Overview
Equivalent Circuits: Thevenin Theorem
• Source Transformation Example
• Review concept of voltage
• Thevenin Equivalent Circuit
o Equivalent Vs-with-series-R
• Norton Equivalent Circuit
EGR 220, Chapter 4.5 - 4.11
(not section 4.9)
February 16, 2016
o Equivalent Is-with-parallel-R
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Source Transformation
Source Transformation
v s = is R
⇔
is =
• Find Vs + series-R equivalent circuit:
vs
R
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Concept of Voltage
Equivalent Resistance
• Definition – discuss at tables
• Voltage à Voltage potential à
Potential difference
• The energy required to move electrons
through a resistance
• Equivalent resistance is terminaldependent
• V = I*R so…
o What is needed to move electrons through
an ideal wire? (i.e., a short circuit)
• If need Vs = 0, replace this source with a
short circuit (a wire), so voltage = 0
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o R = V/I
o The ratio of voltage to current for specific
terminals in a circuit
• Concept of resistance: the ratio of V/I
o Voc/Isc
(open circuit voltage to short circuit current)
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Next Concept…
• Equivalent circuits
o Indistinguishable from each other
o V – I – Req characteristics are the same
o … from the specified terminals (nodes)
• Applications beyond electrical
engineering
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ourelectrician.blogspot.com
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Op Amp Circuit
Lab 2:
Determine equivalent
resistance, Req, when
using meter as
Vmeter and Ammeter
Input voltage 120V in
is 120 V
Output is up
to us…
Vout ≈ VTh
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Thevenin Equivalent – Process
1) Find RTh:
Thevenin: Find VTh, RTh
o Remove the specified load and set all
independent sources in the circuit equal to zero.
• V-source = 0 ð short circuit
• I-source = 0 ð open circuit
o Find the equivalent resistance from the
perspective of the specified nodes
2) Find VTh: Remove the specified load
and find the open-circuit voltage
across the desired/specified nodes
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Thevenin: Find VTh, RTh
Thevenin: Find VTh, RTh
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* Maximum Power Transfer *
Thevenin: Find VTh, RTh
• How would we find the Pmax delivered to RL?
• Do not solve – discuss strategy (chapter example)
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Thevenin Self-­‐‑Review
Concept Question
• Are equivalent circuits all the same?
• Discuss VTh, RTh at a-b and b-c
• What is RL for maximum power transfer, and how do
you find the amount of power transferred?
• What is a Thevenin Equivalent Circuit?
o Draw a generic Thevenin equivalent circuit
o Discuss and write down 3 good uses for a
Thevenin equivalent circuit, or for the
Thevenin theorem
• How might you find the maximum
power that can be delivered to any
load from any circuit?
o Why is this an important question?
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Norton Equivalent Circuit
Summary
• Source transformation
• Equivalency
VTh = voc
o Equivalent resistance
o Thevenin equivalent circuit
o Only need to know the Norton equivalent
exists. We will focus on Thevenin
I N = isc
RTh = R N = Rin =
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voc
isc
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