The formula
V_th = open-circuit voltage ; R_th = resistance with sources removed ; I_N = V_th / R_th
- V_th
- Thévenin voltage — the open-circuit terminal voltage (volts)
- R_th
- Thévenin resistance seen from the terminals, sources zeroed (ohms)
- I_N
- Norton current — the short-circuit terminal current (amperes)
Worked example
A 12 V source with R₁ = 6 kΩ in series, and R₂ = 3 kΩ across the output terminals.
- Open-circuit voltage is a divider: V_th = 12 × 3 / (6 + 3) = 4 V
- Short the source and look back: R_th = 6 kΩ ∥ 3 kΩ = 2 kΩ
- Norton form: I_N = 4 / 2000 = 2 mA in parallel with 2 kΩ
V_th = 4 V with R_th = 2 kΩ, equivalently a 2 mA source in parallel with 2 kΩ.
Where you'll use it
Maximum power transfer problems, working out what a load will actually receive, and any question that asks for the current through one branch while everything else stays fixed. Maximum power reaches the load when R_load = R_th — at 50 % efficiency, which is why power systems never run there.