THEVENIN’S THEOREM Thevenin’s theorem permits the reduction of a two-terminal dc network with any number of resistors and sources (Complex Circuit) to one Equivalent circuit having only one source and one internal resistance in a series configuration shown belw: Original and Thevenin Equivalent Circuit Measuring & Calculating Vth by MultiSim: • Circuit above shows how to measure the Thevenin Voltage (Vth). That is measured 4V. • The Circuit below shows how to measure Thevinin’s Resistance (Rth): • Conclusion: To measure Vth, remove the load resistance (unknown), then measure the voltage across open circuit terminal • (Vo.c. = Vth). • The Circuit below, shows how to measure and calculate the Thevinin Resistance (Rth) Original and circuit to measure Rth Measured and Calculated Rth • To measure Rth, with load resistance removed, also remove and replace source voltage by a short (jumper wire), then measure and calculate the Thevenin resistance zacross the Open Circuit terminals: • Rth is measured 6 Ohms. • Considering the jumper wire, 6 and 3 Ohms resistors are in parallel, and result is in series with 4 Ohm resistor. • Rth (Cal.)= [(6 x 3)/(6+3) + 4] =2 +4 = 6 Ohms Original & Thevenin Equivalent Circuit: Using Ohm’s Law to Calculate Unknown Current • As you can see from this simple Tutorial Example, the load current for original and the Thevenin Equivalent Circuit are Identical. • This verifies our solution and demonstrate how to measure and calculate the unknown element current by using Ohm’s Law for Thevnin Equivalent Circuit: • IL = (Vth)/(Rth + RL) = (4V) / (6+6) • IL = 4/12 = 0.333A => Calculated! • Compare this with measured, they are exactly equal! Now you can apply this to other Problems.