Overview
A coil is simply a conductor wound into turns, but that simple shape gives it an important electrical property: inductance. Current flowing through the turns creates a magnetic field. When the current changes, the magnetic field changes too, and that changing field produces a voltage that opposes the change in current.
What coils do in circuits
- Store energy in a magnetic field.
- Oppose rapid changes in current.
- Filter power-supply ripple and switching noise.
- Block radio-frequency signals while passing DC.
- Form tuned circuits with capacitors.
- Store and transfer energy in switching power converters.
- Create magnetic fields in relays, solenoids, motors, speakers, and electromagnets.
- Couple magnetic energy between windings when two or more coils share a core.
The henry
Inductance is measured in henries, abbreviated H. One henry is a large inductance for many electronic circuits, so millihenries (mH), microhenries (µH), and nanohenries (nH) are common.
| Unit | Value | Common territory |
|---|---|---|
| henry (H) | 1 H | Large filters, audio-frequency magnetics, laboratory coils. |
| millihenry (mH) | 10−3 H | Audio, filters, chokes, some power circuits. |
| microhenry (µH) | 10−6 H | Switching converters, EMI filters, RF chokes. |
| nanohenry (nH) | 10−9 H | High-frequency RF and microwave circuits. |
Cores
A coil may be wound on air, plastic, ceramic, ferrite, powdered iron, laminated steel, or other magnetic material. The core changes inductance, field concentration, losses, current capability, size, and useful frequency range.
| Core style | General characteristics |
|---|---|
| Air core | No magnetic-core saturation; low inductance per turn; useful at high frequencies. |
| Ferrite | High magnetic permeability and useful high-frequency behavior; common in power and EMI magnetics. |
| Powdered iron / composite | Distributed air gap and relatively soft saturation; common in power inductors. |
| Laminated steel or iron | Useful at power and audio frequencies where large magnetic flux is required. |
Important specifications
- Inductance: the nominal magnetic energy-storage property.
- Tolerance: allowed variation from nominal inductance.
- DCR: DC resistance of the winding.
- RMS current: current limited mainly by heating.
- Saturation current: current at which a magnetic core begins losing inductance.
- Q: a measure of stored versus lost energy at a stated frequency.
- Self-resonant frequency: where winding capacitance resonates with the inductance.
- Shielding: how well the magnetic field is contained.
Continue through these pages
The Basics page introduces inductance, magnetic fields, reactance, stored energy, and simple RL behavior. Separate pages cover chokes, RF coils, and power inductors. The History page follows the development of electromagnetic coils, while Advanced introduces flux, permeability, saturation, losses, Q, self-resonance, and practical engineering equations.