What is a coil or inductor?

It is a passive element, 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 establishes:

dλ / dt

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


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

λ= LI

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

V(t)= L
dI / dt

The inductor being formed of cable turns, the magnetic field circulates through the center of the inductor and closes its path on the outside. An interesting feature of the inductors 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 DC power source) it will try to maintain its previous condition.

Typical inductor components vary 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 they have coding systems for the inductors, there is no standard method. Often, the value is printed directly on the device, usually in µH or mH.


The Air core symbol


The symbol for the iron core


The symbol for ferrite core


The symbol for variable 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

  • Depending on the type of assembly:

  1. Insertion

  2. SMD


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

The first step to reading an inductor is the understanding of what each band means:

  1. The first band that corresponds to the left end 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 continue from point 2 or second band to point 5


1.       Magnetic permeability (m):

  • It is a characteristic that has great influence on the core of the coils with respect to the value of the inductance of the coils. 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.

2.       Quality factor (Q):

  • It relates 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 same.