Thevenin's Theorem Lab

Today in class we did the Thevenin's Theorem Lab; the purpose of this lab was to experimentally evaluate Thevenin's theorem, and confirm its claims. This was done by using Thevenin's theorem to analyze a circuit and predict a certain voltage across a resistor that will result from adding the resistor to the circuit. Then, once it was analyzed, we built the circuit, added the resistor and measured the voltage across it. After that, we constructed the Thevenin equivalent of the circuit and connected the resistor to it and measured its voltage, which we compared to the results of the original circuit. Lastly, we varied the value of the load resistance and measured the power across the resistor to confirm the maximum power transfer theorem.

We began by analyzing the circuit using Thevenin's Theorem. The circuit can be seen in the upper left of both of the pictures below.

^^^^ Left: Thevenin resistance calculations Right: Thevenin voltage calculations ^^^^

Thevenin Equivalent Resistance across AB: 7.35K Ohms

Thevenin Equivalent Voltage across AB: 0.463V 

With that completed, we measured the resistances of all resistors and the potentiometer.

6.8K Ohm #1: 6.68K

6.8K Ohm #2: 6.62K

4.7K Ohm     : 4.68K

2.2K Ohm     : 2.16K

1.0K Ohm #1: .998K

1.0K Ohm #2: .998K
1.3K Ohm     : 1.30K
10K Ohm      : 9.93K
8.2K Ohm     : 8.11K
10K Ohm Pot: 9.26K

We then built the complete circuit, but left out the resistor labeled RL in the diagram, and measured the voltage/resistance across points A and B in the diagram.

^^^^ Left: The voltage across AB (hooked up backwards). Right: A closeup of the circuit ^^^^

^^^^ Another view of the completed circuit ^^^^

Voltage across AB without RL      : .455V

Resistance across AB without RL : 7.27K Ohms

Expected Values: .463V and 7.35K Ohms.

As you can see, the experimental values were both incredibly close to the expected values; the voltage varied by 1.73% and the resistance varied by 1.09%. 

After that we connected the 8.2K Ohm resistor across AB and found the voltage across it to be 56.5 mA which is once again quite close to what we were expecting.

Finally, we took the potentiometer, set it to 7.37K Ohms (to imitate the circuit) and connected it to the 8.2K Ohm resistor. Once again we measured the voltage across the resistor and found it was 53.7 mV. That confirms that Thevenin resistance can be calculated and used to replace complicated circuits, when you want to change only one aspect of the circuit.

^^^^ Replacing the circuit with a potentiometer and measuring the voltage across the 8.2K Ohm Resistor ^^^^