Finding the Thevenin and Norton Equivalents of a Circuit

Capture and PSpice can be used to easily calculate the Norton and Thevenin equivalents of a circuit. The method we will use is the same as if we were going to find the equivalent circuits in the lab. We will make two measurements, the open circuit voltage and the short circuit current. The Thevenin resistance is then the open circuit voltage divided by the short circuit current. This will require us to create two circuits, one to find the open circuit voltage, and the second to find the short circuit current. In this example, we will find the Norton and Thevenin equivalent circuits for a DC circuit. This same procedure can be used to find the equivalent circuits of an AC circuit (a circuit with capacitors or inductors). However, instead of finding the open circuit voltage and short circuit current using the DC Nodal Analysis, we would need to use the AC analysis. 

For this example, we will find the Thevenin and Norton equivalent circuits for the circuit attached to the diode in EXERCI5E 3-5. The circuit is repeated below  

This circuit is difficult because it contains a nonlinear element (the diode) and a complex linear circuit. If we could replace VI, Rl, R2, and R3 by a simpler circuit, the analysis of the nonlinear element would be much easier. To simplify the analysis of the diode, we will find the Thevenin and Norton equivalent circuits of the circuit connected to the diode that is, we will find the Thevenin and Norton equivalents of the circuit below  

We will convert this circuit into the Thevenin equivalent  

Once we find numerical values for Vth and Rth the entire circuit of EXEBCISE 3-5 reduces to  

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