Inductor (Coil)

  • Any wire carrying a current is surrounded by a magnetic field; winding the wire into a coil strengthens this field
  • When the current through a coil changes the magnetic field resists the change; this resistance to change is called inductance
  • The unit of inductance is the Henry (H) but as this is a large unit the milli– and micro–Henry (mH, μH) are more commonly used
  • In electronic circuits, inductors are mainly used where the current is alternating – such as in audio frequency or radio frequency circuits
  • However, the resistance to change of the magnetic field will also affect a direct current (i.e. non-alternating) circuit during the switch-on phase: if an inductor is connected to a DC power supply, the sudden increase of current in the inductor is initially resisted, but when the current becomes stable, the inductor no longer resists the current