Side-by-side comparison
| Parameter | Thevenin | Norton Theorem |
|---|---|---|
| Equivalent Circuit | V_th in series with R_th | I_N in parallel with R_N |
| Source Type | Voltage source (V_th) | Current source (I_N) |
| Equivalent Resistance | R_th (same calculation as R_N) | R_N = R_th (identical value) |
| Relationship | V_th = I_N × R_N | I_N = V_th / R_th |
| Best for Load | High-impedance loads; voltage divider analysis | Low-impedance loads; current divider analysis |
| Finding Open-Circuit Value | Measure/calculate V_oc at terminals | Measure/calculate I_sc at terminals |
| Preferred in Transistor Circuits | Common-emitter voltage bias networks | Current mirror and current source analysis |
| Source Conversion | Convert to Norton: I_N = V_th / R_th | Convert to Thevenin: V_th = I_N × R_N |
Key differences
Thevenin and Norton equivalents represent the same network — R_th and R_N are always equal. The difference is source form: Thevenin uses V_oc as a series voltage source, Norton uses I_sc = V_oc / R_th as a parallel current source. For a 12 V source with 600 Ω internal resistance, the Thevenin form is 12 V + 600 Ω series; the Norton form is 20 mA || 600 Ω. Thevenin suits high-load-impedance calculations; Norton suits current-divider chains and op-amp summing junctions.
When to use Thevenin
Use Thevenin's theorem when the load is a high-impedance element like an op-amp input or a voltmeter — for example, finding the voltage across a 10 kΩ load connected to a bridge sensor network.
When to use Norton Theorem
Use Norton's theorem when the load draws significant current or when current dividers dominate — for example, analyzing a BJT current mirror where the collector current is found by current division from I_N.
Recommendation
For most university exam problems, choose Thevenin — question setters ask for "the voltage across the load," which maps directly to the Thevenin equivalent with a voltage divider finish. Switch to Norton only when the problem explicitly involves current sources or current dividers.
Exam tip: The key examiner trap is asking you to find R_th with dependent sources present — you must apply a test voltage V_x, find the resulting I_x, and compute R_th = V_x / I_x; setting independent sources to zero and using ohmmeter rules fails with dependent sources.
Interview tip: A placement interviewer at L&T or ABB will ask you to explain source transformation in one line — say: "V_th and I_N are interchangeable via I_N = V_th / R_th; the parallel or series resistance is the same R_th either way."