ee1 chapter9 capacitors

Apr 12, 2023

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IT2001PAEngineering Essentials (1/2)

Chapter 9 - Capacitors


IT2001PA Engineering Essentials (1/2) 2

Lesson Objectives

Upon completion of this topic, you should be able to: Describe the constructional features of common

capacitors.


IT2001PA Engineering Essentials (1/2)

Specific Objectives

Students should be able to :

List common types of capacitors and its construction. State the application of the common types of

capacitors. Explain the term 'di-electric'. List factors affecting the capacitance of capacitor. Describe the constructional features of common

capacitors.



Capacitors

A capacitor is a device which stores energy in the form of electric charge.

In its simplest form, a capacitor consists of two metal plates separated by a thin layer of insulating material called the dielectric.



Capacitors The basic structure of a capacitor is shown below.



Basic Construction

CONDUCTIVE PLATES- two parallel pieces separated by the dielectric.-metal foil.

CONNECTING LEADS- attached to the plates for external connections.

DIELECTRIC- an insulating (non- conducting) material.- ceramic, plastic film, mica or air.



Capacitance

It is the property of a capacitor to store an electric charge when a potential difference is applied between the two plates.

Symbol : C

Unit : Farad (F) But usually the unit micro-farad (a sub-multiple of Farad) is

used.

1 micro-farad(F) = 10-6 Farad



Capacitor

A capacitor has a capacitance of one farad when 1 V charges it with one coulomb of electricity.

Capacitance (C) = Charge (Q)

P.D. (V)

Or Q = C V

Q

C V



Capacitance

V

Q = C

Capacitance is thus defined as the ratio of the charge stored in a capacitor to the applied voltage.

The larger the capacitance, the larger will be the charge stored.

A measure of a capacitor’s ability to store charge or energy.

Q = ChargeUnit : coulomb(C)



Example 9-1

A capacitor stores 50µC when 10V are applied across its plates. What is its capacitance?

C =QV 50µC 10V=

= 5µF



Example 9-2

A 2µF capacitor has 100V across its plates. How much charge does it store?

Solution

Q = CV

= 2µF x 100V

= 200 µC



Example 9-3

Determine the voltage across a 100pF capacitor which stores 2µC of charge?

V =QC 2µC100pF=

= 20kV



Types of Capacitors

Fixed Capacitors

the capacitance cannot be changed.

Variable Capacitors the capacitance can be changed.



Types of Capacitors Fixed Capacitors

Electrolytic (with polarities) Electrostatic Capacitor (no polarities)

mica ceramic paper

Variable Capacitors air capacitors trimmers



Electrolytic Capacitor

The type commonly used consists of two aluminium foils, one with oxide film (+) and one without.




+ - + -

all electrolytic capacitors are polarized. foil : aluminum; tantalum; etc. used to eliminate ripple in a dc supply.



Electrolytic Capacitor Electrolytic

Aluminum axial-lead construction.

foil (aluminium or

tantalum)paper or gaugesaturated with electrolyte

oxide dielectic




Capacitance value very high. over 200 000µF.

Voltage rating very low. typical maximum 350Vdc.



Mica CapacitorIt consist of either of alternate layers of mica and metal foil clamped tightly together or thin films of silver sputtered on the two sides of a mica sheet.

Stacked layer arrangement.Layers are pressed together

and encapsulated.



Mica Capacitor

Capacitance value very small. from 1pF to 0.1µF.

Voltage rating high. from 100 to 2500 Vdc.



Ceramic Capacitor

films of metal are deposited on each side of a thin ceramic disc and copper wire terminals are connected to the metal.

the entire unit is encapsulated in a protective coating of plastic.

used for frequency discriminating filters; DC blocking and circuits requiring close tolerance and stability.



Ceramic CapacitorThere are two types:

ceramic disc capacitors and ceramic tubular capacitors.

A film of silver is deposited on both sides of the ceramic, acting as the plates.



Ceramic Capacitor

Ceramic disk form.

ceramic disc

metal film



Ceramic Capacitor

Capacitance value Small. from 1pF to 2.2µF.

Voltage rating very high. Up to 6000Vdc.



Paper Capacitor

The paper capacitor consists of aluminium foil and paper dielectric rolled together and impregnated with wax or oil, and then placed in a plastic or aluminium container.



Variable Air Capacitor

Constructed of several plates that mesh together. One set of plates can be moved relative to the other,

thus changing the effective plate area and the capacitance.

Used in applications requiring frequency selection radio and television receivers. RF amplifier.



Variable Air Capacitor

The movable plates are all linked together mechanicallyso that they all move when a shaft is rotated.

movable plates (rotor)

fixed plates (stator)



Capacitor Applications

Capacitors are very widely used in electrical and electronic applications. They can be used:

in filter circuits for tuning circuits as coupling and bypass capacitors in transistor

amplifiers for power factor correction in single phase motors in automobile ignition systems and for other purposes.



Factors Affecting Capacitance

Factors that determine the capacitance of a capacitor are :

plate cross-sectional area (a ). Distance distance between plates (d). dielectric permittivity ().

C =a d



Plate Area Capacitance (C) is directly proportional to

plate cross-sectional area (a).

a - C

full plate areamore capacitance

reduced plate arealess capacitance

a - C



Plates Separation Capacitance (C) is inversely proportional to

the distance between plates (d).

d - C

narrow plate distancemore capacitance

increased dielectric thicknessless capacitance

d - C



Dielectric Permittivity

It is a measure of the ability of a dielectric material to establish an electric field.

- C - C

Capacitance (C) is directly proportional to the dielectric permittivity ().



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