E40M Device Models, Resistors, Voltage and Current Sources, Diodes, Solar Cells. M. Horowitz, J. Plummer, R. Howe 1

Transcription

1 E40M Devce Models, Resstors, Voltage and Current Sources, Dodes, Solar Cells M. Horowtz, J. Plummer, R. Howe 1

2 Understandng the Solar Charger Lab Project #1 We need to understand how: 1. Current, voltage and power behave n crcuts 2. Electrcal devces constran current and voltage 3. Dodes ncludng solar cells work 4. Voltage converter works (later n the quarter). M. Horowtz, J. Plummer, R. Howe 2

3 Readng For These Topcs Chapter 2 n the course reader A&L Two termnal elements Voltage source; resstor; wres M. Horowtz, J. Plummer, R. Howe 3

4 Devce Models A general way to show the relatonshp between two varables That s what we wll do for our dfferent types of electrcal devces 1 A + Sgn conventon: postve current flows nto the termnal wth the + voltage label +, v - +, v + 5 V v - -, v - -, v + M. Horowtz, J. Plummer, R. Howe 4

5 Devce Models Note that the energy s dsspated by the devce n quadrants 1 and 3, and power s generated by the devce n quadrants 2 and 4. 1 A + +, v - +, v + 5 V v -, v - -, v + M. Horowtz, J. Plummer, R. Howe 5

6 Devce Models Battery, Voltage Source A battery or a voltage source provde a fxed out put voltage no matter what current they are asked to provde or consume ( snk ). In quadrant 1 energy s consumed, n quadrant 4 energy s provded. Quadrant 1 = battery chargng, quadrant 4 = battery dschargng V v 5 V M. Horowtz, J. Plummer, R. Howe 6

7 Devce Models Resstors Current s proportonal to voltage V = R Ohm s Law The book also uses G Conductance = 1/R = G V Symbol v + V M. Horowtz, J. Plummer, R. Howe 7

8 Why Does Resstance Exst? (What Physcal Effect Does t Model?) Conductors are not perfect They use a lttle energy to get current to flow through a wre 1 Snce the energy flow nto the wre s ( ΔV) There must be a voltage drop along the wre Generally ths drop s proportonal to the current V = k We call the constant of proportonalty Resstance Make resstors by usng materal that doesn t conduct well 1 Well except for superconductors whch are magcal. They have nterestng propertes, ncludng that current can flow n a loop forever! Superconductors are used n MRI machnes to generate large magnetc felds effcently. M. Horowtz, J. Plummer, R. Howe 8

9 Resstors You ll begn to work wth these ths Frday n the Prelab lecture. M. Horowtz, J. Plummer, R. Howe 9

10 Resstance Problem #1 =? What current flows n the loop? 1kΩ 1 V + 1kΩ What s the voltage across the bottom resstor? M. Horowtz, J. Plummer, R. Howe 10

11 Resstance Problem #2 What s the current? =? + 1 V 1kΩ 1kΩ M. Horowtz, J. Plummer, R. Howe 11

12 New Devce: Current Source Current s constant, ndependent of voltage; s negatve n ths case because t flows out of the + termnal In quadrant 4 the current source s provdng energy, n quadrant 3 the current source consumes energy v -, v - -, v + M. Horowtz, J. Plummer, R. Howe 12

13 New Devce Dode Dode s a one-way street for current Current can flow n only one drecton An deal dode If the current s postve Voltage drop s zero ndependent of current Looks lke a wre (short crcut) If the voltage s negatve Current s always zero ndependent of voltage Looks lke the devce s not there (open crcut) v The plus end of the dode s called the anode The mnus end of the dode s called the cathode M. Horowtz, J. Plummer, R. Howe 13

14 There Are Many Types of Dodes M. Horowtz, J. Plummer, R. Howe 14

15 Real Dodes Do conduct current n only one drecton But they have some forward voltage drop And ther voltage does ncrease wth current, but Current s exponental on voltage! and voltage s logarthmc on current. So the voltage s not very dependent on current level I = I o exp qv kt Ther drop depends on the type of dode Schottky dodes are around 0.3 V Normal slcon PN dodes are generally around 0.6 V Other semconductor materals have larger voltages M. Horowtz, J. Plummer, R. Howe 15

16 Dode v v M. Horowtz, J. Plummer, R. Howe 16

17 Idealzed Dode v Ths s the model we wll mostly use n E40M Matches the behavor of a real dode pretty well Just need to choose the rght value of V f For any postve current The voltage drop across the dode s V f V f For any voltage less than V f The current through the devce s zero v M. Horowtz, J. Plummer, R. Howe 17

18 Dodes n Smple Crcuts =? =? 1 V + 1kΩ V f = V + 1kΩ V f = 0.6 M. Horowtz, J. Plummer, R. Howe 18

19 Some Dodes Are Lght Senstve These dodes are called solar cells When you shne lght on the cell The lght generates a current whch runs n parallel to the dode The value of the current s proportonal to the lght Ths generates electrcal energy Actually converts energy n the lght to electrcal form M. Horowtz, J. Plummer, R. Howe 19

20 Solar Cell Remember a solar cell s a dode So we represent t by a dode symbol When lght shnes on the dode The lght generates a current We represent ths current by a current source. The value of ths current s proportonal to the lght shnng on the dode Notce the drecton of the current Flows out of + termnal of dode M. Horowtz, J. Plummer, R. Howe 20

21 Solar Cell -V Curve V Optcally generated current M. Horowtz, J. Plummer, R. Howe 21

22 Open Crcut Voltage Short Crcut Current What s the voltage when zero current flows out of the devce? What s the current when there s no voltage across the devce? Open Crcut Voltage v Short Crcut Current M. Horowtz, J. Plummer, R. Howe 22

23 What Sets the Open Crcut Voltage and the Short Crcut Current? If there s no path for current (open source voltage case) It wll flow nto the dode KCL must stll hold V dode = V F If you short the dode out (short crcut current) You measure all the optcally generated current M. Horowtz, J. Plummer, R. Howe 23

24 Extractng Power from a Dode Power s V So n nether of these cases do we get power from the dode Open Crcut Voltage v Actual power wll be less than: V OC, SC You ll actually measure these parameters on your solar array next week. Short Crcut Current M. Horowtz, J. Plummer, R. Howe 24

25 FYI How Do Lght Emttng Dodes and Solar Cells Actually Work? M. Horowtz, J. Plummer, R. Howe 25

26 Generatng Enough Voltage There s one weak pont for solar cells Each cell provdes < 0.5 V We need around 5 V, so we bought a panel wth many cells stacked n seres Commercal photovoltac arrays also use ths approach How do we fgure out voltages and currents here? M. Horowtz, J. Plummer, R. Howe 26

27 Or Here? Dode Solar Cell L Bat Volt Conv R In the next set of lecture notes we ll develop methods to analyze crcuts by extendng the KCL and KVL deas we ve already dscussed. M. Horowtz, J. Plummer, R. Howe 27

28 Learnng Objectves for These Notes Understand the devce -V curve of a resstor Understand the devce -V curve of a voltage source Understand the devce -V curve of a current source Understand the operaton of a dode, and ts symbol M. Horowtz, J. Plummer, R. Howe 28