Power Adapters 1

Power AdaptersPower Adapters

Power Adapters 2

Introductory QuestionIntroductory Question

If you install a pocket radio’s batteries If you install a pocket radio’s batteries backward, it won’t work because itsbackward, it won’t work because its

A.A. speaker will move the wrong direction.speaker will move the wrong direction.

B.B. parts can only conduct current one parts can only conduct current one way.way.

C.C. batteries will absorb power and batteries will absorb power and recharge.recharge.

Power Adapters 3

Observations about Power Observations about Power AdaptersAdapters

They obtain power from AC They obtain power from AC electrical outletselectrical outlets

They provide DC power to electronic They provide DC power to electronic devicesdevices

They somehow fix the AC versus DC They somehow fix the AC versus DC problemproblem

They come in various voltages and They come in various voltages and other ratingsother ratings

Power Adapters 4

5 Questions about5 Questions aboutPower AdaptersPower Adapters

Why isn’t a power adapter just a Why isn’t a power adapter just a transformer?transformer?

How does a metal differ from an insulator?How does a metal differ from an insulator? How does charge move in a How does charge move in a

semiconductor?semiconductor? Why does a diode carry current only one Why does a diode carry current only one

way?way? How does a capacitor store electric How does a capacitor store electric

charges?charges?

Power Adapters 5

Question 1Question 1

Why isn’t a power adapter just a Why isn’t a power adapter just a transformer?transformer?

Power Adapters 6

Power Adapter Power Adapter ComponentsComponents

A power adapter provides low-voltage DCA power adapter provides low-voltage DC Starting with high-voltage AC, it mustStarting with high-voltage AC, it must

use a transformer to obtain low-voltage AC,use a transformer to obtain low-voltage AC, use diodes to obtain low-voltage pulsed DC,use diodes to obtain low-voltage pulsed DC, and a capacitors to obtain low-voltage DC.and a capacitors to obtain low-voltage DC.

A diode is a one-way device for currentA diode is a one-way device for current A capacitor is a charge-storage deviceA capacitor is a charge-storage device

Power Adapters 7

Question 2Question 2

How does a metal differ from an How does a metal differ from an insulator?insulator?

Power Adapters 8

Metals, Insulators, and Metals, Insulators, and DiodesDiodes

A metal carries current in any directionA metal carries current in any direction An insulator won’t carry current in any An insulator won’t carry current in any

directiondirection A diode carries current only in one A diode carries current only in one

directiondirection To understand a diode,To understand a diode,

we need to understanding metals and we need to understanding metals and insulatorsinsulators

and a bit of quantum physics.and a bit of quantum physics.

Power Adapters 9

Quantum Physics Quantum Physics (Part 1)(Part 1)

Things travel as waves, but interact as Things travel as waves, but interact as particlesparticles

For example, lightFor example, light travels as waves (electromagnetic waves)travels as waves (electromagnetic waves) but is emitted and absorbed as particles but is emitted and absorbed as particles

(photons).(photons). For example, electronsFor example, electrons

are emitted and detected as particlesare emitted and detected as particles but travel as waves.but travel as waves.

This wave-particle “duality” is universal!This wave-particle “duality” is universal!

Power Adapters 10

Quantum Physics Quantum Physics (Part 2)(Part 2)

Particles come in two types: fermions or bosonsParticles come in two types: fermions or bosons Fermions obey the “Pauli exclusion principle”Fermions obey the “Pauli exclusion principle”

Only one indistinguishable fermion per waveOnly one indistinguishable fermion per wave Indistinguishable fermions require different wavesIndistinguishable fermions require different waves Electrons, protons, and neutrons are fermionsElectrons, protons, and neutrons are fermions

Bosons have no exclusion principleBosons have no exclusion principle Indistinguishable bosons prefer the same waveIndistinguishable bosons prefer the same wave Light particles (photons) are bosons, hence the laserLight particles (photons) are bosons, hence the laser

Power Adapters 11

Electrons in Solids Electrons in Solids (Part 1)(Part 1)

Electrons exist in a solid as standing wavesElectrons exist in a solid as standing waves Only certain electron standing waves fit in solidOnly certain electron standing waves fit in solid Each possible standing wave has an energy “level”Each possible standing wave has an energy “level” Up to two electrons can “occupy” each levelUp to two electrons can “occupy” each level

Electrons have two spin states: up and downElectrons have two spin states: up and down Spin-up is distinguishable from spin-downSpin-up is distinguishable from spin-down

Levels are filled from lowest to highest energyLevels are filled from lowest to highest energy ““Fermi level” divides last filled and first Fermi level” divides last filled and first

unfilledunfilled

Power Adapters 12

Electrons in Solids Electrons in Solids (Part 2)(Part 2)

Power Adapters 13

MetalsMetals

In a metal,In a metal, the Fermi level has empty levels just the Fermi level has empty levels just

above itabove it Like patrons in a partly filled theatre, Like patrons in a partly filled theatre,

electrons can move in response to electrons can move in response to electric fieldselectric fields

Currents can flow through a metal in Currents can flow through a metal in any directionany direction

Power Adapters 14

InsulatorsInsulators

In an insulator,In an insulator, The Fermi level has no empty levels The Fermi level has no empty levels

nearbynearby Like patrons in a full theatre, electrons Like patrons in a full theatre, electrons

can’t move in response to forcescan’t move in response to forces Current can’t flow through an insulatorCurrent can’t flow through an insulator

Power Adapters 15

Question 3Question 3

How does charge move in a How does charge move in a semiconductor?semiconductor?

Power Adapters 16

SemiconductorsSemiconductors

Semiconductors are “poor Semiconductors are “poor insulators”insulators” Narrow gap from valence to conduction Narrow gap from valence to conduction

bandband Like patrons in a full theatre with a low Like patrons in a full theatre with a low

empty balcony, electrons can hop to the empty balcony, electrons can hop to the balcony and movebalcony and move

Currents can sometimes flow in a Currents can sometimes flow in a semiconductorsemiconductor

Power Adapters 17

PhotoconductorsPhotoconductors

In cold and dark, a semiconductor is In cold and dark, a semiconductor is insulatinginsulating Electrons can’t cross into the conduction band,Electrons can’t cross into the conduction band, so the semiconductors behaves like an insulator.so the semiconductors behaves like an insulator.

In heat or light, a semiconductor may conductIn heat or light, a semiconductor may conduct Light energy or thermal energy bridges the gap,Light energy or thermal energy bridges the gap, so electrons cross from valence to conduction so electrons cross from valence to conduction

bandband and the semiconductor behaves like a metal.and the semiconductor behaves like a metal.

Power Adapters 18

Doped SemiconductorsDoped Semiconductors

Pure semiconductors are normally Pure semiconductors are normally insulatinginsulating Valence levels are filled and can’t conductValence levels are filled and can’t conduct Conduction levels are empty and can’t Conduction levels are empty and can’t

conductconduct Impure semiconductors can be conductingImpure semiconductors can be conducting

Extra valence levels permit valence-band Extra valence levels permit valence-band conductionconduction

Extra electrons permit conduction-band Extra electrons permit conduction-band conductionconduction

Power Adapters 19

p-Type Semiconductorsp-Type Semiconductors

When the doping atoms have fewer When the doping atoms have fewer electrons,electrons, they produce extra empty valence levelsthey produce extra empty valence levels and current can flow via those valence and current can flow via those valence

levels.levels.

Power Adapters 20

n-Type Semiconductorsn-Type Semiconductors

When doping atoms have more When doping atoms have more electrons,electrons, they produce extra full conduction they produce extra full conduction

levelslevels and current can flow via those and current can flow via those

conduction levels.conduction levels.

Power Adapters 21

Question 4Question 4

Why does a diode carry current only Why does a diode carry current only one way?one way?

Power Adapters 22

pn-Junction pn-Junction (before contact)(before contact)

Before p-type semiconductor meets Before p-type semiconductor meets n-type,n-type, each material can conduct electricityeach material can conduct electricity and each material is electrically neutral and each material is electrically neutral

everywhere.everywhere.

Power Adapters 23

pn-Junction pn-Junction (after contact)(after contact)

After p-type semiconductor meets n-After p-type semiconductor meets n-type,type, an insulating depletion region appears an insulating depletion region appears

at junctionat junction and that depletion region is electrically and that depletion region is electrically

polarized.polarized.

Power Adapters 24

Forward ConductionForward Conduction

When electrons are added to the n-When electrons are added to the n-type end and removed from the p-type end and removed from the p-type end,type end, the depletion region shrinksthe depletion region shrinks and the diode conducts current.and the diode conducts current.

Power Adapters 25

Reverse ConductionReverse Conduction

When electrons are added to the p-When electrons are added to the p-type end and removed from the n-type end and removed from the n-type end,type end, the depletion region growsthe depletion region grows and the diode does not conduct current.and the diode does not conduct current.

Power Adapters 26

Question 5Question 5

How does a capacitor store electric How does a capacitor store electric charges?charges?

Power Adapters 27

CapacitorsCapacitors

A capacitorA capacitor consists of two conducting platesconsists of two conducting plates that are separated by an insulator.that are separated by an insulator.

Plates store equal but opposite chargesPlates store equal but opposite charges Stored charge is proportional to voltage Stored charge is proportional to voltage

differencedifference Constant of proportionality is called capacitanceConstant of proportionality is called capacitance Bigger or closer plates increases capacitanceBigger or closer plates increases capacitance

Along with charge, capacitors store energyAlong with charge, capacitors store energy

Power Adapters 28

Complete Power AdapterComplete Power Adapter

A transformer provides lower A transformer provides lower voltage AC,voltage AC,

diodes convert that AC to pulsed DC,diodes convert that AC to pulsed DC, and a capacitor smoothes out the and a capacitor smoothes out the

ripples.ripples.

Power Adapters 29

Introductory Question Introductory Question (revisited)(revisited)

If you install a pocket radio’s batteries If you install a pocket radio’s batteries backward, it won’t work because itsbackward, it won’t work because its

A.A. speaker will move the wrong direction.speaker will move the wrong direction.

B.B. parts can only conduct current one parts can only conduct current one way.way.

C.C. batteries will absorb power and batteries will absorb power and recharge.recharge.

Power Adapters 30

Summary about Power Summary about Power AdaptersAdapters

Use transformers to obtain low-Use transformers to obtain low-voltage ACvoltage AC

Use diodes to obtain low-voltage Use diodes to obtain low-voltage pulsed DCpulsed DC

Use a capacitor to obtain low-voltage Use a capacitor to obtain low-voltage DCDC

Semiconductor diodes make them Semiconductor diodes make them inexpensiveinexpensive