# Inductor

The inductor is a passive component like the capacitor and the resistor. The inductor has two terminals and are sometimes called a choke or a coil. The inductor is a conductor wound into a coil around a core. The core is often made from plastic or of a ferromagnetic material. There is also inductor with an air core instead of a plastic or a ferromagnetic material. The inductor stores energy in a magnetic field when current flows through it. Just like the capacitor, it stores energy but in a different way. The capacitor stores energy in an electric field while the inductor stores energy in a magnetic field. The main property of an inductor is its inductance. The inductance is a property that oppose an instantaneous change of current in the circuit. If we compare with the capacitor, the capacitance of the capacitor oppose an instantaneous change of voltage in the circuit. The unit of inductance is the Henry (H). Generally, inductors have values from 100nH to a couple of Henry. Inductors are used in multiples applications : switching mode power supplies, filter, transformers, etc.

The inductor has two symbols:

The symbol on the left is an inductor with an air core. The symbol on the right with the line above the inductor is an inductor with a magnetic core. Often, the air core symbol is used in electric schematic even if the inductor is a magnetic core for simplicity. There is also a third symbol for variable inductor. It is the symbol on the left or right with an arrow at 45 degrees going through the inductor. Variable inductors are use mostly for antenna/radio tuning applications.

##### Inductors in Series

The total inductance of inductors placed in series is calculated like series resistors. We just need to add the values of each inductors to get the equivalent inductor :

$Ltot=L1+L2+L3$

This means that we could replace all 3 inductors with one inductor of inductance Ltot and it would be an equivalent circuit.

The generic formula for inductors in series would be :

$Ltot=L1+L2+L3+Ln+\dots$

##### Inductors in Parallel

The total inductance of inductors placed in parallel is calculated the same way as resistors in parallel.

In the image above, the total inductance would be :

$Ltot=\cfrac{1}{\cfrac{1}{L1}+\cfrac{1}{L2}+\cfrac{1}{L3}}$

This means that we could replace all three inductors with one inductor of inductance Ltot and it would be an equivalent circuit.

The generic formula for inductors in parallel would be :

$Ltot=\cfrac{1}{\cfrac{1}{L1}+\cfrac{1}{L2}+\cfrac{1}{L3}+\cfrac{1}{Ln}+\dots}$