ENGR 225 Circuit Analysis Name_________________________
Lab #3  Circuit Simulation and Thevenin’s Date__________________________
Equivalent Circuits
Objectives:
To see how Electronics Workbench can be used to simulate circuits.
To verify Thevenin’s theorem calculations with measurements.
To experimentally verify the requirements for maximum power transfer to a load.
Procedures:
1. Enter the attached circuit into Multisim and take measurements needed to determine the Thevenin equivalent circuit for everything but R4 looking into the terminals ab. Measure the Voc and Isc and then use Vth = Voc and Rth = Voc/Isc, or you can measure the resistance between terminals a and b with the sources set to 0V to get Rth. Sketch the Thevenin equivalent circuit.
2. Build the same complete circuit on a breadboard and measure the open circuit voltage and the short circuit current to verify that the simulation and the actual circuit give the same results. Show both simulation and breadboard results in a table for easy comparison. Keep this circuit for later use.
3. Can you use the Thevenin equivalent to determine if it would be appropriate to connect an LED (a very nonlinear element) in place of R4? Note that the LED will drop about 1.7V with current in the forward direction and will operate efficiently with 1020mA. If your calculations show it is ok, then hook up the LED and see how it works. How would you go about making this determination without using Thevenin’s theorem?

4. Then along side the original circuit build the Thevenin equivalent circuit on your breadboard. Select 9 resistors ranging in value from Rth/10 to 10*Rth being sure to include one equal to Rth. Connect each of these resistors first across the original circuit between terminals a and b. Record the resistance value and the voltage Vab in a table for each resistor. I would suggest using Mathcad or Excel to record this data. Then repeat these measurement with the Thevenin equivalent circuit. From this data you can show that the two circuits are equivalent (or not). Also, you can compute the power absorbed by the connected resistor load using P = V^{2}/R for both sets of data. Plot this calculated power P vs. R and verify the maximum power theorem.