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What is an Inductor Used for? Function Overview

Author: Apogeeweb
Date: 5 Jul 2021
current through an inductor


Inductor is a passive component that used extensively with capacitors and resistors to create filters for analog circuits and in signal processing. Also it is an energy storage device in many switched-mode power supplies. As a major value of inductor, inductance is the ratio of wire current and the magnetic flux which is created by the flow of electrical current in the magnetic field. When a DC current passes through the inductor, there are only fixed magnetic lines around it, which do not change with time. However, when an alternating current is passed through the coil, the magnetic lines around inductor that will change with time.

Inductors and Inductance Definition



Ⅰ Inductor Working Principle

Ⅱ What Does Inductor Do?

Ⅲ Inductor Main Parameters

Ⅳ Inductor Classification

Ⅴ Chip Inductor

5.1 Purpose of a Chip Inductor

5.2 Chip Inductor Classification

5.3 Three Methods for Reading Chip Inductors

Ⅰ Inductor Working Principle

When ac current is applied to an inductor coil, its own current changes, making its own magnetic flux to change, and then causing induced electromotive force. This phenomenon is called self-inductance. The direction of self-induced current always be affected. When the alternating current increases, the direction of self-inductance current is opposite to that of AC current. When the AC current is weaken, the direction of self-inductance current is the same as that of alternating current, which has blocking effect.
1) Self-induction
When current flows through the coil, a magnetic field is generated around the coil. When the current in the coil changes, the surrounding magnetic field also changes accordingly. This changing magnetic field can cause the coil itself to generate induced electromotive force (EMF is used to represent the terminal voltage of the ideal power supply for active components).
2) Mutual Inductance
When two inductor coils are close to each other, the change of the magnetic field of one coil will affect the other one, and this effect is mutual inductance. The magnitude of the it depends on the degree of coupling between the self-inductance and the two coils. The components made by this principle are called mutual inductors.

inductor coil

Ⅱ What Does Inductor Do?

The inductor mainly plays the role of filtering, oscillating, delaying, tuning and frequency selection in the circuit, as well as filtering signal, filtering noise, stabilizing current and suppressing electromagnetic wave interference. The most common role of inductance in a circuit is to form an LC filter circuit together with a capacitor. Capacitors have the characteristic of "block DC and pass AC", while inductors have the function of "pass DC and block AC". It can be made into low-frequency and high-frequency choke coils by making use of its properties. Common filter inductors are for this purpose.
➡️Pass DC: It means that in a direct current circuit, the inductor acts as a wire and has no effect.
➡️Block AC: In an AC circuit, the inductor will have impedance, that is, XL. The current in the entire circuit will become smaller, which has a certain blocking effect on AC. The self-induced electromotive force is always opposed to the current change in the coil. Mainly can be divided into high-frequency choke coil and low-frequency choke coil.

➡️Inductor has the function of generating self-induced EMF, which is also called energy storage function. Like a capacitor, it is also an important energy storage component, which is widely used in switching power supplies. In addition, the phase relationship between the voltage across the inductor and the current: the voltage leads the current by 90 degrees, which is just the opposite of the capacitor. Using this characteristic, inductors and capacitors form LC series and parallel circuits, which can be used for frequency selection. In reality, they have been widely used in circuits, especially in radio circuits.
If the direct current accompanied by many interference signals is passed through the LC filter circuit, then the AC interference signal will be consumed by the inductance into heat. When the pure DC current passes through the inductor, the AC interference signal in it will also become magnetic induction and heat energy, the higher frequency is most likely to be blocked by the inductor, which used to suppress the higher frequency interference signal.

➡️Tuning and Frequency Selection
The inductance coil and the capacitor are connected in parallel to form an LC tuning circuit. That is, the natural oscillation frequency f0 of the circuit is equal to the frequency f of the non-AC signal, and the inductance and capacitive reactance of the loop are also equal, so the electromagnetic energy oscillates between the inductor and the capacitor. This is the resonance phenomenon of the LC loop. During resonance, since the inductance and capacitive reactance of the circuit are equal and opposite, the inductance of the total current of the loop is the smallest and the current is the largest (referring to the AC signal of f=f0), so the LC resonance circuit has the function of selecting the frequency, therefore an AC signal of a certain frequency is selected.

It is used to block low-frequency alternating current, and pulsating direct current flows to pure direct-current in circuits. Go further, it is commonly used in the middle of two filter capacitors at the output of a rectifier circuit. The choke and capacitor form a filter circuit. In the high-frequency circuit: to prevent the high-frequency current from flowing to the low-frequency end, which is commonly as the high-frequency choke of old regenerative radio.

It also prevents the rectified pulsating DC current from flowing to the pure DC circuit. The choke (to simplify the circuit and reduce the cost, replace the choke with a pure resistance) and two capacitors (electrolytic capacitors) form a filter circuit. The use of capacitor charging and discharging and AC choke coil to block the alternating current to smooth direct current and obtain the pure direct current.

We often say that rectification is to transform AC into DC, so oscillation is the reverse process of it. We call the circuit that completes this process an "oscillator." Oscillator waveform: there are sine wave, sawtooth wave, trapezoidal wave, square wave, rectangular wave, spike wave. The frequency ranges from a few Hz to tens of GHz. It is widely used in the field of wired power and radio.

what is inductor?

Ⅲ Inductor Main Parameters

The main parameters of inductor include inductance, allowable deviation, quality factor, distributed capacitance and rated current.
1) Inductance
Inductance is also called self-inductance, which is a physical quantity that represents the self-inductance of an inductor. The size of the inductance mainly depends on the number of turns  of the coil, the winding method, the core and its material, etc. Generally, the more coil turns 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. What’s more, a coil with a larger magnetic core has a larger inductance.
The basic unit of inductance is Henry, represented by the letter "H". Commonly used units are millihenry (mH) and microhenry (μH). The relationship between them is:
2) Allowable Deviation
The allowable deviation refers to the allowable error value between the nominal inductance and the actual inductance.
Generally, inductors used in circuits such as oscillation or filtering require high accuracy, with an allowable deviation of ±0.2%~±0.5%; while the accuracy requirements of coils used for coupling and high-frequency blocking are not high; the allowable deviation is ±10 %~15%.
3) Quality Factor
Quality factor, also called Q value, is the main parameter to measure the quality of an inductor. It refers to the ratio of the inductance presented by the inductor to its equivalent loss resistance when it works under a certain frequency of AC voltage. The higher the Q value of an inductor, the smaller its loss and the higher its efficiency.
The Q factor is related to the DC resistance of the coil wire, the dielectric loss of the coil frame, and the loss caused by the core and shield.
4) Distributed Capacitance
Distributed capacitance refers to the capacitance that exists between the turns of the coil, the coil and the magnetic core, the coil and the ground, and the coil and the metal. The smaller the distributed capacitance of the inductor, the better its stability. Distributed capacitance can make the equivalent energy dissipation resistance larger. To reduce it, silk-covered wire or multi-strand enameled wire is commonly used, and sometimes honeycomb winding method is also used.
5) 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 inductor will change its performance parameters due to heat, and even burn out due to overcurrent.

inductor symbols

Ⅳ Inductor Classification

According to the form of inductor: fixed inductance, variable inductance.
According to the nature of the magnetic conductor: air core coil, ferrite coil, iron core coil, copper core coil.
According to work nature: antenna coil, oscillating coil, choke coil, trap coil, deflection coil.
According to winding structure: single-layer coil, multi-layer coil, honeycomb coil.
According to working frequency: high frequency coil, low frequency coil.
According to structural characteristics: magnetic core coil, variable inductance coil, color code inductor coil, non-magnetic core coil, etc.


Ⅴ Chip Inductor

5.1 Purpose of a Chip Inductor

Chip inductors are electromagnetic induction components wound with insulated wires. It is a commonly used electronic component. The function of the chip inductor: it is simple to say that it can isolate and filter the AC signal or form a resonant circuit with capacitors, resistors, etc. The inductor coil and capacitor in parallel can form an LC tuning circuit. Any current flowing through the chip inductor will generate a magnetic field, and its magnetic flux will act on the circuit.
When the current passing through the chip inductor changes, the DC voltage potential generated in the chip inductor will prevent the current from changing. When the current passing through the inductor coil increases, and the current passing through the inductor coil decreases, the self-induced electromotive force is in the same direction as the current, which prevent the current from decreasing and release the stored energy at the same time. The direction of flow is opposite to prevent the increase of current, and at the same time, part of the electric energy is converted into magnetic field and stored in the inductor. Therefore, with inductor filtering, not only the pulsation of load current and voltage is reduced, the waveform becomes smooth, and the rectifier diode is turned on.
The role of shielded chip inductors is different from that of the general one. The general chip inductors are not shielded in the circuit to achieve the desired effect. The shielded current instability of this kind inductor in some circuits plays a good blocking role. A metal shield surrounds the positively charged conductor, and the inside of the shield will induce the same amount of negative charge as the charged conductor. A positive charge equal to that of a charged conductor appears on the outside. If the metal shield is grounded, the positive charge on the outside will flow into the ground, and there will be no electric field on the outside, that is, the electric field of the positive conductor is shielded.
The shielding inductance also plays a role of coupling in the circuit. In order to reduce the coupling interference voltage of the alternating electric field to the sensitive circuit, the inductance can be set with a metal shield with good conductivity between the interference source and the sensitive circuit, in addition, the metal shield is grounded. The coupling interference voltage to the sensitive circuit depends on the product of the alternating electric field voltage, the coupling capacitance and the ground resistance of the metal shield. As long as the metal shield is well grounded, the coupling interference voltage can be reduced. The electric field shielding is mainly based on reflection, so the thickness of the shielding body does not need to be too large, and the structural strength is the main consideration.

5.2 Chip Inductor Classification

1) Winding Type
It is characterized by a wide range of inductance (mH~H), high inductance accuracy, low loss (that is, large Q), large allowable current, strong manufacturing process inheritance, simplicity, and low cost, and the shortcoming is size. For example, the ceramic core winding type chip inductor can maintain a stable inductance and a fairly high Q value at such a high frequency, so it occupies a place in the high-frequency circuit.
NL series inductors are wire-wound type, 0.01~100uH, accuracy 5%, high Q value, which can meet general needs. 
NLC type is suitable for power circuit, rated current up to 300mA.
NLV type is high Q value, environmentally friendly (reconstituted plastic), and can be interchanged with NL.
NLFC has a magnetic screen and is suitable for power cords.

2) Layer Type
It has good magnetic shielding, high sintering density and good mechanical strength. The disadvantages of it are low pass rate, high cost, small inductance, and low Q value.
Compared with wire wound chip inductors, it has many advantages: 
Small size is helpful to the miniaturization of the circuit.
Closed magnetic circuit will not interfere with surrounding components, and will not be interfered by neighboring components, which is beneficial to high-density installation.
Integrated structure, high reliability; good heat resistance and solderability.
Regular shape is suitable for automatic surface mounting production.
MLK type inductor has the characteristics of small size, good solderability, magnetic screen, high-density design, monolithic structure, and high reliability.
MLG type has a small inductance and uses high-frequency ceramics, which is suitable for high-frequency circuits.
MLK type working frequency is 12GHz, with high Q and low inductance (1n~22nH).

3) Film Type
It has the characteristics of maintaining high Q, high precision, high stability and small size in the microwave frequency band. The internal electrodes are concentrated on the same layer, and the magnetic field distribution is concentrated, which can ensure that the device parameters after mounting do not change much, and show good frequency characteristics above 100MHz.

4) Weaving Type
Its characteristic is that the inductance per unit volume at 1MHz is larger than other chip inductors, small in size, and easy to install on the substrate. It is usually used as a miniature magnetic component for power processing.
In actual applications, the inductor should be selected according to the situation, circuit requirement, and the material cost.

5.3 Three Methods for Reading Chip Inductors

1) Digital Position Identification (generally rectangular chip resistors use this nominal method)
This method is to use three digits on the resistor to indicate its resistance. Its first and second digits are significant digits, and the third digit represents the number of "0"s added after the significant digits, no letters will appear in this place.
For example: "472'" means "4720Ω"; "151" means "1510Ω". 
If it is a decimal, use "R" to mean "decimal point". It occupies one significant digit, and the remaining two are significant digits.
For example: "2R4" means "2.4Ω"; "R15" means "0.15Ω".

2) Resistor Color Code (generally cylindrical fixed resistors use this nominal method)
Chip resistors are the same as general resistors. Most of them use four rings (sometimes three rings) to indicate their resistance. The first ring and the second ring are significant numbers, and the third ring is the magnification. For example: "brown, green and black" means "15Ω"; "blue, gray, orange and silver" means "68kΩ", the error is ±10%.

3) E96 Number Mixes with Letter
This method also uses three digits to indicate the resistance value, that is, "two digits plus one letter". Two digits represent the E96 series resistance. Its third digit is the magnification expressed by the letter code. For example: "51D" means "332×103; 332kΩ"; "249Y" means "249×10-2; 2.49".


Frequently Asked Questions about Inductor Uses

1. What is inductor and its function?
An inductor is arguably the simplest of all electronic components. It's a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. Typically, an inductor will consist of an insulated wire that's wound into a coil, much like a resistor.


2. What is the basic principle of inductor?
An inductor is a passive electronic component which is capable of storing electrical energy in the form of magnetic energy. Basically, it uses a conductor that is wound into a coil, and when electricity flows into the coil from the left to the right, this will generate a magnetic field in the clockwise direction.


3. What is the function of inductor in AC circuit?
Inductors store their energy in the form of a magnetic field that is created when a voltage is applied across the terminals of an inductor. The growth of the current flowing through the inductor is not instant but is determined by the inductors own self-induced or back emf value.


4. Does an inductor block AC?
We know that inductor has inductive reactance property by which it opposes the flow of current through it. The equation of inductive reactance is, ... For this reason, an inductor can totally block the very high-frequency AC.


5. Why AC is blocked by inductor?
Since inductor behaves like a resistor, DC flows through an inductor. The AC flowing through L produces timevarying magnetic field which in turn induces self- induced emf (back emf). ... For an ideal inductor of zero ohmic resistance, the back emf is equal and opposite to the applied emf.

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