Inductors are passive elements like resistors and capacitors, but they have the characteristic of energy storage in the form of a magnetic field. The simplest form of inductor is a coil of wire that has a tendency to maintain its magnetic field once established. The characteristics of the inductor are a direct result of Faraday’s law of induction , which states:

V (t) =


onde lambda (λ) is the total magnetic flux through the coil windings due to the current. Magnetic flux is measured in webers (Wb). The figure shows the magnetic field lines surrounding an inductor. The south-north direction of the magnetic field lines, shown with arrowheads in the Figure, are found using the right-hand ruler for a coil. The rule states that if the fingers of the right hand are wound in the direction of current flow through the coil, the thumb will point in the direction of magnetic north.

For an ideal coil, the flux is proportional to the current:

λ = LI
Where 𝐿 is the inductance of the coil, which is assumed to be constant. The unit of measurement for inductance is the henry (𝐻 = 𝑊𝑏 / 𝐴). Using the above two equations, the voltage-current relationship of an inductor can be expressed as:

V (t) = L



  • V is the voltage across the inductor [Volt].
  • L is the inductance of the inductor [Henry].
  • dI / dt is the rate of change of the current transformation [(Volt) (sec) / Amper].

The inductor being made up of turns of cable, the magnetic field circulates through the center of the inductor and closes its path on the outside.

An interesting feature of the inductor is that they oppose sudden changes in the current flowing through them. This means that, when modifying the current flowing through them (example: being connected and disconnected to a direct current power supply) it will try to maintain its previous condition.

Typical inductor components range in value from 1 𝜇𝐻 to 100 𝑚𝐻. Inductance is important to consider in motors, relays, solenoids, some power sources, and high frequency circuits. Although some manufacturers have coding systems for inductors, there is no standard method. Often the value is printed directly to the device, usually en or 𝑚𝐻.


Inductance is the opposition to the change in current flowing through the inductor, its value depends on the physical dimensions of the inductor and the permeability of the material with which the core is made.

For an inductor, the inductance is expressed by:

L =

2 µA


  • N corresponds to the number of turns.
  • ℓ is the length.
  • A is the area of ​​the cross section.
  • µ is the permeability of the nucleus.

The inductance value can increase by three factors:

  • A larger cross-sectional area or shorter coil length.
  • A greater number of turns of the coil.
  • Greater permeability of the conductive material than that of the core.


  • According to the core or support:
    1. Air core.
    2. Iron core.
    3. Ferrite core.
  • According to the frequency of the applied current:
    1. High frequency.
    2. Low frequency.
  • According to the coating:
    1. Plastic.
    2. Resin.
    3. Metal.
  • According to the characteristic of its value:
    1. Fixed.
    2. Adjustable.
  • According to the type of mounting:
    1. Inserted.
    2. SMD.


As the electrical resistances and the capacitors, the inductors can also be fixed or variable type, the symbology of these is presented in the following Figures:

  • The air core symbol .
  • The symbol for the iron core .
  • The symbol for ferrite core .
  • The symbol for variable iron core .


Theoretically, inductors adopt any value such as electrical resistances or capacitors, but in the market only certain values ​​are adopted that normally go in the range of microHenrys to Henrys, as well as the material with which the core is built can be air, plastic, iron.

“A non-ideal inductor has a model with a series winding resistance due to the conductive material as well as a winding capacitance due to the capacitive coupling between the conductive coils. Due to their small value, they can be neglected in most applications, except for the high frequency capacitance. ”


The important values ​​that we must know are the electrical inductance, and tolerance. These values ​​are indicated on the package depending on the type of package.

The first step in reading an Inductor is understanding what each band means.

  1. The first band that corresponds to the extreme left is the one that represents the most significant digit of the inductor.
  2. The second band represents the second most significant digit.
  3. The third band represents the third most significant digit of the inductor.
  4. The fourth band represents the power of 10 raised to the corresponding color and multiplied by the first, second and third bands.
  5. The fifth band represents the tolerance of the inductor.

“In a 4-band inductor, the first point or first band must be eliminated and continuity must be given from point 2 or second band to point 5.”


Magnetic permeability (m) :

  • It is a characteristic that has great influence on the core of the coils with respect to the value of their inductance. Ferromagnetic materials are very sensitive to magnetic fields and produce high inductance values, however other materials have less sensitivity to magnetic fields.
  • The factor that determines the greater or lesser sensitivity to these magnetic fields is called magnetic permeability.
  • When this factor is large, the value of the inductance is also large.

Quality factor (Q) :

  • Relate the inductance to the ohmic value of the coil material. The coil will be good if the inductance is greater than the ohmic value due to the material of the coil.

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