Inductor
An inductor opposes changes in current through its magnetic-field behaviour. An ideal inductor has no voltage drop at steady DC, but a real winding has resistance and losses. Its useful frequency and current range depend on the construction.
A wire wound into a coil, sometimes around a magnetic core, builds a magnetic field as current flows. That field stores energy. When the current tries to drop, the collapsing field induces a voltage that keeps it going for a moment longer.
In plain terms
Imagine a flywheel spinning in line with the current. Once it's up to speed it wants to keep going; ask it to suddenly stop and it pushes back hard.
Also called: coil, L.
Why designers use it
- Smooth the output of a switching regulator into a clean DC rail.
- Block high-frequency noise from getting onto a sensitive supply.
- Tune resonant circuits in radios and wireless chargers.
- Energy-transfer element in buck, boost, and flyback converters.
- Choke EMI on a power or signal line.
Best for
- Switching power supplies
- EMI / RFI suppression
- Resonant tank circuits
- Filter networks
Key specifications
- Inductance: Check tolerance and value at operating frequency, temperature and current
- Saturation current: Read the manufacturer's stated percentage inductance drop and curve
- Thermal current: Check permitted temperature rise and actual AC/DC losses
- DC resistance: Creates winding voltage drop and heat
- Self-resonance and Q: Check the useful frequency range and parasitic effects
When not to use it
- As a complete voltage regulator. An inductor is only one element of a suitable power-conversion circuit.
- When the inductance at peak current or the operating temperature falls outside the design requirements.
- Where unaddressed magnetic coupling or layout constraints would disturb nearby circuits.
Common mistakes
- Picking a switching-regulator inductor by inductance alone and ignoring saturation current.
- Mounting two inductors broadside-to-broadside so their fields couple.
- Confusing a ferrite bead with an inductor — the bead is lossy on purpose.
Where you will find it
- A switching regulator uses a power inductor to help control ripple current. The inductance at operating current matters, not just its unloaded nominal value.
- A tuned RF circuit uses an inductor with a capacitor. Tolerance, Q and parasitic capacitance affect the response.
- A filter uses an inductor's frequency-dependent impedance. Winding resistance and the complete circuit determine the actual loss and attenuation.
A short history
Induction-coil experiments helped make the effects of changing current visible. IEEE's milestone for Nicholas Callan describes his development of an induction coil in 1836 at Maynooth. Modern inductor selection still starts with that magnetic behaviour, but also considers winding loss, temperature, saturation and parasitic capacitance.
Good to know
- Saturation current and thermal current answer different questions: how much inductance remains, and how hot the component becomes.
- Some cores saturate gradually. Reaching a published saturation-current rating does not instantly turn every inductor into a bare wire.
- Parasitic capacitance helps create self-resonance, even though the component is called an inductor.