EGR 214 Lab Experiment 7
Thevenin’s Theorem
Lee C. Groeneweg
&
Steve Adamczyk
Date: February 25, 1998
Objective:
Materials:
1 – Heath Circuit Design Trainer (CDT)
1 – Digital Multimeter (DMM)
3 – ¼ W resistors with values to be selected in lab
1 – Multiturn potentiometer ("pot") with value to be selected in lab
Miscellaneous leads and connectors
In this lab the circuit to be considered for analysis will be resistive with a single independent voltage source. For a circuit with "output " terminals designated a and b, the Thevenin equivalent will replace the circuit at these terminals as shown in Figure 1.
In this lab we will determine the Thevenin equivalent circuit using both analytical and experimental methods. Analytically, the Thevenin equivalent circuit can be found as follows:
We apply the above steps for the circuit shown in Figure 1. The open circuit voltage can be found by applying the voltage divider rule:
V_{oc} = V_{s * }R_{2} / (R_{1}+R_{2})
The Thevenin’s equivalent impedance can be found by deactivating the source and looking back into terminals a and b as shown in Figure 2. The Thevenin’s equivalent resistance can then be calculated by simply adding the resistance of R_{3} to the resistance of the parallel combination of R_{1 }and R_{2}. Part of this experiment is to verify that the two parameter Thevenin’s equivalent circuit is indeed equivalent to the original circuit.
In the first procedure we were to measure the values of the resistors used in this lab. These values are located in Table 1.
Resistor 
Resistance 
R #1 
2002 W 
R # 2

3545 W 
R #3 
4252 W 
Table 1. Resistor values used in this experiment.
In the second procedure we were asked to calculate the theoretical values of V_{Nl}
and R_{T } for the circuit in Figure 1. The formula used to calculate the value of V_{NL} and R _{T} are as follows:
V_{NL} = V_{s * }R_{2} / (R_{1}+R_{2}) & R_{T }=R_{3 }+R_{1 }// R_{2}
In the third procedure we were asked to find a suitable resistive load R_{L} for our circuit. That means to determine a value for R_{L} such that the current flow through R_{L} from terminal a to b causes the voltage from terminal a to b to drop to around onehalf of the noload value.
The fourth procedure required us to set up the circuit of Figure 1 using the resistor values we have selected. We were to set the value of V to the value we had selected.
In the fifth procedure we were to measure and record V_{NL}. Apply the load R_{L }we had selected to the terminals of our circuit and determine D V and D i. We were then to calculate the Thevenin’s resistance. R_{T }= D V/D I. These values are found in Table 2.
V_{NL} (calculated) = 3.2037 V 
V_{NL} (measured) = 3.204V 
R_{T } (theory) = 5531.446 W 
R_{T } (calculated) = 5555.446 W 
D V = 1.6 Volts 
D I = .288 mA 
Table 2. Voltage and Resistance values calculated and measured. 
Procedure Six asked us to compare the calculated theoretical and measured values of V_{NL }and R_{T }for our circuit.
Procedure Seven asked us to replace the actual circuit used in the previous three steps with the Thevenin equivalent circuit. We were to use a multiturn "pot" for R_{T }. These values are found in Table 3.
V_{T } = V_{NL} 
V_{L} 
R_{T} 
3.201 Volts 
1.609 Volts 
5555.40 W 
Table 3. Voltage and Resistance values when using the "pot"
In the eighth procedure we were to load the Thevenin equivalent circuit with the same load R_{L} used in step 5 and compare the values of D V measured in step 5 with the values measured here.
In the final procedure we were asked to find the Thevenin equivalent of:
These results are explained in the conclusion.
In this experiment we learned to use Thevenin’s equivalent circuit theorem as applied to an actual circuit. We first set up a circuit with three known resistors and a voltage source. From this circuit we calculated the open circuit voltage, V_{oc }and the Thevenin equivalent resistance, R_{T }. Measuring V_{NL }and R_{T} with the DMM revealed values matching those calculated. Using the potentiometer to create the Thevenin equivalent circuit showed us that the same values could be obtained when the circuit was given the same load.
The last procedure of this experiment involved measuring the V_{NL} and R_{T }of a DMM set to measure voltage, current and resistance. We found that the R_{T } is very large when measuring voltage or resistance and the voltage is zero when the DMM is set as a voltmeter or ammeter.
This lab experiment gave us hands on experience using Thevenin’s Theorem to analyze a real circuit and showed us the limits of the DMM.