(c) p.hsu 2007 sjsu engr 10 electrical power sources prof. ping hsu
TRANSCRIPT
(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.
+
-
Light bulb
Sourcing energy
Consuming energy
Energy flow
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A hydraulic analogy
`
Pump
Hydraulic Motor
Sourcing power Consuming power
Power flow
High pressure
Low pressure
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• Voltage is the force (or pressure) that forces the positive electrical charge to flow (current) through a circuit.
• Voltage is measured in ‘Volts’..
+ +
- -
Light bulb 1.5v
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+ +
- -
Light bulb 1.5v
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.
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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.
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AnalogyElectrical Circuit Hydraulic system Voltage (V) Pressure (psi) Current (A) Fluid flow rate (Charge flow rate)
Pump
Hydraulic Motor
+ +
-
Light bulb
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Ohm’s Law
+ +
- -
V R
I
If the ‘circuit’ is a simple resistor, the voltage, current, and the resistance of the resistor is related by Ohm’s Law:
VI
R
Resistance is measured in Ohm (Ω)
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Two extreme cases
+ +
- -
V R= infinity
I=0
Open Circuit(R=infinite)
+ +
- -
V=0 R= 0
I
Short Circuit(R=0)
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About Ohm’s Law
+ +
- -
V iPod
I
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.
V
IR
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Equivalent Loading Resistance
+ +
- -
V iPod
I
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.
V
RI
oeq
o
VR
I
+ +
- -
Vo Req
Io
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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
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+ +
- -
3v IPod
0.1A
(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?
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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
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Power = VI = V0 = 0
(Open Circuit)
+ +
- -
Light bulb V
I=0
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`
Pump
Hydraulic Motor
No Power
High pressure
No hydraulic fluid flow
Power = Pressure 0 = 0
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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.
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1000psi PUMP Hydraulic
Motor
A B High side
Low side
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
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Q3: Is box A consuming energy or sourcing energy?
a) consumingb) sourcingc) neitherd) both
-
+
V=10
I=2
A B
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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).
+ +
- -
any circuit
V=1.5
Theoretical voltage source can provide any amount of current drawn by the circuit
A theoretical voltage source guarantees output voltage at the specified value.
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Theoretical voltage source V vs. IOutput Voltage
Load
Current
Power (w)
Load
3 0 0 No Load
3 0.1 0.3 IPod
3 0.2 0.6 Walkie Talkie
3 0.3 0.9 -
3 0.4 1.2 Flashlight
3 0.5 1.5 -
3 0.6 1.8 Digital Camera
↓ ↓ ↓
3 10100 3×10100 Only in theory
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Theoretical voltage source V vs. I curve (as the load varies)
Output Current
power curve V 4v 3v 2v 1v 0v
W 2w 1,5w 1w 0.5w 0w
Regardless load current, output voltage remains constant.
The operating point when a MP3 player is connected to the source.
The operating point when a light bulb is connected to the source.
0A 0.2A 0.3A 0.4A 0.5A no |MP3| |flashlight | |camera | load
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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”.
+ +
- -
Light bulb V=1.5
I=0.2
+ +
- -
V=1.1
I=0.8
Bigger Light bulb
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Practical voltage sourceVoltage
Current
Power
Equivalent loading resistance
10 0 0 infinite (open circuit)
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 0 Ω (Output Shorted)
Higher current drawn from the source (lower load resistance)
Output voltage is loaded down.
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Loading resistance
V
V, I, P power
I
Max power Loading condition
In a graphic form(V, I, P vs. Req)
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Current (Amp)
V vs. I curve
V
power curve
W=V*I
Max power point
Max power voltage
Max power current
Max power operating point
In a graphic form(V, P vs. I)
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A solar panel is a practical voltage source.
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IV curve of a solar panel
• There are three set of curves shown, for three different sun light intensities.
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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.
POT + V _
I
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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
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Setup in the lab
cell #1
cell #2
cell #3
cell #4
cell #5
cell #6
POT
Power meter
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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.)
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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.
Voltage (V) 9v
Current (Amp) 2 1
9v @ 2A 9v @ 1A
Either adaptor works for, for example, a CD player that needs 0.5A at 9V.
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This adaptor is rated at 9V at 0.2A.
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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
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Q4. The label on a CD player shows [email protected]. Which of the follow adaptors will work with this CD player?