Loading…
Calculate inductive reactance (XL), energy stored in an inductor, RL time constant, and LC resonant frequency. Essential for filter design, power electronics, RF circuit design, and motor control applications.
Inductors are the least intuitive passive component for most beginners — unlike resistors (which simply oppose current) or capacitors (which store charge), inductors oppose changes in current and store energy in a magnetic field. This unique property makes them irreplaceable in power electronics, radio frequency circuits, and filtering applications where energy must be stored and released with precise timing.
An inductor is a coil of wire that stores energy in a magnetic field when current flows through it. Measured in henries (H), inductance determines how strongly the inductor opposes changes in current. Inductors are essential in power supplies, RF circuits, filters, transformers, and motors.
XL = 2πfL in ohms. Unlike resistance, inductive reactance increases with frequency. At DC (f=0), XL = 0 (inductor is a short circuit). At high frequencies, XL is large (inductor blocks high-frequency signals). This is the opposite of capacitors, making inductors low-pass filter components.
In an RL circuit, τ = L/R seconds. Current rises to 63.2% of final value after 1τ, and reaches full value after 5τ. Unlike RC circuits where voltage builds up, RL circuits build up current. τ = L/R = 1H/1Ω = 1 second.
When an inductor and capacitor are combined, they resonate at f₀ = 1/(2π√LC). At resonance, XL = XC and they cancel each other — maximum current flows in series resonance, minimum current in parallel resonance. Radio tuning circuits use variable capacitors to select resonant frequency.
| Inductor, Frequency | XL (Ω) | Application |
|---|---|---|
| 1 µH, 100 MHz | 628 Ω | RF circuits, VHF |
| 10 µH, 1 MHz | 62.8 Ω | AM radio, switching PSU |
| 100 µH, 100 kHz | 62.8 Ω | DC-DC converters |
| 1 mH, 10 kHz | 62.8 Ω | Audio crossovers |
| 100 mH, 60 Hz | 37.7 Ω | Power line filters |
Inductors are critical in switching power supplies, RF circuits, and motor drives. Unlike capacitors and resistors, inductor design requires careful attention to core saturation, winding resistance, self-resonant frequency, and shielding. A poorly chosen inductor can cause a switching supply to become unstable or reduce efficiency by 10-20%.
DC-DC switching converters (buck, boost, buck-boost) use inductors to store and transfer energy. The inductor value determines ripple current: ΔI = (Vin - Vout) × D / (f × L), where D is duty cycle and f is switching frequency. Typical: 10-30% ripple current. Too small L → high ripple → more output capacitance needed. Too large L → slower transient response.
Inductors have a maximum current (Isat) beyond which the ferrite core saturates and inductance collapses — effectively becoming a short circuit. In switching supplies, inductor current includes both DC load current and AC ripple. Choose an inductor with Isat rating of at least 1.25× peak current. Core saturation causes sudden efficiency drops, excessive heating, and potential component damage.
Unshielded inductors radiate electromagnetic interference (EMI) from their open magnetic field. In noise-sensitive circuits (audio, precision ADC), use shielded inductors (toroidal core or closed core design). The shielded construction contains the magnetic field, reducing EMI by 20-30 dB. Power over Ethernet (PoE) and medical devices mandate shielded magnetics for EMI compliance.