1. Power Adapters 1 Power Adapters

2. Power Adapters 2 Introductory Question If you install a pocket radio’s batteries backward, it won’t work because its If you install a pocket radio’s batteries backward, it won’t work because its A. speaker will move the wrong direction. B. parts can only conduct current one way. C. batteries will absorb power and recharge.

3. Power Adapters 3 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 They come in various voltages and other ratings

4. Power Adapters 4 5 Questions about Power Adapters Why isn’t a power adapter just a transformer? Why isn’t a power adapter just a transformer? How does a metal differ from an insulator? How does a metal differ from an insulator? How does charge move in a semiconductor? How does charge move in a semiconductor? Why does a diode carry current only one way? Why does a diode carry current only one way? How does a capacitor store electric charges? How does a capacitor store electric charges?

5. Power Adapters 5 Question 1 Why isn’t a power adapter just a transformer? Why isn’t a power adapter just a transformer?

6. Power Adapters 6 Power Adapter Components A power adapter provides low-voltage DC A power adapter provides low-voltage DC Starting with high-voltage AC, it must Starting 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 current A diode is a one-way device for current A capacitor is a charge-storage device A capacitor is a charge-storage device

7. Power Adapters 7 Question 2 How does a metal differ from an insulator? How does a metal differ from an insulator?

8. Power Adapters 8 Metals, Insulators, and Diodes A metal carries current in any direction A metal carries current in any direction An insulator won’t carry current in any direction An insulator won’t carry current in any direction A diode carries current only in one direction A diode carries current only in one direction To understand a diode, To understand a diode, we need to understanding metals and insulators we need to understanding metals and insulators and a bit of quantum physics. and a bit of quantum physics.

9. Power Adapters 9 Quantum Physics (Part 1) Things travel as waves, but interact as particles Things travel as waves, but interact as particles For example, light For example, light travels as waves (electromagnetic waves) travels as waves (electromagnetic waves) but is emitted and absorbed as particles (photons). but is emitted and absorbed as particles (photons). For example, electrons For example, electrons are emitted and detected as particles are 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!

10. Power Adapters 10 Quantum Physics (Part 2) Particles come in two types: fermions or bosons Particles come in two types: fermions or bosons Fermions obey the “Pauli exclusion principle” Fermions obey the “Pauli exclusion principle” Only one indistinguishable fermion per wave Only one indistinguishable fermion per wave Indistinguishable fermions require different waves Indistinguishable fermions require different waves Electrons, protons, and neutrons are fermions Electrons, protons, and neutrons are fermions Bosons have no exclusion principle Bosons have no exclusion principle Indistinguishable bosons prefer the same wave Indistinguishable bosons prefer the same wave Light particles (photons) are bosons, hence the laser Light particles (photons) are bosons, hence the laser

11. Power Adapters 11 Electrons in Solids (Part 1) Electrons exist in a solid as standing waves Electrons exist in a solid as standing waves Only certain electron standing waves fit in solid Only 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 level Up to two electrons can “occupy” each level Electrons have two spin states: up and down Electrons have two spin states: up and down Spin-up is distinguishable from spin-down Spin-up is distinguishable from spin-down Levels are filled from lowest to highest energy Levels are filled from lowest to highest energy “Fermi level” divides last filled and first unfilled “Fermi level” divides last filled and first unfilled

12. Power Adapters 12 Electrons in Solids (Part 2)

13. Power Adapters 13 Metals In a metal, In a metal, the Fermi level has empty levels just above it the Fermi level has empty levels just above it Like patrons in a partly filled theatre, electrons can move in response to electric fields Like patrons in a partly filled theatre, electrons can move in response to electric fields Currents can flow through a metal in any direction Currents can flow through a metal in any direction

14. Power Adapters 14 Insulators In an insulator, In an insulator, The Fermi level has no empty levels nearby The Fermi level has no empty levels nearby Like patrons in a full theatre, electrons can’t move in response to forces Like patrons in a full theatre, electrons can’t move in response to forces Current can’t flow through an insulator Current can’t flow through an insulator

15. Power Adapters 15 Question 3 How does charge move in a semiconductor? How does charge move in a semiconductor?

16. Power Adapters 16 Semiconductors Semiconductors are “poor insulators” Semiconductors are “poor insulators” Narrow gap from valence to conduction band 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 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 Currents can sometimes flow in a semiconductor

17. Power Adapters 17 Photoconductors In cold and dark, a semiconductor is insulating In cold and dark, a semiconductor is insulating 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 conduct In 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 band so electrons cross from valence to conduction band and the semiconductor behaves like a metal. and the semiconductor behaves like a metal.

18. Power Adapters 18 Doped Semiconductors Pure semiconductors are normally insulating Pure semiconductors are normally insulating Valence levels are filled and can’t conduct Valence levels are filled and can’t conduct Conduction levels are empty and can’t conduct Conduction levels are empty and can’t conduct Impure semiconductors can be conducting Impure semiconductors can be conducting Extra valence levels permit valence-band conduction Extra valence levels permit valence-band conduction Extra electrons permit conduction-band conduction Extra electrons permit conduction-band conduction

19. Power Adapters 19 p-Type Semiconductors When the doping atoms have fewer electrons, When the doping atoms have fewer electrons, they produce extra empty valence levels they produce extra empty valence levels and current can flow via those valence levels. and current can flow via those valence levels.

20. Power Adapters 20 n-Type Semiconductors When doping atoms have more electrons, When doping atoms have more electrons, they produce extra full conduction levels they produce extra full conduction levels and current can flow via those conduction levels. and current can flow via those conduction levels.

21. Power Adapters 21 Question 4 Why does a diode carry current only one way? Why does a diode carry current only one way?

22. Power Adapters 22 pn-Junction (before contact) Before p-type semiconductor meets n-type, Before p-type semiconductor meets n-type, each material can conduct electricity each material can conduct electricity and each material is electrically neutral everywhere. and each material is electrically neutral everywhere.

23. Power Adapters 23 pn-Junction (after contact) After p-type semiconductor meets n-type, After p-type semiconductor meets n-type, an insulating depletion region appears at junction an insulating depletion region appears at junction and that depletion region is electrically polarized. and that depletion region is electrically polarized.

24. Power Adapters 24 Forward Conduction When electrons are added to the n-type end and removed from the p-type end, When electrons are added to the n-type end and removed from the p-type end, the depletion region shrinks the depletion region shrinks and the diode conducts current. and the diode conducts current.

25. Power Adapters 25 Reverse Conduction When electrons are added to the p-type end and removed from the n-type end, When electrons are added to the p-type end and removed from the n-type end, the depletion region grows the depletion region grows and the diode does not conduct current. and the diode does not conduct current.

26. Power Adapters 26 Question 5 How does a capacitor store electric charges? How does a capacitor store electric charges?

27. Power Adapters 27 Capacitors A capacitor A capacitor consists of two conducting plates consists of two conducting plates that are separated by an insulator. that are separated by an insulator. Plates store equal but opposite charges Plates store equal but opposite charges Stored charge is proportional to voltage difference Stored charge is proportional to voltage difference Constant of proportionality is called capacitance Constant of proportionality is called capacitance Bigger or closer plates increases capacitance Bigger or closer plates increases capacitance Along with charge, capacitors store energy Along with charge, capacitors store energy

28. Power Adapters 28 Complete Power Adapter A transformer provides lower voltage AC, A transformer provides lower voltage AC, diodes convert that AC to pulsed DC, diodes convert that AC to pulsed DC, and a capacitor smoothes out the ripples. and a capacitor smoothes out the ripples.

29. Power Adapters 29 Introductory Question (revisited) If you install a pocket radio’s batteries backward, it won’t work because its If you install a pocket radio’s batteries backward, it won’t work because its A. speaker will move the wrong direction. B. parts can only conduct current one way. C. batteries will absorb power and recharge.

30. Power Adapters 30 Summary about Power Adapters Use transformers to obtain low-voltage AC Use transformers to obtain low-voltage AC Use diodes to obtain low-voltage pulsed DC Use diodes to obtain low-voltage pulsed DC Use a capacitor to obtain low-voltage DC Use a capacitor to obtain low-voltage DC Semiconductor diodes make them inexpensive Semiconductor diodes make them inexpensive