Chapter 4-7
•Thevenin and Norton
Equivalents
Thevenin and Norton Equivalents:: Useful when interested at what is taking
place at a specific pair of terminals
Thevenin equivalent circuit : is a voltage source VTh in series with a
resistance RTh . The combination is equivalent to the original circuit
at the terminals of interest
To find the Thevenin voltage compute the open circuit voltage of the original
circuit at the terminals of interest.
To find the Thevenin resistance we compute the short circuit current at the
same terminals isc then use the relationship:
𝑅𝑇ℎ =
𝑉𝑇ℎ
𝑖𝑠𝑐
Example VTh
Example RTh
The Value? if apply a load across a and b, the voltage across the load, and
current will be the same as in the original circuit.
You Do
The Norton equivalent circuit consist of a current source in parallel with a
Norton resistance. The Norton current is equal to the short circuit current
at the terminals of interest, and the Norton resistance is equal to the
Thevenin resistance. So we can derive the Norton equivalent, once we
know the Thevenin values, from
𝑉
𝑖𝑁 = 𝑅𝑇ℎ and 𝑅𝑁 = 𝑅𝑇ℎ
𝑇ℎ
Alternative method for finding the Thevenin resistance for a network that
contains only independent sources:
1. Deactivate all independent sources
A. Short circuit Voltage sources
B. Open circuit Current sources
2. Find the equivalent resistance looking into the terminals of interest
You Do
Alternative method for finding the Thevenin resistance for a network that
contains dependent sources:
1. Deactivate all independent sources
A. Short circuit Voltage sources
B. Open circuit Current sources
2. Leave dependent sources in circuit
3. Apply a small test voltage (vT), or small test current (iT), at the terminals of
interest
4. Then the Thevenin resistance is the ration of the voltage across the
test source to the current delivered by the test source
Why did I choose vT ?
Write your equations
and solve for 𝑣𝑇 𝑖𝑇
What is the
Thevenin
voltage?
What you should know
1.
Know how to find simple Thevenin and Norton
equivalents.