3. Thevenin’s Equivalent Circuit
The purpose of this laboratory exercise is to investigate Thevenin’s theorem, which states that
any two-terminal network can be replaced by a single resistor and voltage source. The Thevenin
circuit to replace a simple voltage divider is constructed, and both the Thevenin circuit and the
voltage divider should have the same load line.
I. Determining the Thevenin equivalent circuit.
A. Construct the voltage divider shown to the
right with R1 and R2 in the range of 1 to 10 k Ω
and Vbb in the range of 10 to 15 V.
B. Measure VAB using the Digital Multimeter for
at least 10 different load resistors (RL’s). Half
of your measurement should be made with RL
< R2 and half with RL > R2. Determine the
current (IL) from the measured values and plot
the load line (IL vs. VAB) for this circuit.
C. Derive an expression for IL as a function of VAB (the load line) in terms of the
components of the Thevenin equivalent circuit. Make a least squares straight line fit to
your data and from the results determine the short-circuit current, the Thevenin
resistance, and the open-circuit voltage.
II. Construct the Thevenin equivalent circuit of the voltage divider above.
A. It may be helpful to use the variable resistors on the Digital Design Workstations. Also
you might need to use your knowledge of serial and parallel resistors to get close to the
exact value for the Thevenin resistance.
B. Using the same load resistors as above, repeat your measurements for VAB.
C. Compare your results with those obtained in part I.
III. Lab Report (group reports are acceptable)
A. Include all you work, data, and results form above.
B. In addition, include a paragraph (or two) explaining why a circuit production company
would want to replace the circuit in part I with the one in part II. Your explanation
should not just focus on the physical aspect of the two circuits being identical, but also on
any monetary considerations such as parts, labor, etc.