Inductors or chokes or coils or chokes are two-terminal passive electrical energy storage devices that are designed with a magnetic core surrounded by insulating wires. When current flows through an inductor, its magnetic field stores that energy. The energy stored in an inductor is called an inductance, and its unit is the henry, named after the American scientist Joseph Henry.

　　Inductors are mainly used in signal processing and analog circuits. Different types of inductors can be used depending on the application such as air core, iron core, toroidal, variable, etc. In this article, we will briefly introduce the basics related to variable inductors.

　　Basic Definition

　　Variable inductors are defined as inductors or coils with continuously adjustable effective inductance. The frequency range of this inductor is usually 10μH-100μH, but most of the currently available inductors range from 10nH-100mH.

　　Basic Construction

　　Variable inductors are constructed by winding copper wire around a hollow winding barrel. The inductance can be simply changed by positioning and moving the brass core or the core of ferromagnetic material. If a ferrite core is used, the inductance of this inductor can be increased by simply moving the core material to the middle of the winding. Alternatively, if a brass core is used, the inductance of the inductor can be reduced by simply moving the core material to the center of the winding.

　　Variable inductors work by allowing the user to control the inductance by simply changing the position of the ferrite core, so the inductance of a variable inductor is continuously adjustable. As the ferrite core is moved deeper in the coil, it enhances the permeability and inductance. Similarly, moving the ferrite core out of the coil decreases the permeability and inductance.

　　Main Types

　　Depending on the application and construction, variable inductors are categorized into two types: Segmented Tuning and Tap Variable Inductors, which are described in the following order.

　　1. Tap Variable Inductor

　　A tap variable inductor is a coil that can be contacted at more points. This type of inductor mainly consists of a coil with a huge number of turns that are wound on a core with a preferred number of taps. Wherein the taps are wires drawn from the coil at a preferred distance so that different mutual inductances can be obtained on similar inductors.

　　2. Segmented Tuned Inductor

　　Variable inductors containing a modifiable ferrite core are called segment tuned inductors, which control the increase or decrease in inductance based on how much the ferrite core is moved into or out of the coil winding. The inductor construction is the same as a fixed ferrite core inductor except that the core is modifiable.

　　As the metal block is moved into the winding of the coil, the inductance will increase and the resonant frequency of the tuned circuit will decrease. Once the metal block is removed from the winding of the coil, the inductance of this inductor will decrease and the resonant frequency of the tuned circuit will increase, so the movable core changes the inductance value. This core can be moved up and down with a screwdriver.

　　Variable Inductor Circuit

　　A typical variable inductor circuit is shown below. The main objective of this circuit is to understand the effect of magnetic permeability on inductance and to observe how inductive reactance controls the current in an AC circuit. The main components needed to make up this circuit include paper tubes, steel or iron rods, 28-gauge enameled wire, a low-voltage AC power supply, and an incandescent lamp. Use these components to connect the circuit according to the circuit diagram shown below.

　　First, take the paper tube and wrap hundreds of turns of enameled wire around it to make a homemade inductor. After that, this inductor needs to be connected in series with an AC power source and an incandescent lamp to form the circuit. Once the paper tube is empty, the incandescent lamp should be bright. When the steel is placed inside the paper tube, the brightness of the incandescent lamp decreases due to an increase in inductance (L), which leads to an increase in reactance (XL).

　　Key Characteristics

　　The characteristics of variable inductors include the following:

　　The inductance of this inductor is continuously adjustable.

　　These inductors can be distinguished by different characteristics, the first characteristic is the type of core it is wound with. The second characteristic is their shape, the coils of the inductor are wound in the structure. Some are wound in a circular shape and some are wound in a cylindrical shape.

　　The last characteristic is whether the variable inductance is variable or adjustable.

　　These inductors offer high quality factor, low parasitic capacitance and excellent high frequency (HF) characteristics.

　　Major Applications

　　The main applications for variable inductors include the following:

　　For use where tuning is required, such as radio and HF based applications.

　　Used in different circuits such as tuning, coupling, oscillators and timing.

　　Can be used in highly sensitive applications where fixed inductors may not be fully aligned under these conditions.

　　Used in power factor (PF) correction panels to adjust inductor values.

　　Used in power supply system based applications.

　　Used in power supply applications with coupled circuits.

　　Variable inductors are used in medium power based applications and also control o/p currents in high frequency (HF) resonant circuits.

　　Used to control electronic ballasts and LED lighting.

　　Used in telecommunications to control the impedance of circuits in order to have multi-band RF circuit control.

　　Summarize

　　The above is an overview of the working principle, main types and application circuits of variable inductors. Because variable inductors are able to change their own inductance, they are very reliable and are mainly used in RF applications.