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Transcript
SJSU ENGR 10 Electrical Power Sources Prof. Ping Hsu (c) P.Hsu 2007 Electrical energy can be transmitted via a pair of wires. + - Light bulb Energy flow Sourcing energy Consuming energy (c) P.Hsu 2007 A hydraulic analogy High pressure Hydraulic Motor Pump Sourcing power Power flow Low pressure Consuming power (c) P.Hsu 2007 ` • Voltage is the force (or pressure) that forces the positive electrical charge to flow (current) through a circuit. • Voltage is measured in ‘Volts’.. + + 1.5v - (c) P.Hsu 2007 Light bulb ‘Voltage’ is a relative quantity. The “+” and “-” symbols in a circuit diagram denote the relative voltage (pressure) between two wires. + + 1.5v - Light bulb - In this circuit, the voltage on the wire marked by ‘+’ is 1.5v higher than that the wire marked by ‘-”. (c) P.Hsu 2007 Current is the flow rate of positive electrical charge. Current is measured in Ampere (or Coulomb per second) + + Light bulb - Current always flows in complete loop. (c) P.Hsu 2007 Analogy Electrical Circuit Hydraulic system Voltage (V) Pressure (psi) Current (A) Fluid flow rate (Charge flow rate) + - Pump + Light bulb Hydraulic Motor (c) P.Hsu 2007 Ohm’s Law I + + V - R - If the ‘circuit’ is a simple resistor, the voltage, current, and the resistance of the resistor is related by Ohm’s Law: V I R Resistance is measured in Ohm (Ω) (c) P.Hsu 2007 Two extreme cases I=0 + + V - I + R= infinity V=0 - - Open Circuit (R=infinite) + - Short Circuit (R=0) (c) P.Hsu 2007 R= 0 About Ohm’s Law Ohm’s Law, I=V/R, ONLY applies to the voltage across and current through a resistor. While any circuit operates at a certain voltage and current, the voltage and current may NOT be related by Ohm’s Law. I + + V - iPod - (c) P.Hsu 2007 V I R Equivalent Loading Resistance While not all circuits’ voltage and current are related by the Ohm’s Law, we often take a circuit’s normal operating voltage and divide it by its normal operating current. This value is the circuit’s equivalent resistance. Io I + + V - + iPod + Vo - - V R I Req - Vo Req Io (c) P.Hsu 2007 Q1. Voltage in an electrical circuit is similar to what physical quantity in a hydraulic system? (a) Fluid flow rate (b) Speed of the hydraulic motor (c) Volume of the hydraulic fluid (d) Pressure (e) Speed of the pump (c) P.Hsu 2007 Q2: From the values given below, what is the equivalent resistance of an IPod? 0.1A + + 3v - (a) (b) (c) (d) (e) 0.3 Ω 30 Ω 0.03 Ω 0.9 Ω 3.1 Ω (c) P.Hsu 2007 IPod The rate of electric energy transfer (power) in an electrical circuit is: Power(w) = V(volts) I(Amps) The light bulb is consuming P=V*I of power. The battery is sourcing the same amount of power. + + Light bulb (c) P.Hsu 2007 I=0 + + Light bulb V - Power = VI = V0 = 0 (Open Circuit) (c) P.Hsu 2007 High pressure No hydraulic fluid flow Hydraulic Motor Pump ` No Power Power = Pressure 0 = 0 (c) P.Hsu 2007 Sourcing or Consuming Power? When a current flows through a circuit experiencing a voltage drop, this circuit is consuming power. When a current flows through a circuit experiencing a voltage rise, this circuit is sourcing power. (c) P.Hsu 2007 A hydraulic system analogy B A 1000psi PUMP High side Hydraulic Motor Low side For Box A: Current flows from low to high => providing power For Box B: Current flows from high to low => consuming power (c) P.Hsu 2007 I=2 - A V=10 B + Q3: Is box A consuming energy or sourcing energy? a) consuming b) sourcing c) neither d) both (c) P.Hsu 2007 Theoretical Voltage Source A theoretical voltage source keeps the output voltage at a constant level regardless of the amount of current drawn by the circuit (load). Theoretical voltage source can provide any amount of current drawn by the circuit + + V=1.5 - any circuit - A theoretical voltage source guarantees output voltage at the specified value. (c) P.Hsu 2007 Theoretical voltage source V vs. I Output Load Voltage Current Power (w) Load 3 3 3 3 0 0.1 0.2 0.3 0 0.3 0.6 0.9 No Load IPod Walkie Talkie - 3 3 3 0.4 0.5 0.6 1.2 1.5 1.8 Flashlight Digital Camera ↓ ↓ ↓ 3 10100 3×10100 (c) P.Hsu 2007 Only in theory Theoretical voltage source V vs. I curve (as the load varies) V W 4v power curve 2w 3v 1,5w 2v 1w 1v 0.5w 0v 0w Regardless load current, output voltage remains constant. Output Current 0A 0.2A 0.3A 0.4A no |MP3| |flashlight | load 0.5A |camera | The operating point when a light bulb is connected to the source. The operating point when a MP3 player is connected to the source. (c) P.Hsu 2007 Practical Voltage Source • A practical voltage source’s output voltage drops as more current is drawn from it. This effect is called being “loaded down”. I=0.2 + + V=1.5 - Light bulb - I=0.8 + + V=1.1 - Bigger Light bulb - (c) P.Hsu 2007 Practical voltage source Output voltage is loaded down. Voltag e Curr Pow ent er Equivalent loading resistance 10 0 0 10 2 20 5 Ω (Light load) 10 3 30 3.3 Ω (Light load) 9.8 4 39.2 2.5 Ω (Normal) 9.4 5 47 1.9 Ω (Normal) 8.5 6 51 1.4 Ω (MAX POWER) 7.2 7 50.4 1 Ω Over load 5.2 8 41.6 0.65 Ω (Over load) 3.0 9 27 0.3 Ω (Over load) 0 15 0 infinite (open circuit) 0 Ω (Output Shorted) (c) P.Hsu 2007 Higher current drawn from the source (lower load resistance) In a graphic form (V, I, P vs. Req) V, I, P power V I Max power Loading condition (c) P.Hsu 2007 Loading resistance In a graphic form (V, P vs. I) Max power voltage V vs. I curve V Max power operating point W=V*I Max power point Current (Amp) power curve Max power current (c) P.Hsu 2007 A solar panel is a practical voltage source. (c) P.Hsu 2007 IV curve of a solar panel • There are three set of curves shown, for three different sun light intensities. (c) P.Hsu 2007 I + V _ POT A variable resistor (potentiometer or POT) is used in experimentally determining the V vs. I curve of a solar cell. The same procedure is used in the wind turbine experiment. (c) P.Hsu 2007 Voltage Current Power Loading condition Vary resistance from 0 to the full resistance of the POT. Take down the voltage, current, and power readings at a small resistance increment (c) P.Hsu 2007 Setup in the lab cell #1 cell #2 cell #3 cell #4 cell #5 cell #6 Power meter POT (c) P.Hsu 2007 Practical voltage source specification A 9V DC adaptor is rated at 9 v @1A means: This voltage source can maintain output voltage close to 9V if the load current is less than 1A. It is capable to power any equipment that needs 9v supply and takes less than 1Amp. It does NOT mean: This voltage source will maintain its output voltage at 9V and its output current at 1A. (Output current depends on the circuit (load), not the voltage source.) (c) P.Hsu 2007 Practical voltage source specification • The following graph are the V-I curves of a adaptor rated at 9v at 1A and one at 9v at 2A. 9v @ 1A Voltage (V) 9v 1 2 9v @ 2A Current (Amp) Either adaptor works for, for example, a CD player that needs 0.5A at 9V. (c) P.Hsu 2007 This adaptor is rated at 9V at 0.2A. (c) P.Hsu 2007 ‘Load’ specification This Radio Shack intercom is labeled 7.5V, 100mA. • It should only be connected to a 7.5v voltage source. A higher voltage can damage the equipment. The equipment may not function correctly with a lower voltage. • When it is connected to 7.5v voltage source, it will draw about 0.1A (100mA) of current from it. (c) P.Hsu 2007 Q4. The label on a CD player shows [email protected]. Which of the follow adaptors will work with this CD player? (a) [email protected] (b) [email protected] (c) [email protected] (d) [email protected] (e) [email protected] (c) P.Hsu 2007