Engineering Essentials Chapter 9 - Capacitors

IT2001PA
Engineering Essentials (1/2)

Chapter 9 - Capacitors

Lecturer Name
lecturer_email@ite.edu.sg
Aug 16, 2012
Contact Number


Lesson Objectives
Upon completion of this topic, you should be able to:
 Describe the constructional features of common
capacitors.

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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.

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Capacitors
 The basic structure of a capacitor is shown below.

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

CONDUCTIVE PLATES
- two parallel pieces CONNECTING LEADS
separated by the dielectric. - attached to the plates
-metal foil. for external connections.

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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(µF) = 10-6 Farad

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Capacitor

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

Charge (Q)
 Capacitance (C) =
P.D. (V) Q

C V
Or Q= CV
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Capacitance

Q
C = Q = Charge
V Unit : coulomb(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.
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Example 9-1
 A capacitor stores 50µC when 10V are applied
across its plates. What is its capacitance?

Q
C =
V
50µC
= 10V
= 5µF

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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

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Example 9-3
Determine the voltage across a 100pF capacitor
which stores 2µC of charge?

Q
V =
C
2µC
= 100pF
= 20kV

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Types of Capacitors
 Fixed Capacitors
 the capacitance cannot be changed.

 Variable Capacitors
 the capacitance can be changed.

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Types of Capacitors
 Fixed Capacitors
 Electrolytic (with polarities)
 Electrostatic Capacitor (no polarities)
 mica
 ceramic
 paper
 Variable Capacitors
 air capacitors
 trimmers

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Electrolytic Capacitor

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

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

+ - + -

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 Electrolytic

foil
(aluminium or
tantalum)
paper or gauge
saturated with electrolyte
oxide dielectic

Aluminum axial-lead construction.

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 Capacitance value
 very high.
 over 200 000µF.

 Voltage rating
 very low.
 typical maximum 350Vdc.

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Mica Capacitor
It 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.

Layers are pressed together
Stacked layer arrangement. and encapsulated.

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Mica Capacitor

 very small.
 from 1pF to 0.1µF.

 Voltage rating
 high.
 from 100 to 2500 Vdc.

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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.

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Ceramic Capacitor
There 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.

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Ceramic Capacitor

metal film

ceramic disc

Ceramic disk form.
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Ceramic Capacitor
 Small.
 from 1pF to 2.2µF.

 Voltage rating
 very high.
 Up to 6000Vdc.

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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.

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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.

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Variable Air Capacitor

movable plates (rotor)

fixed plates (stator)
The movable plates are all linked together mechanically
so that they all move when a shaft is rotated.

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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.

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Factors Affecting Capacitance
 Factors that determine the capacitance of a
capacitor are :

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

a∈
C =
d
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Plate Area
 Capacitance (C) is directly proportional to
plate cross-sectional area (a).

a↑ - C↑
a↓ - C↓

full plate area
reduced plate area
more capacitance
less capacitance
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Plates Separation
 Capacitance (C) is inversely proportional to
the distance between plates (d).

d↑ - C↓
d↓ - C↑

narrow plate distance increased dielectric thickness
more capacitance less capacitance
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Dielectric Permittivity
 It is a measure of the ability of a dielectric
material to establish an electric field.

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

∈↑ - C↑
∈↓ - C↓
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Next Lesson

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Engineering Essentials Chapter 9 - Capacitors

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