Identify the load or the branch across which you want to find the Thevenin equivalent. Mark terminals A and B.
Disconnect the load, leaving terminals A and B open-circuited for voltage calculation.
Find the open-circuit voltage across terminals A and B using circuit analysis techniques.
Deactivate all sources and calculate equivalent resistance between terminals A and B.
Create the Thevenin equivalent with Vth and Rth in series.
Configuration: 5 resistors in series with voltage source
Configuration: Multiple parallel branches with series elements
Features: Combination of series and parallel elements
Thévenin’s theorem says any linear two-terminal network, however complicated, behaves exactly like a single voltage source VTH in series with a single resistance RTH, as seen from those two terminals.
The point is not to simplify the drawing — it is that once you have the equivalent, you can analyse any load connected to those terminals with one division, instead of re-solving the whole network each time. Sweeping a load resistance across a range becomes trivial.
Deactivating sources trips people up. A voltage source becomes a short because an ideal one holds zero volts across itself when its EMF is removed; a current source becomes an open because an ideal one passes zero current when its drive is removed. Dependent sources must be left in place — they are part of the network’s behaviour, not independent excitation. Where they are present, RTH is found instead by applying a test source and taking the ratio of voltage to current.
A Norton equivalent is a current source IN in parallel with RN, and the two forms convert directly: IN = VTH/RTH with RN = RTH. Neither is more correct; choose whichever suits the next step.
RTH is the output impedance of a real source, and it is what determines how much the terminal voltage sags under load. It is also what sets maximum power transfer, which occurs when the load resistance equals RTH. Note that this point is only 50% efficient, so it is the right target for signal and antenna matching but deliberately wrong for power delivery, where low source impedance and high efficiency matter far more.
Any linear two-terminal network can be replaced, as seen from those terminals, by a single voltage source in series with a single resistance. The replacement behaves identically for any load connected there.
Deactivate all independent sources — short voltage sources, open current sources — then calculate the resistance looking back into the terminals. Dependent sources are left in place.
Because an ideal voltage source with its EMF removed holds zero volts across itself, which is what a short circuit does. By the same reasoning an ideal current source with its drive removed passes no current, which is an open circuit.
They stay in the circuit, since they describe the network's behaviour rather than external excitation. With dependent sources present, find RTH by applying a test source and taking the ratio of voltage to current.
They are the same circuit in two forms. IN = VTH/RTH and RN = RTH, so either converts directly into the other.
Maximum power reaches the load when load resistance equals RTH. It is only 50% efficient, so it is appropriate for signal and antenna matching but deliberately avoided in power delivery, where efficiency matters more.
Written and reviewed by
Lead Electronic Engineer (R&D) · Power Electronics Specialist · M.E. Power Electronics
Founder of Power4All. Works in power-electronics R&D and writes the converter guides, calculators and simulators on this site.
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