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THE END