(c) P.Hsu 2007 SJSU ENGR 10 Electrical Power Sources Prof. Ping Hsu

(c) P.Hsu 2007 Electrical energy can be transmitted via a pair of wires.

(c) P.Hsu 2007 A hydraulic analogy

(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’..

(c) P.Hsu 2007 In this circuit, the voltage on the wire marked by ‘+’ is 1.5v higher than that the wire marked by ‘-”. ‘Voltage’ is a relative quantity. The “+” and “-” symbols in a circuit diagram denote the relative voltage (pressure) between two wires.

(c) P.Hsu 2007 Current is the flow rate of positive electrical charge. Current is measured in Ampere (or Coulomb per second) 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)

(c) P.Hsu 2007 Ohm’s Law If the ‘circuit’ is a simple resistor, the voltage, current, and the resistance of the resistor is related by Ohm’s Law: Resistance is measured in Ohm (Ω)

(c) P.Hsu 2007 Two extreme cases Open Circuit (R=infinite) Short Circuit (R=0)

(c) P.Hsu 2007 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.

(c) P.Hsu 2007 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.

(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 (a)0.3 Ω (b)30 Ω (c)0.03 Ω (d)0.9 Ω (e) 3.1 Ω Q2: From the values given below, what is the equivalent resistance of an IPod?

(c) P.Hsu 2007 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.

(c) P.Hsu 2007 Power = V  I = V  0 = 0 (Open Circuit)

(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 For Box A: Current flows from low to high => providing power For Box B: Current flows from high to low => consuming power A hydraulic system analogy

(c) P.Hsu 2007 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).

(c) P.Hsu 2007 Theoretical voltage source V vs. I Output Voltage Load Current Power (w) Load 300No Load IPod Walkie Talkie Flashlight Digital Camera ↓↓↓ × Only in theory

(c) P.Hsu 2007 Theoretical voltage source V vs. I curve (as the load varies)

(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”.

(c) P.Hsu 2007 Practical voltage source Voltag e Curr ent Pow er Equivalent loading resistance 1000 infinite (open circuit) Ω (Light load) Ω (Light load) Ω (Normal) Ω (Normal) Ω (MAX POWER) Ω Over load Ω (Over load) Ω (Over load) Ω (Output Shorted)

(c) P.Hsu 2007 In a graphic form (V, I, P vs. R eq )

(c) P.Hsu 2007 In a graphic form (V, P vs. I)

(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 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 VoltageCurrentPowerLoading 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

(c) P.Hsu 2007 Practical voltage source specification A 9V DC adaptor is rated at 9 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. 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 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. ‘Load’ specification

(c) P.Hsu 2007 Q4. The label on a CD player shows Which of the follow adaptors will work with this CD player? (a) (b) (c) (d) (e)