Variable Inductor : Working, Circuit, Construction, Types & Its Applications

An inductor or a choke or a coil or a reactor is a two-terminal passive electrical energy storage device designed from a core that is enclosed by an insulated wire. When electric current flows throughout the inductor, then its magnetic field stores this energy. The amount of energy stored by an inductor is known as Inductance and its unit is the Henry which is named after American scientist – Joseph Henry. Inductors are used mainly in signal processing & analog circuits. There are different types of inductors available that are used based on applications like air core, iron core, torodial, variable, etc. This article discusses an overview of a variable inductor – working with applications.

What is a Variable Inductor?

A variable inductor definition is an inductor or coil whose effective inductance is adjustable continuously. The frequency range of this inductor typically ranges from 10 μH – 100 μH & currently available inductors range from 10nH – 100 mH. The variable inductor symbol is shown below.

Variable Inductor Symbol
Variable Inductor Symbol


The variable inductor is constructed by winding a copper wire around a bobbin cylinder that is hollow. The inductor value can be simply changed by locating & moving the brass core or ferromagnetic material core. If we use a ferrite core then the inductance of this inductor is increased by simply moving the core material to the middle of the winding. Alternatively, if a brass core is utilized, then the inductance of an inductor is reduced by simply moving the core material to the middle of the winding.


The working principle of the variable inductor is that it allows the user to control the inductance by simply changing the ferrite core position. So the inductance of the variable inductor is adjustable continuously. When the ferrite core is moved very deeper into the coil then it enhances the permeability and the inductance. Similarly, moving the ferrite core out of the coil will reduce the permeability and the inductance

Types of Variable Inductor

Variable inductors are classified into two types slug tuned and tapped variable inductors.

Tapped Variable Inductor

A tapped variable inductor is a coil to which electrical contact is accessible at more points. This inductor mainly includes a coil with a huge number of turns. These turns are wounded on a magnetic core with a preferred number of tappings. Here, a tap is a conducting wire that is taken out from the coil at a preferred distance because of which different mutual inductance can be attained over a similar inductor.

Tapped Type
Tapped Type

Slug Tuned Inductor

A variable inductor that includes a modifiable ferrite core is called a slug-tuned inductor. Based on the ferrite core moving in or out of the coil winding, the inductance value of this inductor increases or decreases. This inductor construction is the same as the fixed ferrite core inductors, except the core is modifiable.

Slug Tuned Inductor
Slug Tuned Inductor

When the slug is moved into the winding of the coil then the value of inductance will be increased & the tuned circuit’s resonant frequency will be decreased. Once the slug is moved from the winding coil, the inductance of this inductor will decrease & the tuned circuit’s resonant frequency will be increased. The inductance value can be changed with the movable core. This core can be moved up/down with a screwdriver.

Variable Inductor Circuit

The variable inductor circuit is shown below. The main objectives of this circuit are; to magnetic permeability effects on inductance and also to observe how inductive reactance controls the flow of current within an AC circuit.

The required components to make this circuit mainly includes a paper tube, steel or iron bar, magnet wire with 28 gauge, low voltage AC power supply, and Incandescent lamp. Connect the circuit using these components as per the circuit diagram shown below.

Variable Inductor Circuit
Variable Inductor Circuit

First, take the paper tube and wrap hundreds of magnet wire turns around the paper tube to make a homemade inductor. After that, this inductor needs to connect with an AC power supply & Incandescent lamp in series to make a circuit. Once the paper tube is empty, the Incandescent lamp should glow very brightly. When the steel bar is placed within the paper tube, the incandescent lamp brightness will reduce from increased inductance (L) and thus inductive reactance (XL) will be increased.

Wiring Diagram
                                          Wiring Diagram Variable Inductor

When we use different material bars made with stainless steel or copper material they don’t have a similar effect because of differences in magnetic permeability.


The variable inductor characteristics include the following.

  • The inductance of this inductor is adjustable continuously.
  • These inductors can be distinguished through different characteristics the first characteristic is, the type of core it is wounded around. The second characteristic is their shape, where the coil of an inductor is wounded in the construction. Some are wounded in circles, although many are in cylindrical shape.
  • The final characteristic is whether the variable inductor is variable or adjustable.
  • These inductors have high-quality factors, low parasitic capacitance & outstanding high frequency (HF) characteristics.

Applications/ Uses

The applications or uses of variable inductors include the following.

  • These types of inductors are applicable where the tuning is necessary like radio & high frequency-based applications.
  • These inductors are used in different circuits like tuning, coupling, oscillator, and timing.
  • This type of inductor is also employed within highly sensitive applications because a fixed inductor may not be completely aligned in these conditions.
  • These are used in power factor (PF) correction panels to adjust the Inductance value.
  • This is appropriate for power system-based applications.
  • These are used in power applications with coupled circuits.
  • Variable inductors are used in mid-power-based applications and also control the o/p current of high-frequency (HF) resonant circuits.
  • These are used for controlling electronic ballasts & LED lighting.
  • These are used in the telecommunication field to control the impedance of circuits in order to have multiband radio frequency circuit control.

Thus, this is an overview of a variable inductor – circuit, its working, types, and characteristics with applications. This inductor is capable of changing its own inductance. These are extremely reliable, so they are used mainly in RF applications. Here is a question for you, what is an inductance?