Thevenins Theorem Lab Report

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ELECTRICAL PRINCIPLES

TITLE: Thevenins Theorem Experiment

Lecturer:

Submission Form Hardcopy: Yes/No Softcopy: Yes/No

Signature: ______________ Date: _________________

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1.0. Abstract:

The experiment that follows is carried out to determine the Thevenins theorem experimentally.

Thevenins theorem is an equivalence principle in circuit theory.This theorem is widely used for

circuit analysis simplification and to study circuit's initial-condition and steady-state response. In

the experiment, the load resistance was removed. The Vth and Rth were found and the resultant

current measured. After the experiment was carried out, it was verified that using the Thevenins

theorem procedure, the current in a particular load (potentiometer) in this case has the same

current flowing through it if it is solved using any other equivalence principle.

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2.0. Objective:

The objective is this experiment is to verify the Thevenins theorem.

3.0. Theory:

In circuit analysis, Thevenins theorem for linear electrical networks states that any combination

of voltage sources, current sources, and resistors with two terminals is electrically equivalent to a

single voltage source V in series with a single series resistor R.To calculate the results for an

equivalent circuit, one needs a resistance and some voltage - two unknowns. Thus two equations

are needed. These two equations are usually obtained by using the following steps:

1. Calculate the output voltage, VAB, when in open circuit condition (no load resistor - meaning

infinite resistance). This is VTh.

2. RTh is found by having the voltage source short circuited and the current source opened. The load

resistance (RL) is kept zero.

3. After both RTh and VTh are obtained, both are placed in series with the RL and the resultant

current is found.

4.0. Apparatus:

S.no Components Ratings Quantity

1 Variable power source 0-20V 1

2

Resistor

4.35k 1

35.1k 1

7.30k 1

3 Variable resistance (pot) 9.00k 1

4 Multimeter 200-2M 2

1A dc

5 Breadboard - 1

6 Connecting wires - few

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5.0. Procedure:

1. An calibrated ohmmeter is used to measure and record the value of the resistor R1, R2, R3

and RL in table 1.

2. The circuit is constructed as shown in the figure below.

3. The voltage (VL) across the resistor (RL) and the current (IL) through the load

aremeasured and recorded in table1 using a voltmeter and an ammeter respectively.

4. The power supply is switchedoff and the load of 9K removed.

5. The supply is switch ON and a voltmeter is connected across the terminals A and B to

measure the Theveninsvoltage (VTH) as shown in the figure below. The reading is

recorded in table 1.

V1 5

R1 1k R2 1k

R3 1

k

A+

AM1

R4 1

k

V+

VM1

V1 5

R1 1k R2 1k

R3 1

k V+

VM1

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6. The power supply is switched OFF. The voltage source is removed and connected in the

circuit as shown in the figure below.Using an ohmmeter, the resistance is found between

A and B which is the Thevenins resistance, RTH. This value is then tabulated in table 1.

7. The circuit is reconnected as shown in the figure below with the voltage source, the

variable pot, ammeter in series with the load 9k. A voltmeter is connected across the

load to measure the load voltage (Thevenins equivalent load voltage). An ammeter is

used to measure the current. This current value is then put in the table 1.

R1 1k R2 1k

+

ZM1 1kR

3 1

k

V1 5

R1 1k

A+

AM1

R2 1

k

V+

VM1

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6.0. Results:

IL (original)

(A)

VL (original)

(V)

VTH (V) RTH () IL

(Thevenin's

method)

VL

(Thevenin's

method)

Mead Valu

e

Cal. value

Mead value

Cal. value

Mead value

Cal. value

Mead

value

Cal. value

Equi value

Cal value

Equi value

Cal. value

0.44mA

0.44mA

4.01V

3.96V

8.98V

8.91V

11.23K

11.17k

0.431mA

0.44mA

3.56V 3.97V

Mead=Measured value, Cal.=Calculated value, Equi= Equivalent circuit

TABLE 1

6.1. Stimulated results:

Computer generated results using the software TINA are presented below to verify the

experimental readings.

6.1.1. VL and IL (without Thevenins method):

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6.1.2. Vth:

6.1.3. Rth:

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6.1.4. VL and IL (Thevenins method):

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6.2. Theoretical results:

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7.0. Discussion:

The experiment was carried out to verify the Thevenins theorem. In the experiment, the

individual resistance of fixed resistors and variable resistance (potentiometer) were measured

using a calibrated ohmmeter. It is to be noted that the resistance of the resistors is determined

with the resistor disconnected from the circuit and not connected to the power supply, as this

would give us a false reading. The voltage and the current were recorded using a calibrated

multimeter. The multimeter acting as an ammeter was connected in series with the load while the

voltmeter was connected across the load to get the voltage across the required load. In the

experiment, the variable resistors resistance was kept to a constant resistance (9k in this case)

throughout the experiment. This yielded appreciable results for our experiment. In the

experiment, the load across which the current was to be determined was removed and Rth found

out using a ohmmeter. The Vth was found with the load resistance kept disconnected for the

circuit. It was measured with a voltmeter. After both Vth and Rth were found, they both were

placed in series with the RL (the removed load). Then the current in RL was found using a n

ammeter. The experimental value of the current was cross checked using computer generated

results and as well as theoretical results. Both these results were very close to the experimental

results indicating that both the human and the apparatus errors were kept to the minimum. The

results from the experiment hold a significant importance in electronic and electrical principles.

Thevenin's Theorem is useful in analyzing power systems and other circuits where one particular

resistor in the circuit (called the load resistor) is subject to change, and re-calculation of the

circuit is necessary with each trial value of load resistance, to determine voltage across it and

current through it. Thevenins theorem is also greatly applied in source modeling and resistance

measurement using the Wheatstone bridge.

8.0. Conclusion:

From the experiment carried out, it was verified that the current flowing through a load

resistance under Thevenins theorem analysis is same as the current flowing through it under

normal condition.

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9.0. References:

John Becker. (2013). Thevenin's Theorem. Available: http://www-

cecs.evansville.edu/mr56/ecce170/Lab4.pdf.

Boylestad L. Robert, 2007, Principles of Electric circuits: Conventional current

Version, 9th version, Pearson international edition. Prentice hall.