Diodes and Capacitors

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What is a diode?

A diodeis a component made

from a semiconducting material

such as silicon.

iodes ha!e the uni"ue

propert# of onl# allowing

current to pass through themin one direction.

$ome diodes emit light when the#

conduct electricit#. %hese are called

Light &mitting iodes 'L&s(.

%he arrowhead on the s#m)ol

shows the direction of current flow.

diode and its symbol


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Investigating diodes


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VIgraph for a diode


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,or materials to conduct electricit#- the# must )e a)le to

transfer a charge when a potential difference is applied

across them.

Conduction in metals

metal ions: fixed positive charge

electrons: mobile negative charge

etals conduct electricit# due to

delocalized electrons.%hese

electrons are not )ound to asingle atom- and act as a source

of mo)ile negati!e charge.

espite this free charge- metals

ha!e a net neutral charge. As freeelectrons are released from the

metal atoms- the atoms are left

with a )alancing positi!e charge.


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Conduction in semiconductors

$ome semiconductors also contain a pool of delocalied

electrons which allow current to flow.

owe!er- semiconductors can also contain groups of

atoms with an unusuall# low num)er of electrons.

$uch regions contain holes.

oles are positi!el# charged- and are a)le to flow through a

semiconductor- ust like delocalied electrons. $imilarl#- their

formation lea!es an opposing fied charge )ehind.

$emiconducting materials can )e artificiall# modified to

adust their conducti!e properties- adding etra holes or

electrons to their structure.


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Ho diodes or!


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"iodes summary



#ypes of current

urrent can )e either6

$lternating current %$C&:

7n an alternating current the direction

of electron flow around a circuit is

continuousl# switching.

"irect current %"C&:7n a direct current the electrons flow

in one direction around a circuit.

%he !oltage remains constant-

producing a flat line on an oscilloscope.

%he !oltage oscillates )etween negati!e

and positi!e- producing a wa!eform.



'rom $C to "C

A is used to carr# electrical

energ# around the countr# as6

7ts !oltage is eas# to change

using transformers.

7t re"uires a simple generator.

ost modern appliances in our home

now use microchips.

ost microchips re"uire with a

near constant !oltage to function.

%his means we must con!ert A from the mains- to in

order to use our appliances.



(ectification

(ectification is the process which makes an A signal flow ina constant direction.

%here are two maor steps re"uired to con!ert an A signal

to a signal6

%he current must )e made to flow in a constant

direction.

%he !ariations in !oltage must )e remo!ed.

%he signal produced is imperfect "C- as its !oltage still !aries.


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Half ave rectification



'ull)ave rectification

B# using a com)ination of four diodes- called a bridge

rectifier- an A signal can )e full# rectified.

input output

%his output signal alwa#s has a positi!e !oltage- meaning

that- unlike half8wa!e rectification- no energ# is lost.


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*ridge rectifier ho does it or!?


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(ectification summary



What is a capacitor?

%he amount of charge a capacitor is

a)le to store- or its capacitance- is

measured in farads ',( or

microfarads ',(.

A capacitor is a component that

stores charge.

apacitors contain two metal plates

with an insulator. %he larger the

plates- the larger the capacitance.

capacitor

symbol

%he charge stored on a capacitor:s

plates can )e discharged into a circuit-

maintaining a current for a short time.



Ho does a capacitor charge?

;hen a discharged capacitor is connected to a circuit- it will

)egin to charge.

%he high potential

difference across the

circuit forces electrons

onto one plate- and pullsthem from the other.

%his increases the

potential difference

across the capacitor.

%he capacitor is full# charged when it has the same potential

difference as the )atter#.

electrons forced

onto plate

electrons forced

from plate



Charging a capacitor


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"ischarging a capacitor


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-ses for capacitors

apacitors are used in

defi)rillators to store

electric charge.

%his results in a large current which can restart the heart.

7nitiall# the capacitor is

charged from a power

source. %here is no powerto the paddles.



Capacitor summary


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.erfect "C

icrochips re"uire a perfect "Csuppl# of around 5=.

>ectification allows the production of a signal from an

A input. owe!er- the is imperfect- as it still has a

great le!el of !ariation in its !oltage.

A rectified signal is not good enough tocontinuousl# run a chip in a circuit-

due to these !oltage fluctuations.

7n order to run most modern

de!ices- this fluctuation in !oltagemust )e remo!ed.

%his process is called smoothing.

/ hi


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/moothing

/moothingturns the )ump# rectified signal into a smooth-

near8perfect direct current- which can )e used )# microchips.

A capacitor is used to smooth the signal.

ow do #ou think a capacitor does this?

input: rectified output: smoothed

0V

1V

H d it th i l ?


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Ho do capacitors smooth signals?


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V i lt


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Varying voltage


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2l


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2lossary

$


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$nagrams

3 lti l h i i


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3ultiple)choice 4uiz

Diodes and Capacitors

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