There are several methods for finding the Thevinin and Norton equivalent circuits, and only experience will allow you to easily identify which technique is the easiest in any given situation. If unsure, pick Method 2 since it will work in all cases.
The Thevenin Equivalent looking into any two terminals of any circuit containing only linear components comprises a voltage source and a resistor (In Chapter 9 this will be generalized to an impedance) in series.
The Norton equivalent comprises a current source in parallel with a resistor (impedance).
The Thevenin and Norton equivalents are interchangeable through source transformation, thus the resistance (impedance) is the same for both.
METHOD 0 (Used only if the circuit contains NO dependent sources.)
Use combinations of source transforms and resistor combinations (possibly including delta-Y and Y-delta transforms) until the circuit tis reduced to the Thevenin or Norton equivalent. If the circuit contains only resistors (no sources) then the Thevenin and Norton equivalents will simply be a single resistor.
If a load is connected to the two terminals in question, remove it, leaving only the circuit for which you wish the equivalent. Denote the voltage between the terminals as VOC (open-circuit voltage). Determine VOC by any means you wish (nodal analysis, mesh analysis, circuit reduction, etc.). VOC is the value of the Thevinin equivalent voltage source (VTh=VOC), and the positive terminal of the source is oriented toward the terminal which was labelled as the positive reference of VOC.
Next, connect a wire between the two terminals, and define a current ISC (short-circuit current) with the arrow going from the terminal that was previously the positive reference for VOC and toward the terminal that was the negative reference for VOC. (Note that VOC no longer exists since we shorted out the terminals!) Determine this current by any method you wish (mesh, node, reduction, etc.). ISC is the value of the Norton equivalent source. Note that the arrow of this source must point toward the terminal from which ISC was leaving (i.e., the one that was originally marked with the positive reference of VOC).
The Thevinin equivalent resistance may now be determined using Ohm's Law from VOC and ISC. RTh = VOC/ISC. Remember that the Norton resistance is the same as the Thevenin resistance.
Determine either VOC (VTh) or ISC (IN) as in Method 1 above. If there are no independent sources, both of these values will be zero, and the Thevenin and Norton equivalent circuits consist of a single resistor (impedance).
Determine RTh. There are two cases.
Case 1 - No dependent sources present.
- Kill all independent sources (remove current sources, replace voltage sources with a wire).
- Find RTh using circuit reduction techniques.
Note that case 1 may also be solved using the technique explained for Case 2 below.
Case 2 - Dependent sources may be present.
- Kill any independent sources as mentioned in case 1.
- Connect a one volt source between the terminals on the circuit in question. *
- Determine by any means (mesh, node, reduction, etc.) the current I leaving the positive terminal of the one-volt source.
- Calculate the Thevenin equivalent resistance by dividing one volt by the current it generated into the circuit. (Note that if dependent sources are present you might get a negative resistance!) RTh = 1 / I
*Alternately, you could connect a one ampere source and determine the voltage V created across the terminals, then the numerical value of the resistance equals that of the generated voltage (RTh = V / 1 ).
This material still in preparation.
This material still in preparation.