Inductor

What is an Inductor?

 

An Inductor, also called a Coil, Choke, or Reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil.

When the current flowing through the coil changes, the time-varying magnetic-filed induces an electromotive force (EMF voltage) in the conductor, described by Faraday’s Law of Induction. According to Lenz’s Law, the induced voltage has a polarity (direction) which opposes the change in current that created it. As a result, inductors oppose any changes in current through them. It is also determined by several factors such as:

 

• The shape of the coil
• The number of turns and layers of the wire
• The space that is given between the turns
• Permeability of the core material
• The size of the core

 

 

An inductor is characterized by its inductance, which is the ratio of the voltage to the rate of change of current. In the international SI, the unit of inductance is the henry (H) named for 19^th century American scientist Joseph Henry. In the measurement of magnetic circuits, it is equivalent to weber/ampere. Inductors have values that typically range from 1Μh (10^-6H) to 20H. Many inductors have a magnetic core made of iron or ferrite inside the coil, which serves to increase the magnetic field and thus the inductance. Along with capacitors and resistors, inductors are one of the three passive linear circuit elements that make up electronic circuits. Inductors are widely used in alternating current (AC) electronic equipment, particularly in radio equipment. They are used to block AC while allowing DC to pass; inductors designed for this purpose are called chokes. They are also used in electronic filters to separate signals of different frequencies, and in combination with capacitors to make turned circuits, used to tune radio and TV receivers.

 

Construction of an Inductor

 

If we look at the construction of an inductor it usually consists of a coil of conducting material (widely used ones include insulated copper wire) that is wrapped around a core that is made up of plastic material or ferromagnetic material. One advantage of using a ferromagnetic core is that it has high permeability which helps in increasing the magnetic field and at the same time confining it closely to the inductor. Ultimately this results in higher inductance.

On the other hand, inductors with low frequency are usually constructed like transformers. They have cores made up of electrical steel that is laminated to help prevent eddy currents. ‘Soft’ ferrites are also widely used for cores above audio frequencies.

Inductors do come in many shapes and types. In some inductors, you will find an adjustable core that allows changing the inductance. Inductors that are used in blocking very high frequencies are mostly made by stringing a ferrite bead on a wire.

Planar inductors are made using a planar core while small value inductors are built on integrated circuits using the processes of making interconnects. Typically, an aluminum interconnect is used and fixed in a spiral coil pattern. However, small dimensions have some limitations. They restrict the inductance.

There are also shielded inductors which are commonly used in power regulation systems, lighting, and other systems requiring low-noise operating conditions. These inductors are often partially or fully shielded.

Surface mount (SM) inductors are placed on the top of a printed circuit board (PCB) on pads with solder paste, and then reflow soldered. Through-hole (TH) inductors are mounted to the top of a PCB with the leads fed through via holes in the circuit board, and then wave soldered on the backside.

 

Different Types of Inductors

 

Depending on the type of material used inductors can be classified as follows:

 

• Iron Core Inductor: As the name suggests the core of this type of inductor is made of iron. These inductors are low space inductors that have high power and high inductance value. However, they are limited in high-frequency capacity. These inductors are used in audio equipment.
• Air Core Inductor: These inductors are used when the amount of inductance required is low. Since there is no core, it does not have a core loss. But the number of turns the inductor must have is more for this type when compared to the inductors with the core. This results in a high-Quality factor. Usually, ceramic inductors are often referred to as air-core inductors.
• Iron Powder Inductor: In this type of inductor, the core is Iron Oxide. They are formed by very fine and insulating particles of pure iron powder. High magnetic flux can be stored in it due to the air gap. The permeability of the core of this type of inductor is very less. They are usually below 100. They are mainly used in switching power supplies.
• Ferrite Core Inductor which is divided into Soft Ferrite and Hard Ferrite. In this type of Inductor, ferrite materials are used as core. The general composition of ferrites is XFe2O4. Where X represents transition material. Ferrites can be classified into two types. Soft ferrites and hard ferrites. (1) Soft Ferrite: Materials that have the ability to reverse their polarity without any external energy. (2) Hard Ferrite: These are permanent magnets. That is their polarity will not change even when the magnetic field is removed.

 

What is a Choke?

 

An inductor placed in series (in line) with a conductor, such as a wire or circuit board trace, blocks or impedes changes in current and functions as a low pass filter. Because inductors restrict or choke changes in current, they are also called “chokes”. For example, a broadband (wideband) bias choke in line with the DC bias of an amplifier blocks a wide range of high frequencies while allowing pass-through of the dc current. In this way, a bias choke isolates the DC bias from the RF signal to the amplifier.

The Federal Communications Commission (FCC) has created standards and certifies electronic devices sold or manufactured in the United States for meeting electromagnetic interference (EMI) requirements. Worldwide electromagnetic compatibility (EMC) standards organizations include CISPR, IEC, ISO, and EN. FCC regulations are mandatory and apply to devices such as computer, switched-mode power supplies, television receivers, transmitters, and industrial, scientific, and medical (ISM) devices that emit RF radiation. Inductors are employed in electrical circuits to reduce EMI by attenuating high-frequency noise in order to meet EMC emission and immunity requirements.

 

Functions of an Inductor

 

Inductors can be used for two primary functions.

 

• Controlling Signals: Coils in an inductor can be used to store energy. The function of the inductor depends upon the frequency of the current passing through it. That is for higher frequency signals will be passed less easily and vice versa. This function tells that it blocks AC Current and passes DC Current. Hence, it can be used to block AC signals. Inductors can be used along with capacitors to form LC filters.
• Storing Energy: Inductor stores energy in the form of magnetic energy. Coils can store electrical energy in a form of magnetic energy using the property that an electric current flowing through a coil produces a magnetic field, which in turn produces an electric current. In other words, coils offer a means of storing energy on the basis of inductivity.