Variable Inductor: Types, Construction, Application, etc

Types of variable Inductor | Slug-tuned inductor | Tapped inductor | What is the Frequency Range inductor | variable inductor application and construction |

Types of variable inductor:

In electronic circuits, sometimes it becomes necessary to adjust the value of frequency. For example, we want to change the radio station, set different timing, etc. Such types of adjustments will handle by using variable inductors.

The inductance whose inductance value varied by manually from minimum to its maximum specified limit called a variable inductor. The value of inductance varied using a movable core which can be moved up or down by using a screwdriver, there are two types of variable inductor:

1. Slug-tuned inductor

2. Tapped inductor

The circuit symbol of variable inductor is as shown in the figure.

1. Slug-tuned inductor:

The tuning inductors is the type of variable inductor which is either air core, ferrite, or powdered iron core coils.

The air-core type of variable inductor is not adjusted unless either an expensive roller of contact mechanism system or a clumsy tap system on the windings of the coil.

However, the ferrite and powdered iron slug-tuned core coils are adjustable. The figure shows the construction of the slug-tuned inductor.

Slug tuned inductor
Figure: Slug tuned inductor


The construction of the slug-tuned coil variable inductor is similar to the ferrite core coil, but the core is adjustable. The form made of materials like plastic, phenolic, fiberglass, nylon, or ceramic, and this is internally threaded.

The windings of the coil have wound on the form. The tuning slug made up of ferrite or powdered iron coil core that matters with the internal threads in the form of a coil. A screwdriver slot or hex hole in either (or both) ends allow you to adjust it.

The inductance of the coil will depend on how much size of the core is inside the coil windings. The permeability of the laminated core will increase or decrease on the basis of inductance according to how much of the core is inside the coil.

If the core has made with the shape of either a hexagonal hole or screwdriver slot, then its inductance value of the coil adjusts by moving the core in or out of the coil, and this coil called a slug-tuned inductor.


  • Its size is small
  • It has low cost


  • Slug-tuned inductor used in timing, calibration.
  • It will also used in tuning and filter circuit.
  • It will use at all Radio frequencies (R.F) as well as for width and linearity controls on television sets.

2. Tapped Inductor:

The construction of tapped inductor is as shown in the figure.

Tapped inductor
Figure: Tapped inductor


It consists of a coil having a large number of turns and wound on a magnetic core, with a desired number of tapping. Here the inductance of a coil is varied by placing taps along the winding and using a selector switch.

Provisions should make to short out the unused portion of the inductor.  Since taps cannot provide smooth and continuous adjustment, the tapped inductor should probably be termed as an adjustable inductor rather than a variable inductor.

But this arrangement will not give the continuous inductance value.

What are the types of frequency Range variable inductor:

Following are the some types of frequency range inductors:

i. Audio frequency inductors (A.F)

ii. Radio frequency inductor (R.F)

iii. Intermediate frequency inductor (I.F)

iv. Toroidal inductor.

1. Audio frequency inductor (A.F) choke:

Audio frequency coil, usually called A.F chokes can have a ferromagnetic iron core to increase their inductance. This Audio frequency inductor used to provide a high impedance to audio frequencies (60 Hz to 5 kHz)

Audio frequency inductor (A.F) choke is the type of low-frequency variable inductor. This choke is large in size and inductance value. But compared to filter chokes, they are smaller in size and have lower impedance value.

2. Radio frequency inductors (RF choke):

A Radio frequency inductor or choke is a high-frequency variable inductor. Chokes having still lesser inductance value and smaller size used as a radiofrequency choke to block the radiofrequency.

The laminated cores have not used these chokes, since the core losses are proportional to the square of the frequency. The core losses will be excessive at such high frequencies some of these chokes have solid or powdered iron cores.

For every ultra-frequency, plastic or steatite cores are used. These winding are commonly built up in a number of sections, each section being several layers deep, but not very wide as a means of reducing capacitance that exists between wires turns.

The windings of many R.F chokes are left exposed, but smaller and flat windings are enclosed. These inductors use air core, powdered iron slugs and ferrite cores.

As cores are small in size and hence ferrite cores are used for high frequency applications due to their high permeability and high resistivity.

Mica Bakelite, polystyrene, phenolics, and ceramic are used as former or core for R.F coils. Compared to filter chokes and A.F chokes, R.F chokes are smaller in size and have lower inductance value.

These chokes have used to block radio frequencies and hence these chokes called R.F chokes. These will use in oscillator circuits.

3. Intermediate frequency inductor (IF choke):

An intermediate frequency coil or inductor or choke has designed for use in F.M superheterodyne receiver. The copper coil wound on grooved polystyrene forms and tuning accomplished by movable iron cores. Applications of intermediate frequency inductor is it will used in double conversion receivers filters, mixers etc.

4. Toroidal Inductor:

Toroidal Inductor usually available in a smaller size, a lower-cost choice, and made from a variety of alloys. The construction of the toroidal inductor is as shown in the figure. In this type of inductor, the core is toroid or doughnut shape.

Toroidal inductor
Figure: Toroidal inductor

The main advantage of this shape provides a continuous magnetic path and flux confined to the volume enclosed by windings. They offer the most magnetic material choices with very little current loss.

They manufacture from the following materials like cobalt alloy, ferrite metallic glass, and nickel alloys. The result is a large inductance value for its size.

Depending on the frequency of applications, the coil has few or more turns of the coil on core. The only difficulty is in the windings of this coil. The main advantage of this coil is that it has a less stray magnetic field.

Fixed type of inductor

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