Inductors Basics

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