ECE 225 Experiment #8

Thevenin's Theorem

Set up the circuit in Figure 1, which is supposed to represent a moderately complex linear circuit.

Figure 1.

Measure the open circuit voltage V_OC      (V_AB of this circuit) with the DMM. Then measure the short circuit current I_SC by attaching the DMM, used as a DC current meter, directly to the output terminals A  -  B. Calculate R_TH = V_OC /I_SC.

Set up a graph with voltage on the horizontal axis and current on the vertical axis, and plot the current-voltage combinations you have obtained from the open circuit voltage measurement (one point on the graph) and the short circuit current measurement (another point.) Attach a variety of values of load resistance R_L (ranging from 10 ohms to 100K. See Figure 2.) to the output terminals; for each value of R_L, first determine the load voltage and load current which result and then plot the combination as a point on the graph. Comment on the nature of the graph.

Figure 2.

Now construct the Thevenin equivalent of this circuit, using a DC source set equal to the measured V_OCmeasured above, and a resistance equal to R_TH calculated above. See Figure 3.

Attach the same set of R_L values you used earlier, and record the load voltages and currents which result. See Figure 4. If this simplified circuit is in fact equivalent to the original more complex circuit, these values should be the same as before. Are they? Comment.

Figure 3.

Figure 4.