Notes on Experiment #8

Thevenin's Theorem

Be sure to print a copy of experiment #8 and bring it with you to your lab session. There will not be any copies available in the lab.

Measure the Resistors First!

The resistors must be accurate in this experiment. Discard any with an error greater than 5%. Ask your lab instructor for a replacement.

The element values are:

• Part 1:

  R₁ = 10K; R₂ = 6.8K; R₃ = 10K; R₄ = 3.3K, and R₅ = 2.7K

• V₁ = 10 Volts and V₂ = 6 Volts.

Procedure

Use a DC source for V₁ and V₂.

Procedure

1. Build the circuit but do not connect a load resistor.
2. Measure V_OC.
3. Measure I_SC.
4. Compare these values to the values from your circuit analysis. There should be almost no error. If there is error then:
   1. you did not build the circuit correctly or
   2. you did not measure correctly.
5. If the data is OK then use the above data values of V_OC and I_SC to calculate R_TH.
6. Now measure R_TH! Just set the voltage sources to zero and use an Ohm meter to measure the resistance at the output terminals. PLEASE NOTE: Normally you can not and should not measure R_TH in the above manner. Usually we can't turn of the the internal sources in a circuit. Try measuring R_TH while the sources are connected. You should get a very large error.
7. Does the calculated R_TH equal the measured R_TH? It should!
8. DO NOT GO ON. show your data to your lab instructor. If all the data is OK then you may go on.
9. Connect the following load resistors R_L (one at a time) and measure and record:
   1. V_L and
   2. I_L

   R_L = {100 Ohms, 470 Ohms, 1K, 4.7K, 10K, 20K}

   IMPORTANT: Do not use the power resistor decade box for R_L. Use the extra resistors supplied in your kit.

10. DO NOT GO ON. show your data to your lab instructor. If all the data is OK then you may go on. DO NOT DISMANTLE THE CIRCUIT.
11. Now build the Thevenin Equivalent Circuit (TEC) of the elaborate circuit you just worked on.
    1. Set the voltage source V_OC equal to the open circuit voltage V_OC YOU measured and
    2. Use the power resistor decade box as R_TH. Do not trust the dials. Measure the resistance on the decade box so that you know that it is set correctly.
    3. Now repeat steps 2 to 10 above. Be sure to use exactly the same load resistors.
12. Compare the data from the original circuit and the TEC. Do error analysis.
13. Plot the suggested graph using the values of R_L from above.
14. You're done. Dismantle the circuits, put the parts away, and turn in your report.

Circuit Analysis

Calculate the values for V_OC, I_SC, and R_TH using any method you like. Use the values given at the top of this page. You do not need to calculate the load resistor voltages and currents. That's all.

Have fun.