Core equations
| V = L · dI/dt | Voltage across inductor from changing current |
|---|---|
| E = ½ L I² | Energy stored (joules) |
| X_L = 2π f L | Inductive reactance (Ω) |
| Impedance | Z = jωL (pure inductor) |
| Quality factor Q | = ωL / R |
| Time constant τ | = L / R |
Series / parallel
| Series | L_total = L1 + L2 + … |
|---|---|
| Parallel | 1 / L_total = 1/L1 + 1/L2 + … |
| Two parallel | L = (L1 · L2) / (L1 + L2) |
Practical notes
- Saturation current (I_sat): above this, core magnetically saturates and L drops sharply.
- DCR: DC resistance of windings — causes copper loss and voltage drop.
- Self-resonance: parasitic capacitance makes inductors resonant — above SRF they behave as capacitors.
- Core types: powdered iron (gradual saturation), ferrite (hard saturation), air (no saturation but low L).
- Flyback: interrupting current through an inductor generates large voltage spikes; always include a clamp (diode, snubber, or TVS).
Common values
| Value | Use |
|---|---|
| nH range | RF chokes, matching networks |
| µH range | Switching power supply, filters |
| mH range | AC filters, audio crossovers |
| H range | Line-frequency chokes |
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