Side-by-side comparison
| Parameter | Voltage Source | Current Source |
|---|---|---|
| Ideal internal resistance | Zero ohms | Infinite ohms |
| Terminal voltage | Fixed regardless of load current | Varies with load — V = I_s × R_L |
| Terminal current | Varies with load — I = V_s / R_L | Fixed regardless of load voltage |
| Practical internal resistance | Very low — e.g. 0.1 Ω for a lab bench supply | Very high — e.g. 1 MΩ for a transistor collector |
| Series/Parallel element | Short circuit in parallel has no effect; open in series blocks it | Open circuit in series has no effect; short across it is dangerous |
| Thevenin equivalent | Is the Thevenin model itself | Converted: V_th = I_s × R_th |
| Norton equivalent | Converted: I_N = V_s / R_N | Is the Norton model itself |
| Real component example | LM317 voltage regulator, 7805 IC (5 V fixed) | BJT collector (β×I_B), LM334 current source IC |
| Power delivery | Maximum to load when R_L >> R_internal | Maximum power transfer at R_L = R_source |
| Waveform linearity | Exponential capacitor charging | Linear capacitor charging (constant current) |
Key differences
An ideal voltage source has zero internal resistance and holds its terminal voltage at, say, exactly 5 V whether the load draws 10 mA or 1 A; the 7805 regulator approximates this. An ideal current source has infinite internal resistance and forces a fixed current — the BJT collector forcing β×I_B = 100×1 mA = 100 mA regardless of V_CE is the textbook example. Source transformation swaps them: a 12 V source with 4 Ω becomes a 3 A current source in parallel with 4 Ω. The fundamental duality — short kills current source, open kills voltage source — is tested repeatedly.
When to use Voltage Source
Use a voltage source model when the load resistance is much smaller than the source internal resistance would cause a significant drop — for example, a regulated 5 V, 1 A bench supply (LM317-based) driving digital logic ICs.
When to use Current Source
Use a current source model when the load resistance is much larger and you need constant current regardless of voltage — for example, an LM334 providing a stable 1 mA bias current to a photodiode over varying illumination conditions.
Recommendation
For network analysis problems involving source transformation or Thevenin/Norton equivalents, choose the model that reduces the circuit faster: voltage source for mesh analysis, current source for nodal analysis. Most exam circuits become simpler when you match the source type to the analysis method.
Exam tip: Examiners test source transformation — expect to convert a voltage source with series resistance to a Norton equivalent and verify the answer by calculating load current both ways; getting the same answer proves your conversion.
Interview tip: Placement interviewers at Infosys or core electronics companies ask you to explain why a BJT collector behaves as a current source — answer using the Early effect and the flat I_C vs V_CE characteristics in the active region.