Transcript ppt

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 = VI = V0 = 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