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Thevenin Notes, 170203 The three cases to consider are Case 1. All sources are independent Case 2. The circuit has dependent sources and independent sources Case 3. The circuit has only dependent sources Depending on the case, one or more of the following methods can be used to find the Thevenin equivalent: Direct Rth. (Applies only to Case 1). Turn off all independent sources (i.e., set V = 0 for voltage sources, and I = 0 for current sources). Note - this is the same thing as replacing voltage sources with short circuits, and current sources with open circuits. Connect a fictitious ohmmeter across terminals a-b, and “measure” Rth directly. Find Isc (or, alternatively, find Voc = Vth ). Compute Vth = Voc = Isc • Rth (or, alternatively, Isc = Voc / Rth ). If time permits, find Vth (or, alternatively, Isc ) directly from the circuit, and then double-check with the above. Voc , Isc (Applies to Cases 1 and 2). Find Voc = Vth . Find Isc . Compute Rth = Vth / Isc . Fictitious Source (Applies to all Cases) Attach a fictitious source Vab across terminals a-b. Find a linear equation with the following form: Vab A BI ab . By definition the linear equation must match Thevenin equation Vab Vth Rth I ab , term by term. Thus, matching the terms yields Vth A, Rth B . Case Direct Rth. Voc , Isc Case 1 Case 2 Case 3 OK OK OK Fictitious Source OK OK OK Iab + Vth Rth Vab Vth Rth I ab – Note – For Case 3, the Vth should be zero. Thus, for Case 3, you can attach any voltage source Vab to the output (e.g., Vab = 1V), find I ab , and compute Rth Vab . I ab