INDUCTOR

These are the last of the purely passive components.  An inductor is most commonly a coil, but in reality, even a straight piece of wire has inductance.  Winding it into a coil simply concentrates the magnetic field, and increases the inductance considerably for a given length of wire.  Although there are some very common inductive components (such as transformers, which are a special case), they are not often used in audio.  Small inductors are sometimes used in the output of power amplifiers to prevent instability with capacitive loads.

Note: Transformers are a special case of inductive components, and will be covered separately.

Even very short component leads have some inductance, and like capacitance, it is just a part of life.  Mostly in audio, these stray inductances cause no problems, but they can make or break a radio frequency circuit, especially at the higher frequencies.

An inductor can be considered the opposite of a capacitor.  It passes DC with little resistance, but becomes more of an obstacle to the signal as frequency increases.

There are a number of different symbols for inductors, and three of them are shown below.  Somewhat perversely perhaps, I use the "standard" symbol most of the time, since this is what is supported best by my schematic drawing package. 

 Inductor Symbols

There are other core types not shown above.  Dotted lines instead of solid mean that the core is ferrite or powdered iron, rather than steel laminations or a toroidal steel core.  Note that pure iron is rarely (if ever) used, since there are various grades of steel with much better magnetic properties.  The use of a magnetic core further concentrates the magnetic field, and increases inductance, but at the expense of linearity.  Steel or ferrite cores should never be used in crossover networks for this reason (although many manufacturers do just that, and use bipolar electrolytic capacitors to save costs).

Inductance is measured in Henrys (H) and has the symbol "L" (yes, but ... Just accept it :-).  The typical range is from a few micro-Henrys up to 10H or more.  Although inductors are available as components, there are few (if any) conventions as to values or markings.  Some of the available types may follow the E12 range, but then again they may not. 

Like a capacitor, an inductor has reactance as well, but it works in the opposite direction.  The formula for calculating the inductive reactance (X_L) is ...

7.1.1   X_L = 2 π F L  where L is inductance in Henrys

As before, the transposition triangle helps us to realise the wanted value without having to remember basic algebra.

 
 Inductance Triangle

An inductor has a reactance of 8 ohms at 2Khz.  What is the inductance?  As before, cover the wanted value, in this case inductance.  The formula becomes ...

7.1.2   L = X_L / 2 π F

The answer is 636uH.  From this we could deduce that a 636uH inductor in series with an 8 ohm loudspeaker will reduce the level by 3dB at 2kHz.  Like the capacitor, when inductive reactance equals resistance, the response is 3dB down, and not 6dB as would be the case with two equal resistances.  What we have done in these last two examples is design a simple 2kHz passive crossover network, using a 10uF capacitor to feed the tweeter, and a 636uH inductor feeding the low frequency driver.

Like a capacitor, an inductor (in theory) dissipates no power, regardless of the voltage across it or the current passing through.  In reality, all inductors have resistance, so there is a finite limit to the current before the wire gets so hot that the insulation melts.