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Transcript
Power Adapters 1 Power Adapters Power Adapters 2 Introductory Question 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 3 Observations about Power Adapters 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 Power Adapters 4 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? Power Adapters 5 Question 1 Why isn’t a power adapter just a transformer? Power Adapters 6 Power Adapter Components 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. A diode is a one-way device for current A capacitor is a charge-storage device Power Adapters 7 Question 2 How does a metal differ from an insulator? Power Adapters 8 Metals, Insulators, and Diodes 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. Power Adapters 9 Quantum Physics (Part 1) Things travel as waves, but interact as particles For example, light travels as waves (electromagnetic waves) but is emitted and absorbed as particles (photons). For example, electrons are emitted and detected as particles but travel as waves. This wave-particle “duality” is universal! Power Adapters 10 Quantum Physics (Part 2) 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 Bosons have no exclusion principle Indistinguishable bosons prefer the same wave Light particles (photons) are bosons, hence the laser Power Adapters 11 Electrons in Solids (Part 1) 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 Electrons have two spin states: up and down Spin-up is distinguishable from spin-down Levels are filled from lowest to highest energy “Fermi level” divides last filled and first unfilled Power Adapters 12 Electrons in Solids (Part 2) Power Adapters 13 Metals 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 Power Adapters 14 Insulators 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 Power Adapters 15 Question 3 How does charge move in a semiconductor? Power Adapters 16 Semiconductors 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 Power Adapters 17 Photoconductors In cold and dark, a semiconductor is insulating Electrons can’t cross into the conduction band, so the semiconductors behaves like an insulator. 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. Power Adapters 18 Doped Semiconductors Pure semiconductors are normally insulating Valence levels are filled and can’t conduct Conduction levels are empty and can’t conduct Impure semiconductors can be conducting Extra valence levels permit valence-band conduction Extra electrons permit conduction-band conduction Power Adapters 19 p-Type Semiconductors When the doping atoms have fewer electrons, they produce extra empty valence levels and current can flow via those valence levels. Power Adapters 20 n-Type Semiconductors When doping atoms have more electrons, they produce extra full conduction levels and current can flow via those conduction levels. Power Adapters 21 Question 4 Why does a diode carry current only one way? Power Adapters 22 pn-Junction (before contact) Before p-type semiconductor meets n-type, each material can conduct electricity and each material is electrically neutral everywhere. Power Adapters 23 pn-Junction (after contact) After p-type semiconductor meets n-type, an insulating depletion region appears at junction and that depletion region is electrically polarized. Power Adapters 24 Forward Conduction 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. Power Adapters 25 Reverse Conduction 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. Power Adapters 26 Question 5 How does a capacitor store electric charges? Power Adapters 27 Capacitors A capacitor consists of two conducting plates that are separated by an insulator. 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 Along with charge, capacitors store energy Power Adapters 28 Complete Power Adapter A transformer provides lower voltage AC, diodes convert that AC to pulsed DC, and a capacitor smoothes out the ripples. Power Adapters 29 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 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