[PDF] - Power Adapters Power Adapters speaker will move the wrong PDF Document

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Power Adapters 1

Power Adapters Power Adapters

Power Adapters 2

Introductory Question Introductory Question

If you install a pocket radio’s batteries

If you install a pocket radio’s batteries backward, it won’t work because its backward, it won’t work because its k ill h di i k ill h di i

A. A.

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

B. B.

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

C. C.

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

Power Adapters 3

Observations about Power Adapters Observations about Power Adapters

They obtain power from AC electrical outlets

They obtain power from AC electrical outlets

They provide DC power to electronic devices

They provide DC power to electronic devices

They somehow fix the AC versus DC problem

They somehow fix the AC versus DC problem

They come in various voltages and other ratings

Power Adapters 4

5 Questions about 5 Questions about Power Adapters Power Adapters

Why isn’t a power adapter simply a transformer?

Why can electrons move in metals not insulators?

How does charge move in a semiconductor?

How does a diode carry current only one way?

How does a capacitor store electric charges?

Power Adapters 5

Question 1 Question 1

Why isn’t a power adapter simply a transformer?

Why isn’t a power adapter simply a transformer?

Power Adapters 6

Power Adapter Components Power Adapter Components (part 1)

(part 1)

A basic power adapter completes three steps

to convert household AC power

into low

into low-

voltage DC power.

voltage DC power.

A tr n f rm r p rf rm

nl n f th t p A tr n f rm r p rf rm nl n f th t p

A transformer performs only one of those steps.

Step 1: household AC to low

Step 1: household AC to low-

voltage AC

voltage AC

Performed by a step

Performed by a step-

down transformer

down transformer

Household AC flows into the primary coil

Low

Low-

voltage AC flow out of the secondary coil

voltage AC flow out of the secondary coil

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2

Power Adapters 7

Power Adapter Components Power Adapter Components (part 2)

(part 2)

Step 2: low

Step 2: low-

voltage AC to low

voltage AC to low-

voltage pulsed DC

voltage pulsed DC

Performed 1 to 4 diodes: one

Performed 1 to 4 diodes: one-

way devices for current

way devices for current

Low

Low-

voltage AC flows into the system of diodes

voltage AC flows into the system of diodes

Th di d

it h th rr nt th t it l fl Th di d it h th rr nt th t it l fl

The diodes switch the current so that it always flows

ut through one wire and returns through the other
ut through one wire and returns through the other

Because the AC reversals cause the voltage to pulse,

low

low-

voltage pulsed DC flows out of the diodes

voltage pulsed DC flows out of the diodes

Power Adapters 8

Power Adapter Components Power Adapter Components (part 3)

(part 3)

Step 3: low

Step 3: low-

voltage pulsed DC to low

voltage pulsed DC to low-

voltage DC

voltage DC

Performed by a capacitor: a charge storage device

Low

Low-

voltage pulsed DC flows into the capacitor

voltage pulsed DC flows into the capacitor

C p

it r t r h r h n lt i in r in C p it r t r h r h n lt i in r in

Capacitor stores charge when voltage is increasing

Capacitor releases charge when voltage is decreasing

The capacitor smoothes out the voltage pulses

Low

Low-

voltage DC flows out of the capacitor

voltage DC flows out of the capacitor

Power Adapters 9

Question 2 Question 2

Why can electrons move in metals not insulators?

Power Adapters 10

Metals, Insulators, and Diodes Metals, Insulators, and Diodes

Three different electrical behaviors:

Metals can carry current in any direction

Insulators can’t carry current

Di d

rr rr nt nl in n dir ti n Di d rr rr nt nl in n dir ti n

Diode carry current only in one direction

To understand a diode,

we need to understanding metals and insulators

so we must take a peek at quantum physics.

Power Adapters 11

Quantum Physics Quantum Physics (Part 1)

(Part 1)

Classical physics (pre

Classical physics (pre-

1900) thought that

1900) thought that

everything in nature is a particle

everything in nature is a particle or

r a wave

a wave

electrons, atoms, and billiard balls are particles

li ht nd

nd r li ht nd nd r

light and sound are waves

Modern physics (post

Modern physics (post-

1900) recognizes that

1900) recognizes that

everything in nature is both particle

everything in nature is both particle and and wave wave

things are most wave

things are most wave-

like as they move unobserved

like as they move unobserved

things are most particle

things are most particle-

like when they interact

like when they interact

Power Adapters 12

Quantum Physics Quantum Physics (Part 2)

(Part 2)

Example 1: light

travels as waves (electromagnetic waves)

is emitted and absorbed as particles (photons)

E l 2 l E l 2 l

Example 2: electrons

are emitted and detected as particles (electrons)

travel as waves (matter waves)

reside in atoms and solids as standing waves

SLIDE 3

3

Power Adapters 13

Electrons in Solids Electrons in Solids (Part 1)

(Part 1)

In a solid, each electron is a standing wave

Similar to the vibration of a string or a drumhead

Only certain standing waves fit properly in the solid

Each allowed standing wave is called a “level”

Each allowed standing wave is called a level

An electron’s level determines its energy

At most two electrons can occupy a single level

A consequence of the Pauli exclusion principle

and the electron’s two possible spins: up and down

Power Adapters 14

Electrons in Solids Electrons in Solids (Part 2)

(Part 2)

Electrons settle into a solid’s lowest

Electrons settle into a solid’s lowest-

energy levels

energy levels

Fermi level is between last filled and first unfilled

The levels in a solid

The levels in a solid if l if l are not uniformly are not uniformly distributed in energy: distributed in energy: they clump together they clump together in “bands” that are in “bands” that are separated by separated by “band gaps” “band gaps”

Power Adapters 15

Metals Metals

In a metal,

the Fermi level has empty levels just above it

Like patrons in a partly filled theatre, electrons can

Like patrons in a partly filled theatre, electrons can move in response to electric fields move in response to electric fields move in response to electric fields move in response to electric fields

Currents can flow through a metal in any direction

Power Adapters 16

Insulators Insulators

In an insulator,

The Fermi level has no empty levels nearby

Like patrons in a full theatre, electrons can’t move in

Like patrons in a full theatre, electrons can’t move in response to electric fields response to electric fields response to electric fields response to electric fields

Current can’t flow through an insulator

Power Adapters 17

Question 3 Question 3

How does charge move in a semiconductor?

Power Adapters 18

Semiconductors Semiconductors

Pure semiconductors are “poor insulators”

A narrow band gap separates the full “valence” band

A narrow band gap separates the full “valence” band below from the empty “conduction” band above below from the empty “conduction” band above

Like patrons in a full theatre with a low empty

Like patrons in a full theatre with a low empty balcony, electrons can hop to the balcony and move balcony, electrons can hop to the balcony and move

Light or heat can allow current in a semiconductor

SLIDE 4

4

Power Adapters 19

p-

Type Semiconductor

Type Semiconductor

Adding impurity atoms to a semiconductor

changes the number of electrons it contains

alters the filling of its valence and conduction bands

I i i h d h b f l I i i h d h b f l

Impurities that reduce the number of electrons

leave some of the valence

leave some of the valence levels empty levels empty

and current can flow

and current can flow via those valence levels. via those valence levels.

p-

type semiconductor

type semiconductor

Power Adapters 20

n-

Type Semiconductor

Type Semiconductor

Impurities that increase the number of electrons

fill some of the conduction levels

and current can flow via those conduction levels.

i d i d

n-

type semiconductor

type semiconductor

Power Adapters 21

Question 4 Question 4

How does a diode carry current only one way?

Power Adapters 22

pn pn-

Junction

Junction (before contact)

(before contact)

Before p

Before p-

type semiconductor meets n

type semiconductor meets n-

type,

type,

each material can conduct electricity

and each material is electrically neutral everywhere.

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pn pn-

Junction

Junction (after contact)

(after contact)

After p

After p-

type semiconductor meets n

type semiconductor meets n-

type,

type,

electrons migrate from the n

electrons migrate from the n-

type to the p

type to the p-

type

type

an insulating depletion region appears at junction

nd th t d pl ti n r

i n i l tri ll p l riz d nd th t d pl ti n r i n i l tri ll p l riz d

and that depletion region is electrically polarized.

Power Adapters 24

Forward Conduction Forward Conduction

When electrons are added to the n

When electrons are added to the n-

type end and

type end and removed from the p removed from the p-

type end,

type end,

the depletion region shrinks

and the diode conducts current

and the diode conducts current.

SLIDE 5

5

Power Adapters 25

Reverse Conduction Reverse Conduction

When electrons are added to the p

When electrons are added to the p-

type end and

type end and removed from the n removed from the n-

type end,

type end,

the depletion region grows

and the diode does not conduct current

and the diode does not conduct current.

Power Adapters 26

Question 5 Question 5

How does a capacitor store electric charges?

Power Adapters 27

Capacitors Capacitors

A capacitor consists of

A capacitor consists of

two conducting plates

an insulator that separates those plates.

Th i Th i

The capacitor can

accumulate equal but opposite charges on its plates

develop a voltage difference between its plates

store electrostatic potential energy

Charge and voltage difference are proportional:

charge on positive plate = voltage difference · capacitance charge on positive plate = voltage difference · capacitance

Power Adapters 28

Complete Power Adapter Complete Power Adapter

A transformer provides low

A transformer provides low-

voltage AC,

voltage AC,

diodes convert that AC to pulsed DC,

and a capacitor smoothes out the pulses.

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 its backward, it won’t work because its k ill h di i k ill h di i

A. A.

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

B. B.

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

C. C.

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

Power Adapters 30

Summary about Power Adapters Summary about Power Adapters

Use transformers to obtain low

Use transformers to obtain low-

voltage AC

voltage AC

Use diodes to obtain low

Use diodes to obtain low-

voltage pulsed DC

voltage pulsed DC

Use a capacitor to obtain low

Use a capacitor to obtain low-

voltage DC

voltage DC

Semiconductor diodes make them practical