Basics of Inductors

An inductor is a coil of wire either hollow, or wound around some ferrous (magnetic) material.

When current flows through the coil a magnetic field is produced. When the current stops flowing the magnetic field collapses.

If the coil is connected to a d.c. supply, a steady current will flow, and the opposition to the flow will be mainly due to the resistance of the wire used to make the coil.



Inductor Coil



However, at the moment when the current is switched on or switched off the rising or falling magnetic field also opposes the flow of current. This means that if an alternating supply is connected to the coil the opposition to the flow is greater than that due to the resistance alone.

The amount of opposition to a.c. depends upon the wire used, the number of turns, type of material inside the coil etc., and the effect is known as the inductance of the coil. Inductance is measured in henrys, and a small inductor may have a value of, say, 10mH.

There is also some capacitance associated with the coil, and this too affects the way it behaves with a.c. So the whole effect due to the resistance of the wire, the inductive effect and capacitive effect is summed up by referring to the impedance of the coil.

Impedance is the total opposition to a.c. and will depend upon the a.c. frequency. You will probably know that one of the important loudspeaker measurements is its impedance. If your amplifier has an output impedance of 8 ohm, then you need a speaker of 8ohm impedance if you wish to extract the maximum power.

Inductors are often used to reduce voltage spikes in a circuit – in fact you often see ferrous material wrapped around mains leads or other leads associated with computers, video recorders etc. Inductors are also used in radio tuning, and combined with capacitors can form a “tuned circuit” i.e. one which resonates with a particular frequency – to tune in your favorite radio station for instance.