Thevenin’s Theorem Formula and Examples Electric Circuits

Why do we need to learn about Thevenin’s theorem formula? It is very useful when analyzing a complex configuration electrical circuit.

Thevenin’s theorem may help us greatly when dealing with electrical analysis in real life experience. It is a normal thing that sometimes a component in a circuit is variable (its load may change from time to time) while other components are fixed.

The simplest example of this thing is our household electrical load. We will plug our electrical devices to our electrical and it can be a lamp, charger, TV, refrigerator, computer, and many more. Our point here is, the load will be a variable component for our household’s electrical circuit.

It will be a tiresome thing if we want to analyze our electrical system every time we plug in different devices. This is where Thevenin’s theorem kicks in! We will analyze the fixed component in the electrical circuit into a simplified equivalent circuit and we don’t have to analyze it all over again when the variable component changes.

This will save a lot of our precious time.

What is Thevenin’s Theorem

Observe the illustration below where the linear two terminal circuit is our fixed components while the load may change frequently. This is also true for Norton’s theorem.

thevenin's theorem formula 1

We can replace the linear two terminal circuit into its Thevenin equivalent circuit as shown below:

thevenin's theorem formula 2

The load may be just a resistor or even another circuit. This Thevenin equivalent circuit is our main focus to analyze electric circuits. If the load is replaced by any component, the Thevenin equivalent circuit will remain as it is.

Thevenin’s theorem states that:

A linear two terminal circuit can be simplified into a circuit that consists only of a voltage source VTh, connected series with an equivalent resistance RTh between the two observed terminals.

The true essence of this theorem is to simplify a circuit analysis, that is to make a substitute circuit consisting of voltage source connected in series with its equivalent resistance.

What is the Thevenin’s Theorem Formula

After learning how Thevenin’s theorem works, we still need to understand how we implement mathematical solutions to understand it completely.

Our main objectives are to find:

  • Thevenin equivalent voltage, VTh
  • Thevenin equivalent resistance, RTh

Keep in mind that we can call them equivalent if the voltage and current are still the same at their terminals.

thevenin's theorem formula 3

From the illustration above, with the substitution theorem we can see the circuit B can be replaced with a voltage source with the same value when the current flowing through circuit B at the two terminals we are observing (terminal a-b)

After we get the substitution circuit, using Superposition theorem we get that:

1. If a voltage source V is active then the linear circuit A is not active (all its independent sources are replaced with their inner resistance), thus we can calculate the equivalent resistance. This step is about finding Thevenin resistance or finding R Thevenin.

thevenin's theorem formula 4

2. If the linear circuit A is active then the independent voltage source is replaced with its inner resistance that is zero or short circuit.

thevenin's theorem formula 5

After unify these two conditions (superposition theorem) we get:

thevenin 1

When terminal a-b is open circuit (OC), then i flowing in the circuit is zero (i=0), then

thevenin's theorem formula 6

thevenin 2

This is about Thevenin’s Theorem VTh formula. From Equations.(1) and (2), we get

thevenin 3a

The V thevenin formula is equal to V;

 

thevenin 3b

Note:

It is not a weird thing when the Thevenin resistance RTh has negative value. The negative resistance will cause the voltage (v=-iR) negative. It means the circuit is supplying power. Thevenin negative resistance is possible if the circuit has dependent sources.

The summary of how to calculate Thevenin voltage will be covered below.

Thevenin’ Resistance Formula

We need to get the value of Thevenin resistance (R Thevenin formula) as the equivalent resistance for the circuit.

The steps we need to do to understand how to find thevenin resistance are:

  1. First thing is we turn off all the independent sources in linear circuit A,
  2. Replace the voltage source with its inner resistance R=0 or short circuit,
  3. Replace the current source with its inner resistance R=∞ or open circuit,
  4. Calculate the equivalent resistance in the circuit,
  5. If the circuit has dependent sources then we need to find the “short circuit current” (isc),
  6. Thevenin equivalent resistance (RTh) can be calculated from the voltage across the desired terminal divided by the current flowing through the short-circuited terminal (isc).

Summary, thevenin resistance (RTh formula) is the resistance measured in terminal a-b when all of the voltage sources are replaced by a short circuit and the current sources are replaced by an open circuit.

Thevenin’s Theorem Procedures

1. Find and determine the terminal a-b where the parameter is asked or observed.

2. Remove the component at the terminal a-b, make it open circuit at that terminal and calculate the voltage across that terminal a-b (Vab=Voc=VTh).

3. If there are only independent sources, then the resistance is measured at the terminal a-b when all of the sources are turned off and replaced by their inner resistance. We will get Thevenin equivalent resistance (Rab = RTh)

  • Replace voltage source with short circuit.
  • Replace current source with open circuit.

4. If there is dependent source, we use equation below to find the Thevenin equivalent resistance,

thevenin 4

5. In order to find the value of Isc (short circuit current), we make terminal a-b short-circuited and calculate the current flowing through that terminal (Iab=Isc).

6. Redraw the Thevenin equivalent circuit as a series circuit consists of:

  • The removed component we did on Step.(2).
  • Thevenin voltage/ open circuit voltage/ a-b voltage (VTh=Voc=Vab)
  • Thevenin equivalent resistance (RTh).

7. Solve the simplified circuit.

Thevenin’s Voltage Formula

If you find it difficult on understanding how to find Thevenin voltage, these are the simplest steps to do:

  1. Determine the terminal a-b where the parameter is observed.
  2. Remove the component at the terminal a-b, make it open circuit at that terminal and calculate the voltage across that terminal a-b (Vab=Voc=VTh).

Thevenin with Independent Source

1. Find the value of i with Thevenin’s theorem!

thevenin's theorem formula 7

Solution:

Determine terminal a-b on R where i is observed. Remove the component and make it open circuit. Calculate the voltage across terminal a-b when open circuit:

thevenin's theorem formula 8

We get the Vab or Voc :

thevenin 5

Using the RTh thevenin formula, we find the Thevenin resistance RTh when all the independent sources are turned off (replace them with their inner resistance). From the terminal a-b perspective:

thevenin's theorem formula 9

Thus we get the Thevenin’s resistance formula is

thevenin 6

We redraw the circuit into Thevenin equivalent circuit:

thevenin's theorem formula 10

Hence,

thevenin 7

2. Find the value of i with Thevenin’s theorem!

thevenin's theorem formula 11

Solution:

Determine the terminal a-b on R where i is observed. Remove the component and calculate the voltage across terminal a-b when open circuit:

thevenin's theorem formula 12

With nodal analysis:

thevenin's theorem formula 13

Observe node voltage v1:

thevenin 8

Thus

thevenin 9

Find the Thevenin resistance RTh when all the independent sources are turned off and replace them with their inner resistance. From the terminal a-b perspective:

thevenin's theorem formula 14

Then the Thevenin resistance formula RTh is

thevenin 10

Redraw the Thevenin equivalent circuit:

thevenin's theorem formula 15

Thus,

thevenin 11

3. Find the voltage across terminal a-b with Thevenin’s theorem!

thevenin's theorem formula 16

Solution:

Find Vab when terminal a-b is open circuit:

thevenin's theorem formula 17

Then the Vab:

thevenin 12

Thus

thevenin 13

Find the Thevenin resistance when all the independent sources are turned off and replaced with their inner resistance. From the a-b perspective:

thevenin's theorem formula 18

Then

thevenin 14

Then Thevenin equivalent circuit:

thevenin's theorem formula 19

Hence,

thevenin 15

Thevenin with Dependent Source

1. Find the value of V with Thevenin’s theorem!

thevenin's theorem formula 20

Solution:

Find Vab where voltage across the R=3Ω then we make that terminal open circuit:

thevenin's theorem formula 21

Then

thevenin 16

Because the circuit has a dependent source, in order to get the RTh we can’t turn off all the sources. We need to find the short circuit current Isc first:

thevenin's theorem formula 22

Then

thevenin 17

Thus

thevenin 18

We redraw the Thevenin equivalent circuit:

thevenin's theorem formula 23

And we get,

thevenin 19

2. Find the value i with Thevenin’s theorem!

thevenin's theorem formula 24

Solution:

Find Vab when terminal a-b is open circuit:

thevenin's theorem formula 25

We get

thevenin 20

Because there is a dependent source, in order to find RTh we can’t turn off all the sources directly. We first find the value of isc:

thevenin's theorem formula 26

We get:

thevenin 21

Then,

thevenin 22

We redraw Thevenin equivalent circuit:

thevenin's theorem formula 27

And we get

thevenin 23

Thevenin’s Theorem Examples

1. Redraw the circuit below into its Thevenin equivalent circuit to the left of the a-b. Find the current through the RL when RL=6,16,36 Ω.

thevenin's theorem formula 28

Solution:

First we remove the observed terminal and turn off all of its independent sources and replace them with their inner resistances:

  • Turn off the 32 V voltage source and replace it with a short circuit.
  • Turn off the 2 A current source and replace it with an open circuit.

The circuit becomes below:

thevenin's theorem formula 29

We get the Thevenin equivalent resistance, RTh as:

thevenin 24

In order to find the VTh, we can use mesh analysis to the two loops on the left as shown below:

thevenin's theorem formula 30

We obtain

thevenin 25

Use the i2 we can solve the i1 = 0.5 A. Hence,

thevenin 26

We can also use nodal analysis for simpler solutions. Ignore the 1 Ω resistor because there will be no current flowing through the open circuit. From the node VTh with KCL produces:

thevenin 27

The value is the same as before. Next we redraw the circuit into Thevenin equivalent circuit:

thevenin's theorem formula 31

The current flowing through RL is

thevenin 28

RL = 6,

thevenin 29

RL = 16,

thevenin 30

RL = 36,

thevenin 31

2. Redraw the circuit below into its Thevenin equivalent circuit at terminals a-b.

thevenin's theorem formula 32

Solution:

Since the circuit has a dependent voltage source, we still replace all of the independent sources with their inner resistances but leave the dependent source alone.

Because there is a dependent source, we energize the circuit with a voltage source vo connected to the terminal as shown below. We set the voltage source vo=1 V to make it simpler for calculation because the circuit is linear so the voltage-current relationship will not change.

thevenin's theorem formula 33

Next what we need to do is to find the value of io through the terminals a-b to obtain the value of Thevenin equivalent resistance:

thevenin 32

We can also connect current source io on the terminals a-b and find the VTh to get the RTh:

thevenin 33

Using the mesh analysis to the loop 1 produces

thevenin 34

But

thevenin 35

Thus,

thevenin 36

For loop 2, KVL results in

thevenin 37

For loop 3, KVL results in

thevenin 38

Solving these three equations gives

thevenin 39

But

thevenin 40

Thus,

thevenin 41

In order to get the Thevenin equivalent voltage (VTh) we will find the open circuit voltage, voc in the circuit below:

thevenin's theorem formula 34

Using the mesh analysis for the three loops produces:

Loop 1:

thevenin 42

Loop 2:

thevenin 43

Loop 3:

thevenin 44

But

thevenin 45

Solve these equations will produce

thevenin 46

Thus,

thevenin 47

We can redraw the circuit into its Thevenin equivalent circuit as shown below

thevenin's theorem formula 35

3. Convert the circuit below into its Thevenin equivalent circuit at terminals a-b.

thevenin's theorem formula 36

Just like the example before, we will energize the circuit with either 1 V voltage source or 1 A current source. For this case we will go with a current source with nodal analysis. The circuit becomes below

thevenin's theorem formula 37

Assume io = 1 A. Using nodal analysis produces

thevenin 48

Now we have two unknown variables but only one equation. We need the constraint equation

thevenin 49

Substituting Equation.(3.2) into (3.1) produces

thevenin 50

Since

thevenin 51

Then

thevenin 52

This is the example when the resistance has a negative value. It means our circuit is supplying power. Accurately the dependent source is the one supplying power. Thus the Thevenin equivalent circuit becomes

thevenin's theorem formula 38

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