What is an inductor?

Ⅰ. Definition and development of inductors

1. Definition

Inductors are components that can convert electrical energy into magnetic energy and store it. Inductors are similar in construction to transformers, but with only one winding. Inductors have a certain inductance, which only resists changes in current. If the inductor is in a state where no current is flowing, it will try to block the current from flowing through it when the circuit is on; if the inductor is in a state where current is passing, it will try to maintain the current when the circuit is off. Inductors are also called chokes, reactors, and dynamic reactors.

2. Development 

The most primitive inductor is the iron core coil used by British M. Faraday in 1831 to discover the phenomenon of electromagnetic induction.

In 1832, J. Henry of the United States published a paper on the phenomenon of self-induction. People call the unit of inductance Henry, or Henry for short.

In the mid-19th century, inductors found practical applications in devices such as telegraphs and telephones. The inductors used by H.R. Hertz in Germany in 1887 and N. Tesla in the United States in 1890 are very famous, and they are called Hertz coils and Tesla coils respectively.

Ⅱ. Main parameters of the inductors

Allowable deviation, rated current, inductance, distributed capacitance and quality factor are the main parameters of the inductor.

1. Allowable deviation

The allowable deviation refers to the allowable error value between the nominal inductance on the inductor and the actual inductance.

It is generally used for inductors in circuits such as oscillation or filtering, which require high precision, and the allowable deviation is ±0.2%~±0.5%; while the precision requirements for coils such as coupling and high-frequency blocking current are not high; the allowable deviation is ±0.2% 10%~15%.

2. Rated current

The rated current refers to the maximum current value that the inductor can withstand under the allowable working environment. If the operating current exceeds the rated current, the performance parameters of the inductor will change due to heating, and even burn out due to overcurrent.

3. Inductance

Inductance, also known as self-inductance coefficient, is a physical quantity that expresses the self-induction ability of inductors.

The inductance of an inductor depends mainly on the number of turns (number of turns) of the coil, the winding method, whether there is a magnetic core and the material of the magnetic core, etc. Generally, the more coils and the denser the coils, the greater the inductance. A coil with a magnetic core has a larger inductance than a coil without a magnetic core; a coil with a greater magnetic core permeability has a greater inductance.

The basic unit of inductance is Henry (Henry for short), represented by the letter "H". Commonly used units are millihenry (mH) and microhenry (μH), and the relationship between them is:

1H=1000mH

1mH=1000μH

4. Distributed capacitance

Distributed capacitance refers to the capacitance between the turns of the coil, between the coil and the core, between the coil and the ground, and between the coil and the metal. The smaller the distributed capacitance of the inductor, the better its stability. Distributed capacitance can increase the equivalent energy dissipation resistance and the quality factor. To reduce distributed capacitance, silk-covered wire or multi-strand enameled wire is commonly used, and sometimes honeycomb winding method is also used.

5. Quality factor

Quality factor, also known as Q value or figure of merit, is the main parameter to measure the quality of inductors.

It refers to the ratio of the inductive reactance presented by an inductor to its equivalent loss resistance when it operates at a certain frequency of AC voltage. The higher the Q value of an inductor, the smaller its losses and the higher its efficiency.

The quality factor of the inductor is related to the DC resistance of the coil wire, the dielectric loss of the coil bobbin, and the loss caused by the iron core and shielding cover.

Ⅲ. Characteristics of inductors

The characteristics of an inductor are just the opposite of those of a capacitor. It has the property of blocking alternating current and allowing direct current to pass smoothly. The resistance when the DC signal passes through the coil is the resistance drop of the wire itself is very small; when the AC signal passes through the coil, a self-induced electromotive force will be generated at both ends of the coil, and the direction of the self-induced electromotive force is opposite to the direction of the applied voltage, hindering the passage of AC , so the characteristic of the inductor is to pass DC and block AC. The higher the frequency, the greater the coil impedance. Inductors often work together with capacitors in circuits to form LC filters, LC oscillators, etc. In addition, people also use the characteristics of inductance to manufacture choke coils, transformers, relays, etc.

Direct current: 

It means that the inductor is in a closed state to direct current. If the resistance of the inductor coil is not considered, then the direct current can pass through the inductor "unimpeded". For direct current, the resistance of the coil itself has little resistance to direct current, so often neglected in circuit analysis.

Resistance to alternating current: 

When the alternating current passes through the inductive coil, the inductor has a hindering effect on the alternating current, and what hinders the alternating current is the inductive reactance of the inductive coil.

Ⅳ. The structure of the inductors

Inductors are generally composed of iron or magnetic cores, skeletons, shielding cover, winding, and packaging materials.

1. Iron core

The iron core materials mainly include silicon steel sheet, permalloy, etc., and its shape is mostly "E".

2. Magnetic core and magnetic rod

Magnetic cores and rods are generally made of nickel-zinc ferrite (NX series) or manganese-zinc ferrite (MX series), which are available in a variety of shapes, such as "I", column, cap, "E", and can.

3. Skeleton

Skeleton generally refers to the frame on which the coil is wound. Some larger fixed inductors or adjustable inductors (such as oscillating coils, choke coils, etc.), most of them wrap enameled wires (or yarn-covered wires) around the skeleton, and then wrap the magnetic core or copper core, iron the core, etc. are loaded into the inner cavity of the skeleton to increase its inductance.

The skeleton is usually made of plastic, bakelite, and ceramics, and can be made into different shapes according to actual needs. Small inductors (such as color-coded inductors) generally do not use a bobbin, but directly wind the enameled wire on the magnetic core.

4. Shielding cover

In order to prevent the magnetic field generated by some inductors from affecting the normal operation of other circuits and components, a metal screen cover (such as the oscillating coil of a semiconductor radio, etc.) is added to it. Using shielded inductors will increase the loss of the coil and reduce the Q value.

5. Winding

Winding refers to a group of coils with specified functions, which is the basic component of inductors. There are single-layer and multi-layer windings. There are two forms of single-layer winding: close winding (the wires are next to each other when winding) and inter-winding (there is a certain distance between each circle of wires when winding); multilayer windings have layered flat winding and random winding. There are many kinds of winding, honeycomb winding and so on.

6. Packaging material

After some inductors (such as color code inductors, color ring inductors, etc.) are wound, the coils and cores are sealed with packaging materials. The packaging material is plastic or epoxy resin.

Ⅴ. Functions and applications of inductors

1. Functions

Inductors have two main functions: control signals and store energy.

(1) Control signal

The coils in an inductor can be used to store energy. The function of an inductor depends on the frequency of the current passing through it. That is, higher frequency signals will pass less easily, and vice versa. The function indicates that it blocks AC current and passes DC current. Therefore, it can be used to block AC signals.

Inductors can be used with capacitors to form LC filters.

(2) Store energy

Inductors store energy in the form of magnetic energy. The coil can store electrical energy in the form of magnetic energy, and the current flowing through the coil will generate a magnetic field, thereby generating the characteristics of the current. In other words, coils provide a method of storing energy based on inductance.

2. Applications

(1) This inductor can be used in RF circuits such as vintage speakers from the 1980s.

(2) For DC power supply, smooth voltage.

(3) Used in audio frequency division circuit to separate low frequency and high frequency sound.

(4) Induction motors for variable speed applications.

(5) When connected in series with a load resistor, it blocks AC and allows DC to pass. This acts as a line filter.

(6) Compensation for capacitive reactance. It improves power factor.

Ⅵ. Common types of inductor

Inductors can be made of a coiled core of electrically conductive material, typically copper wire, or the core can be removed or replaced with a ferromagnetic material. A core material with a higher permeability than air can more tightly confine the magnetic field around the inductive element, thus increasing the inductance. There are many kinds of inductors, most of which are made of outer enamel coated wire wrapped around a ferrite bobbin, while some shielded inductors have the coil completely inside the ferrite. The core of some inductive components can be adjusted. In this way, the inductance can be changed. Small inductors can be etched directly on the PCB, using a method of laying out spiral traces. Small value inductors can also be used to manufacture transistors in integrated circuits using the same process. In these applications, aluminum interconnects are often used as the conductive material. Regardless of the method used, the most widely used, based on practical constraints, is a circuit called a "rotator", which uses a capacitive and active element that exhibits the same characteristics as an inductive element. Inductive elements for high frequency isolation are often constructed with a wire threaded through a post or bead.

1. Blocking inductor

Blocking inductor refers to the inductance coil used to block the AC current path in the circuit, which is divided into high-frequency blocking coil and low-frequency blocking coil.

① High-frequency choke coil: High-frequency choke coil is also called high-frequency choke coil, which is used to prevent high-frequency AC current from passing through.

High-frequency choke coils work in high-frequency circuits, mostly using hollow or ferrite high-frequency cores, the skeleton is made of ceramic materials or plastics, and the coils are wound in honeycomb segments or multi-layer flat winding segments.

② Low-frequency choke coil: Low-frequency choke coil is also called low-frequency choke coil. It is used in current circuit, audio circuit or field output circuit, and its function is to prevent low-frequency AC current from passing through.

The low-frequency choke coil generally adopts an "E"-shaped silicon steel core (commonly known as a silicon steel core), a permalloy core or a ferrite core. In order to prevent magnetic saturation caused by a large DC current, an appropriate gap should be left in the iron core during installation.

2. Small inductors

Small fixed inductors are usually wound directly on the magnetic core with enameled wires. They are mainly used in circuits such as filtering, oscillation, notch, and delay. They have two types of packaging: sealed and non-sealed. Both vertical and horizontal configurations are available.

① Vertical sealed fixed inductor

The vertical sealed fixed inductor adopts the same direction pin, the domestic inductance range is 0.1~2200μH (directly marked on the shell), the rated working current is 0.05~1.6A, the error range is ±5%~±10%, and the imported The inductance, the current range is larger, and the error is smaller. There are TDK series color-coded inductors imported, and the inductance is marked on the surface of the inductor with colored dots.

② Horizontal sealed fixed inductor

Horizontal sealed fixed inductors adopt axial pins, domestically produced LG1.LGA, LGX and other series.

The inductance range of LG1 series inductors is 0.1~22000μH (directly marked on the shell)

LGA series inductors adopt ultra-small structure, similar in appearance to 1/2W color ring resistors, the inductance range is 0.22~100μH (marked on the shell with a color ring), and the rated current is 0.09~0.4A.

LGX series color-coded inductors also have a small package structure. The inductance range is 0.1~10000μH, and the rated current is divided into four specifications: 50mA, 150mA, 300mA and 1.6A.

3. Adjustable inductor

Commonly used adjustable inductors include oscillating coils for semiconductor radios, horizontal oscillating coils for televisions, linear coils, intermediate frequency trap coils, frequency compensation coils for audio, and choke coils.

① Oscillating coil for semiconductor radio:

This oscillating coil forms a local oscillating circuit with a variable capacitor in a semiconductor radio, and is used to generate a local oscillating signal that is higher than 465kHz from the radio signal received by the input tuning circuit. The outside is a metal shield, and the inside is composed of a nylon bushing, an I-shaped magnetic core, a magnetic cap and a pin seat, etc. There is a winding made of high-strength enameled wire on the I-shaped magnetic core. The magnetic cap is installed on the nylon frame in the shielding cover, which can be rotated up and down, and the inductance of the coil can be changed by changing the distance between it and the coil. The internal structure of the TV IF trap coil is similar to that of the oscillating coil, except that the magnetic cap can adjust the magnetic core.

② Line oscillating coil for TV:

The horizontal oscillating coil is used in early black and white TV sets. It forms a self-excited oscillating circuit with peripheral RC elements and horizontal oscillating transistors to generate a rectangular pulse voltage signal with a frequency of 15625HZ.

There is a square hole in the center of the magnetic core of the coil, and the row synchronization adjustment knob is directly inserted into the square hole. Turning the row synchronization adjustment knob can change the relative distance between the magnetic core and the coil, thereby changing the inductance of the coil and maintaining the row oscillation frequency. It is 15625HZ, and it generates synchronous oscillation with the line synchronization pulse sent by the automatic frequency control circuit (AFC).

③ Linear coil:

Linear coil is a nonlinear magnetically saturated inductance coil (its inductance decreases with the increase of current), which is generally connected in series in the line deflection coil circuit, and uses its magnetic saturation characteristics to compensate the image linear distortion.

Linear coils are made of enameled wires wound on "I"-shaped ferrite high-frequency cores or ferrite rods, and adjustable permanent magnets are installed next to the coils. By changing the relative position of the permanent magnet and the coil to change the size of the coil inductance, so as to achieve the purpose of linear compensation.

4. Shielded inductors

Magnetically shielded inductors minimize EMI while balancing small size, low DCR, and high current rating. It is ideal for use as power chokes and for eliminating noise in power supplies in automotive, commercial, industrial and high reliability designs.

Shielded inductors are especially useful in the following situations:

(1) Reduce electromagnetic interference: 

Inductors will generate a magnetic field when they are working, and changes in the magnetic field may cause interference to nearby circuits and equipment. By using shielded inductors, this interference can be reduced, increasing the stability and reliability of the circuit。

(2) Prevent mutual coupling: 

On high-density circuit boards, inductors may interact with each other, resulting in mutual coupling effects. Using shielded inductors can reduce this mutual coupling and improve the isolation performance of the circuit.

(3) Suppress external interference: 

Inductors are susceptible to interference from external electromagnetic fields, affecting their performance. Shielded inductors can prevent external electromagnetic fields from affecting them to a certain extent.

Ⅶ. Working principle of inductors

An inductor in a circuit resists changes in the current flowing through it by inducing a voltage across it that is proportional to the rate of change of the current. To understand how an inductor works in a circuit, consider the diagram below.

The lamp, coil (inductor) and switch are connected to the battery as shown. If we remove the inductance from the circuit, the light will work normally. The behavior of the circuit is completely different when using an inductor.

An inductor or coil has much lower resistance compared to a lamp, so when the switch is closed most of the current should start flowing through the coil as it provides a low resistance path for the current to flow. Therefore, we expect the lamp to emit a very dim light. But due to the behavior of the inductor in the circuit, when we turn off the switch, the light glows brightly, then dims, and when we turn on the switch, the bulb glows very brightly, then quickly turns off. The reason is that when a voltage or potential difference is applied across an inductor, the current flowing through the inductor creates a magnetic field. According to Lenz's law, this magnetic field again induces a current of opposite polarity in the inductor. The induced current due to the magnetic field of the inductor tries to prevent any change, increase or decrease in current. Once the magnetic field is established, current can flow normally. Now, when the switch is closed, the magnetic field around the inductor keeps the current flowing in the inductor until the magnetic field disappears. This current keeps the lamp glowing for a certain period of time even when the switch is turned on.

In other words, an inductor stores energy in the form of a magnetic field and tries to resist any change in the current flowing through it. So the overall result is that the current through the inductor cannot change instantaneously.