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Power Adapters 1
Power Adapters
Power Adapters 2
Introductory Question
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If you install a pocket radio’s batteries
backward, it won’t work because its
A.
speaker will move the wrong direction.
parts can only conduct current one way.
batteries will absorb power and recharge.
B.
C.
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Observations about Power Adapters
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They obtain power from AC electrical outlets
They provide DC power to electronic devices
They somehow fix the AC versus DC problem
They come in various voltages and other ratings
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5 Questions about
Power Adapters
Why isn’t a power adapter just a transformer?
How does a metal differ from an insulator?
How does charge move in a semiconductor?
Why does a diode carry current only one way?
How does a capacitor store electric charges?
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Question 1
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Why isn’t a power adapter just a transformer?
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Power Adapter Components
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A power adapter provides low-voltage DC
Starting with high-voltage AC, it must
use a transformer to obtain low-voltage AC,
 use diodes to obtain low-voltage pulsed DC,
 and a capacitors to obtain low-voltage DC.
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A diode is a one-way device for current
A capacitor is a charge-storage device
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Question 2
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How does a metal differ from an insulator?
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Metals, Insulators, and Diodes
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A metal carries current in any direction
An insulator won’t carry current in any direction
A diode carries current only in one direction
To understand a diode,
we need to understanding metals and insulators
 and a bit of quantum physics.
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Quantum Physics (Part 1)
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Things travel as waves, but interact as particles
For example, light
travels as waves (electromagnetic waves)
 but is emitted and absorbed as particles (photons).
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For example, electrons
are emitted and detected as particles
 but travel as waves.
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This wave-particle “duality” is universal!
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Quantum Physics (Part 2)
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Particles come in two types: fermions or bosons
Fermions obey the “Pauli exclusion principle”
Only one indistinguishable fermion per wave
 Indistinguishable fermions require different waves
 Electrons, protons, and neutrons are fermions
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Bosons have no exclusion principle
Indistinguishable bosons prefer the same wave
 Light particles (photons) are bosons, hence the laser
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Electrons in Solids (Part 1)
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Electrons exist in a solid as standing waves
Only certain electron standing waves fit in solid
 Each possible standing wave has an energy “level”
 Up to two electrons can “occupy” each level
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Electrons have two spin states: up and down
 Spin-up is distinguishable from spin-down
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Levels are filled from lowest to highest energy
“Fermi level” divides last filled and first unfilled
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Electrons in Solids (Part 2)
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Metals
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In a metal,
the Fermi level has empty levels just above it
 Like patrons in a partly filled theatre, electrons can
move in response to electric fields
 Currents can flow through a metal in any direction
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Insulators
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In an insulator,
The Fermi level has no empty levels nearby
 Like patrons in a full theatre, electrons can’t move in
response to forces
 Current can’t flow through an insulator
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Question 3
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How does charge move in a semiconductor?
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Semiconductors
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Semiconductors are “poor insulators”
Narrow gap from valence to conduction band
 Like patrons in a full theatre with a low empty
balcony, electrons can hop to the balcony and move
 Currents can sometimes flow in a semiconductor
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Photoconductors
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In cold and dark, a semiconductor is insulating
Electrons can’t cross into the conduction band,
 so the semiconductors behaves like an insulator.
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In heat or light, a semiconductor may conduct
Light energy or thermal energy bridges the gap,
 so electrons cross from valence to conduction band
 and the semiconductor behaves like a metal.
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Doped Semiconductors
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Pure semiconductors are normally insulating
Valence levels are filled and can’t conduct
 Conduction levels are empty and can’t conduct
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Impure semiconductors can be conducting
Extra valence levels permit valence-band conduction
 Extra electrons permit conduction-band conduction
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p-Type Semiconductors
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When the doping atoms have fewer electrons,
they produce extra empty valence levels
 and current can flow via those valence levels.
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n-Type Semiconductors
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When doping atoms have more electrons,
they produce extra full conduction levels
 and current can flow via those conduction levels.
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Question 4
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Why does a diode carry current only one way?
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pn-Junction (before contact)
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Before p-type semiconductor meets n-type,
each material can conduct electricity
 and each material is electrically neutral everywhere.
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pn-Junction (after contact)
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After p-type semiconductor meets n-type,
an insulating depletion region appears at junction
 and that depletion region is electrically polarized.
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Forward Conduction
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When electrons are added to the n-type end and
removed from the p-type end,
the depletion region shrinks
 and the diode conducts current.
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Reverse Conduction
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When electrons are added to the p-type end and
removed from the n-type end,
the depletion region grows
 and the diode does not conduct current.
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Question 5
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How does a capacitor store electric charges?
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Capacitors
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A capacitor
consists of two conducting plates
 that are separated by an insulator.
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Plates store equal but opposite charges
Stored charge is proportional to voltage difference
 Constant of proportionality is called capacitance
 Bigger or closer plates increases capacitance
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Along with charge, capacitors store energy
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Complete Power Adapter
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A transformer provides lower voltage AC,
diodes convert that AC to pulsed DC,
and a capacitor smoothes out the ripples.
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Introductory Question (revisited)

If you install a pocket radio’s batteries
backward, it won’t work because its
A.
speaker will move the wrong direction.
parts can only conduct current one way.
batteries will absorb power and recharge.
B.
C.
Power Adapters 30
Summary about Power Adapters
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Use transformers to obtain low-voltage AC
Use diodes to obtain low-voltage pulsed DC
Use a capacitor to obtain low-voltage DC
Semiconductor diodes make them inexpensive