Coils, Inductors and Toroids

A coil, or inductor, is a length of wire wound into turns so that current flowing through it creates a concentrated magnetic field. The defining property of an inductor is inductance, which measures its tendency to oppose changes in the current through it. When the current changes, the changing magnetic field induces a voltage that resists that change, an effect described by the laws of electromagnetic induction developed by Faraday and formalised in Maxwell’s equations. This makes coils essential for storing energy magnetically, for filtering, and for tuning circuits.

Winding a coil around a magnetic core greatly increases its inductance. Ferromagnetic materials such as iron and ferrite concentrate the magnetic field within the core, so far fewer turns are needed for a given inductance. A toroid is a coil wound on a ring-shaped core; this closed loop keeps almost all of the magnetic field inside the core, which reduces stray field and interference with nearby circuits and makes toroidal coils efficient and compact.

Inductors appear throughout electronics. A choke is an inductor used to block high-frequency alternating current while allowing direct current or lower frequencies to pass. Transformers use two or more coils sharing a common core to transfer energy between circuits and change voltage levels, and ignition coils are a specialised transformer that produces the high voltage needed to create a spark. Solenoids are coils built to produce mechanical motion, pulling an iron core when energised.

Frequently asked questions

What is inductance?

Inductance is a coil’s opposition to changes in the current through it. A changing current produces a changing magnetic field, which induces a voltage that resists the change.

Why are toroidal cores used?

A ring-shaped core keeps almost all of the magnetic field inside the core, reducing stray field and interference. This makes toroidal coils efficient and compact for a given inductance.

What is a choke?

A choke is an inductor used to block or impede high-frequency alternating current while passing direct current or lower frequencies, often to filter or smooth a supply.