Transcript Unit One: AC Electronics - Helderberg Hilltowns Association

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ET115 DC Electronics
Unit Three:
Ohm’s Law
John Elberfeld
[email protected]
WWW.J-Elberfeld.com
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Schedule
Unit Topic
Chpt Labs
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2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
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2
3
3
4
5
6
6
6
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Quantities, Units, Safety
Voltage, Current, Resistance
Ohm’s Law
Energy and Power
Series Circuits
Exam I
Parallel Circuits
Series-Parallel Circuits
Thevenin’s, Power Exam 2
Superposition Theorem
Magnetism & Magnetic Devices
Course Review and Final Exam
2 (13)
3 + 16
5 (35)
6 (41)
7 (49)
9 (65)
10 (75)
19 (133)
11 (81)
Lab Final
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Unit 3 Objectives - I
• Describe the relationship among voltage,
current, and resistance.
• Given two of the three variables in Ohm’s
Law, solve for the remaining quantity.
• Solve Ohm’s Law problems using metric
prefixes.
• Construct basic DC circuits on a
protoboard.
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Unit 3 Objectives – II
• Use a digital multimeter (DMM) to measure
a predetermined low voltage on a power
supply.
• Measure resistances and voltages in a DC
circuit using a DMM.
• Explain the Multisim workbench and show
how to construct a basic circuit.
• Test circuits by connecting simulated
instruments in Multisim
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Reading Assignment
• Read and study
• Chapter 3: Ohm’s Law
Pages 71-80
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Lab Assignment
• Lab Experiment 5:
• Ohm’s Law Pages 35-38
• Complete all measurements, graphs,
and questions and turn in your lab
before leaving the room
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Written Assignments
• Answer all questions on the
homework handout
• Be prepared for a quiz on questions
similar to those on the homework.
• If there are any calculations, you
must show ALL your work for credit:
– Write down the formula
– Show numbers in the formula
– Circle answer with the proper units
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Ohms Law
• MEMORIZE: V = I R
• Ohm’s Law
• If you increase the voltage, you
increase the current proportionally
– 3 times the voltage gives you three
times the current
– Resistance (ohms) is the proportionality
constant and depends on the atomic
structure of the material conducting the
current
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Experimental Results
Current (x)
Voltage (y)
0A
0V
2A
6V
4A
12 V
6A
18 V
8A
24 V
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Graph of Data
V
V
o
l
t
a
g
e
x
x
x
x
x
I – Current in Amps
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Reasoning
V=IR
• Ohms Law: V = I R
• High voltage produces high current for a
given resistance
• Low voltage produces low current for a
given resistance
• For a given voltage, a high resistance
produces a low current
• For a given voltage, a low resistance
produces a high current
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Electronic Circuit
• A battery with the voltage V pushes a
current I through a resistor R
V=IR
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Ohm’s Law
V=IR
• This is the BIG IDEA for the day (year)!
•V=IR
• What if we divide both sides by R?
• V=IR
R R
• But R/R = 1, so we don’t need to write it
down:
• I= V
I=V/R
R
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Ohm’s Law
V=IR
•V=IR
• What if we divide both sides by I?
• V=IR
I I
• But I / I = 1, so we don’t need to write
it down:
• R=V R=V/I
I
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Ohm’s Law
•
•
•
•
•
Memorize: V = I R
Use algebra to find:
I=V/R
R=V/I
If you can, learn all three variations,
but you can get by if you memorize:
V=IR
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Practice
•V = I R
• What voltage (V) is needed to push a
current of 2 Amperes (I) through a
resistance of 18 Ohms (R) ?
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Practice
•V = I R
• What voltage (V) is needed to push a
current of 2 Amperes (I) through a
resistance of 18 Ohms (R) ?
• V=IR
• V = 2 A x 18 Ω
• V = 36 V
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Examples
• Ohms Law: V = I R k = 103 μ = 10-6
• How much voltage must be connected
across a 1.2 k Ω resistor to cause 575
μA of current to flow?
• V=IR
?V
1.2k Ω
575
μA
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Examples
• Ohms Law: V = I R k = 103 μ = 10-6
• How much voltage must be connected
across a 1.2 k Ω resistor to cause 575
μA of current to flow?
• V=IR
• V = 575 μA 1.2 k Ω
• V = .69V = 690 x 10-3V = 690 mV
?V
1.2k Ω
575
μA
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Examples
103=k
10-3 = m
10-6 = μ
• Ohms Law: V = I R
• How much current flow through a 25
Ω resistor with 10 V across it?
• V=IR
I=V/R
10 V
25 Ω
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Examples
103=k
10-3 = m
10-6 = μ
• Ohms Law: V = I R
• How much current flow through a 25
Ω resistor with 10 V across it?
• V=IR
I=V/R
• 10 V = I 25 Ω
• I = 10 V / 25 Ω
• I = .4 A or 400 x 10-3A = 400 mA
10 V
25 Ω
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Examples
103=k
10-3 = m
10-6 = μ
• Ohms Law: V = I R
• If a certain resistor allows 250 mA to
flow when 35 V are across it, what is
the resistance?
• V=IR
R=V/I
35 V
250 mA
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Examples
103=k
10-3 = m
10-6 = μ
• Ohms Law: V = I R
• If a certain resistor allows 250 mA to
flow when 35 V are across it, what is
the resistance?
• V=IR
R=V/I
• 35 V = 250 mA R
• R = 35 V / 250 ma
• R = 140 Ω
35 V
250 mA
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Examples
103=k
10-3 = m
10-6 = μ
• Ohms Law: V = I R
• How much current flow through a
3.3k Ω resistor with 4.5 mV across it?
• V=IR
I=V/R
4.5 mV
3.3k Ω
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Examples
103=k
10-3 = m
10-6 = μ
• Ohms Law: V = I R
• How much current flow through a
3.3k Ω resistor with 4.5 mV across it?
• V=IR
I=V/R
• 4.5 mV = I 3.3k Ω
• I = 4.5 mV / 3.3k Ω
• I = 1.36 μ A
4.5 mV
3.3k Ω
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DIRECT AND INVERSE
RELATIONSHIPS
• I=V
R
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DIRECT AND INVERSE
RELATIONSHIPS
• I=V
R
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And Still More Practice
V=IR
I=V/R
1.6 mA
250 μA
500 mA
850 μA
75 μA
3 mA
5 μA
2.5 A
R=V/I
2.2 kΩ
1.0 kΩ
1.5 MΩ
10 MΩ
47 Ω
27 kΩ
100 MΩ
47 Ω
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Practice
•V = I R
• What current (I) flows through a
resistance of 8 ohms when the
resistor is connect to a 24 volt
battery?
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Practice
V=IR
• What current (I) flows through a
resistance of 8 ohms when the
resistor is connect to a 24 volt
battery?
• V=IR
I=V/R
• 24 V = I x 8 Ω
I = 24 V / 8 Ω
• I = 24 V / 8 Ω
I=3A
• I=3A
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And Still More Practice
V=IR
40 V
1 kV
66 kV
12 V
25 V
5V
15 V
I=V/R
R=V/I
68 kΩ
2 kΩ
10 MΩ
10 Ω
10 kΩ
2.2 MΩ
1.5 kΩ
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Practice
V=IR
• What size resistor allows 2 amperes
of current through it when it is
connected to a 10 Volt power
supply?
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Practice
V=IR
• What size resistor allows 2 amperes
of current through it when it is
connected to a 10 Volt power
supply?
• V=IR
R=V/I
• 10 V = 2 A x R
R = 10 V / 2 A
• R = 10 V / 2 A
R=5Ω
• R=5Ω
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And Still More Practice
V=IR
500 V
50 V
1 kV
6V
8V
12 V
39 V
I=V/R
250 mA
500 μA
1 mA
2 mA
2A
4 mA
150 μA
R=V/I
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Lab 5 - Ohm’s Law
• Ohm’s Law describes the
relationship among voltage, current,
and resistance – it does not control
it!
• In lab, you will prove to yourself that
Ohm’s Law applies to circuits
• Use the special handout to organize
your information
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Select and Measure Resistors
• Your resistors can off by +/- 5% from
the marked value
• You must measure as accurately as
possible the real resistance used in
your experiment
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Use TWO meters
• Use TWO DMMs in your experiment
• Record as many digits as possible
for both voltage and current
A
V
• You must BREAK the circuit to
measure current
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Plot Your Points
• Your lab handout says to plot I along
the x axis and V along the y axis
• The slope is Δy / Δx = ΔV/ ΔI
• Based on Ohm’s Law, R = V / I, just
like the slope
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Lab 4 – Voltage Measurement
1. Select the correct voltage mode (ac or dc).
2. Select range higher than expected
voltage.
3. Connect the meter across the points. Red,
positive (+),
Black,
common (–)
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Next Steps
• 4. Reduce the range setting until the
reading fails
• 5. Increase the range setting one
step and record all the numbers, with
the proper units, shown on the meter
• 34.67 mV, for example
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Voltage Notation
• Voltage is always the difference
between TWO points.
• Measure VBC by attaching the RED
lead to B and the BLACK lead to C
A
B
V
D
C
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Voltage
• If only one letter is given, attach the
RED lead to that letter, and the
BLACK lead to the reference point or
ground.
• If D is your reference point, VB is:
A
B
D
C
V
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Voltage Differences
•
•
•
•
•
•
If D is your reference point, then
VB is really VBD
VC is really VCD
Electrically, then
VBC = VBD - VCD
Voltage is the difference between two
points
• Choosing a different reference point
does NOT change the real voltage
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Unit 3 Summary
1. Ohm’s Law
2. Solving for voltage, current, or resistance
in a one-load circuit
3. Ohm’s Law using metric prefixes