Inductor Basics: Structure, Parameters, and Measurement

Catalog

I Inductor Structure

An inductor is generally composed of a skeleton, a winding, a magnetic core, an iron core, a shielding case, and a package.

1. Skeleton

Skeleton usually refers to a bracket for coiling. Most of the enameled wires of large fixed inductors or adjustable inductors(such as oscillating coils, choke coils, etc.) are wrapped on the skeleton, and then the magnetic core, copper core, or iron core, etc. is installed into the inner cavity of the skeleton to increase the inductors.

Generally, the skeleton is made of plastic, bakelite and ceramic, and can be made into different shapes according to actual needs. Small inductors (such as color code inductors) do not have a skeleton, and the enameled wires are directly wrapped on the magnetic core. For air-core inductors, there is no magnetic core, skeleton, and shielding case. The wires are first wound around a mold, and then the mold is removed, and there will be a certain distance between the coils.

2. Winding

Winding refers to a group of coils with specified functions, which is the basic component of the inductor. The winding is divided into single-layer types and multiple layer types. And the single-layer windings can be subdivided into dense winding and space winding, while the multilayer winding can be further divided into the flat winding, random winding and honeycomb winding.

An Inductor

3. Magnetic Core and Magnetic Rod

The magnetic core and magnetic rod are generally made of Ni-Zn ferrite and Mn-Zn ferrite materials, which are usually in the shape of a pillar, cap or can.

4. Iron core

The material of the iron core mainly includes silicon steel sheet, permalloy, etc., and its shape is mostly "E" type.

5. Shielding Case

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) is added. The use of the shielding case will increase the loss of the coil and reduce the Q value.

6. Package

After the inductors are wound, the coils and magnetic cores are packaged with plastic or epoxy resin.

II Common Types of Inductors

1. Adjustable inductor

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

(1) Oscillating Coil for Semiconductor Radios

In a semiconductor radio, the oscillating coil is connected with a variable capacitor to generate a local oscillation signal higher than 465 kHz for the input radio signal received by the tuning circuit. The outer part is a metal shield, and the inner part is composed of a nylon lining, an H-shaped magnetic core, a magnetic cap, and a pin outlet. There are windings with high-strength enameled wire on the H-shaped core. The magnetic cap is mounted on the nylon frame inside the shield, which can be rotated up and down. By changing the distance between the cap and the coil, we can also change the inductance.

(2) Horizontal Oscillating Coils for TV

Horizontal oscillating coils were used in early black and white TV sets, which can form self-excited oscillation circuits (three-point oscillator, intermittent oscillator, or multivibrator) with peripheral resistor-capacitor units and horizontal oscillation transistors, to generate a rectangular pulse voltage signal of 15625HZ.

There is a square hole in the center of the magnetic core, and the line synchronization adjustment knob is directly inserted into it. If rotating the adjustment knob, we can change the relative distance between the core and the coil, thereby changing the inductance of the coil and keeping the line oscillation frequency at 15625HZ. In this way, this oscillation frequency with the line synchronization pulse sent by the automatic frequency control circuit (AFC) will generate asynchronous oscillation.

Block Diagram of Receiver Showing Automatic Frequency Control

(3) Horizontal Linear Coil

The horizontal linear coil is a nonlinear magnetic saturation inductance coil, whose inductance decreases with the increase of current. It is generally connected in series in the line deflection coil circuit to compensate for linear distortion of the image with its magnetic saturation characteristics.

The horizontal linear coil is wound with enameled wires on an H-shape high-frequency ferrite core or ferrite bar, and an adjustable permanent magnet is installed beside the coil. By adjusting the relative position of the permanent magnet and the coil, we can change the size of the coil inductance, so as to achieve linear compensation.

2. Choke Inductors

Choke inductors refer to the inductive coils used to block the AC path in the circuit,

It is divided into a high-frequency choke coil and a low-frequency choke coil.

(1) High-frequency choke coil

A high-frequency choke coil is used to prevent high-frequency alternating currents. It works in high-frequency circuits and is mostly with hollow or ferrite cores. The skeleton is made of ceramic materials or plastics, and the coils are wound with honeycomb segment winding or multilayer flat segment winding.

Choke Inductors

(2) Low-frequency Choke Coil

Low-frequency choke coil is used in the current circuit, audio circuit or field output circuit. Its function is to prevent low-frequency AC from passing.

Generally, the low-frequency choke coil used in the audio circuit is called the audio choke coil, the low-frequency choke coil used in the field output circuit is called the field choke coil, and that used in the current filter circuit is named the smoothing choke coil.

Low-frequency choke coils generally use "E" shaped silicon-steel sheet iron core, permalloy iron core or ferrimagnetic core. In order to prevent magnetic saturation caused by a large DC, an appropriate gap should be left in the core during the installation.

III Characteristics and Functions

1. Characteristics

The property of an inductor is just the opposite of that of the capacitor. It can prevent alternating current from passing and allow direct current to pass through smoothly.

When the DC signal passes through the coil, the resistance is the resistance of the wire itself, and the voltage drop is very small. When the AC signal passes through the coil, the self-induced electromotive force will be generated at both ends of the coil. The direction of self-induced electromotive force is opposite to the direction of applied voltage, hindering the passage of AC. The higher the frequency, the greater the coil impedance.

Inductors often work with capacitors in circuits to form LC filters, LC oscillators, etc. In addition, people also use the characteristics of inductors to manufacture choke coils, transformers, relays and so on.

2. Functions

In the circuit, the inductor mainly plays the role of filtering, oscillation, delay and notching. Besides, it can also filter signal and noise, stabilize current and suppress electromagnetic wave interference.

The most common function of the inductor in the circuit is to form an LC filter circuit together with capacitors. If the DC with many interference signals is passing through the LC filter circuit, then the AC interference signal will be changed into heat energy by the inductor. The signal with a higher frequency is the easiest to be impeded, thereby the interference signal of higher frequency is inhibited.

IV Parameters of Inductors

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

1. Inductance

Inductance is also called the self-inductance coefficient, which is a physical quantity that represents the self-induction ability of the inductor.

The value of the inductance mainly depends on the 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 greater the number of coil turns, the denser the coils, and 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 larger magnetic permeability will have a larger inductance.

The basic unit of inductance is Henry, which is represented by the letter "H". Other commonly used units are millihenry (mH) and microhenry (μH), and the relationship among them is:

2. Allowable Deviation

Allowable deviation refers to the allowable tolerance between the nominal inductance and the actual inductance.

In general, the inductor used in oscillation or filtering circuits requires high precision, so the allowable deviation is ±0.2% - ±0.5%, while the precision of the coils used for coupling and the high-frequency choke is not high, thus the allowable deviation is usually ±10% - 15%.

3. Quality factor

The quality factor, also known as Q value or optimal value, is the main parameter to measure the quality of the inductor.

It is the ratio of the inductance to its equivalent loss resistance when the inductor is operating at the AC voltage of a certain frequency.

The higher the Q value of the inductor, the smaller the loss and the higher the efficiency.

The quality factor of the inductor is related to the DC resistance of the coil wires, the dielectric loss of the coil frame and the loss caused by the iron core and the shielding case.

4. Distributed Capacitance

Distributed capacitance refers to the capacitance between the turns of the coil, between the coil and the magnetic 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. The distributed capacitance can increase the equivalent loss resistance and the quality factor.

And to reduce the distributed capacitance, silk-covered wires or multistrand enameled wires are commonly used, sometimes the honeycomb winding method is also used.

Enameled Wires

5. Rated Current

The rated current is the maximum current value that the inductor can withstand under permitted working conditions.

If the working current exceeds the rated current, the performance parameters of the inductor will change due to the heat, and the inductor will even be burned out due to the overcurrent.

V Calculation Formula

1. General Formula

L 一 inductance;

μ  一 permeability of the magnetic core;

Ae 一 the cross-sectional area of the magnetic core;

N 一 the number of turns of the coil;

I一 the length of the magnetic path of the magnetic core.

2. Empirical Formula

L 一 inductance, in Henry (H)

μ0一 vacuum permeability. μ0=4π*10-7

μs一 relative permeability of the magnetic core. For air core coil, μs=1.

N 一 the number of turns of the coil;

S一 the cross-sectional area of the coil, in square meters

I 一 the length of the coil in meters

k一 a coefficient depends on the ratio of the radius (R) to the length (l) of the coil.

VI Measurement of Inductance

1. Measurement Procedures

Types of inductance measurement instruments: RLC(resistance, inductance and capacitance) meter and inductance measurement meter.

Measurement method: measurement with no-load current (theoretical value) and measurement in the actual circuit (actual value).

Here, we discuss the no-load measurement with RLC meter. The specific measurement procedures are:

(1) Get familiar with the instructions and precautions of the instrument.

(2) Turn on the meter and let it prepare for 15-30 minutes.

(3) Select the L gear, and choose inductance measurement.

(4) Clamp the two clips together and reset to zero.

(5) Clamp the two clips at both ends of the inductor, and read the value and record the inductance.

(6) Repeat steps 4 and 5 to record 5-8 data.

(7) Compare these measured values: if the difference is not large, take the average value as the theoretical value of the inductor; if the difference is too large, repeat step 2 to 6 until we get the theoretical value.

Note: Because the inductance parameters measured by different instruments will be somewhat different, it is necessary to be familiar with the instruments before the measurement. After understanding the specific functions of the meter, we can operate in accordance with the instructions.

An RLC Meter

2. Marking Method on Circuit Diagram

(1) Direct Marking Method

The main parameters, such as the inductance, allowable tolerance and maximum working current of the inductor coil is directly marked with numbers and words on the housing of the inductor coil.  

(2) Marking with Color Code

This method uses the color ring to represent the inductance in the unit of mH. The first and second ring represent the significant digits, the third ring represents the multiplier, and the fourth ring represents tolerance.

Marking with Color Code

3. Judge the Performance

(1) Measure the inductor with a multimeter. Adjust the gear to the place of buzzer diode, put the test lead on the two pins, and read the number.

(2) For chip inductors, the reading should be zero. If the reading is too large or infinite, it means that the inductor is damaged.

For the inductor with many coil turns and short wire diameter, the reading will reach tens to several hundred ohms, but usually, the DC resistance of the coil is only a few ohms.

4. Precautions

(1) The inductance value of the iron core and the winding of the inductance element are easy to change when the temperature rises, it is necessary to keep the temperature of the inductor within the range of specifications.

(2) It is easy to form an electromagnetic field after the current passes through the winding of the inductor. When we place the components, it is necessary to keep the inductors away from each other, or keep the winding groups at right angles to each other, so as to reduce the amount of induction between each other.

(3) There will be gap capacitance between the winding layers of the inductor, especially the winding with multiturn and thin lines, which will cause the high-frequency signal bypass and reduce the actual filtering effect of the inductor.

(4) When we measure the inductance value and Q value with the meter, the test lead should be as close as possible to the component body in order to obtain the correct data.

VII Inductor VS. Magnetic Bead

1. The inductor is the energy storage element, while the magnetic bead is the energy conversion (consumption) device;

2. The inductor is mostly used in power supply filter circuit, and the magnetic bead is mostly used for EMC (electromagnetic compatibility) in the signal circuit;

3. Magnetic beads are mainly used to suppress electromagnetic interference, while inductors are used to suppress conductive interference. Both can be used to deal with EMC and EMI(Electro-Magnetic Interference) problems.

There are two EMI paths: radiation and conduction. Different suppression methods are adopted for different paths. Magnetic beads are used for the former and inductance is used for the latter.

4. Magnetic beads are used to absorb UHF(ultra-high-frequency) signals. Magnetic beads are often added in the power input part of some RF circuits, PLL(Phase Locking Loop), oscillation circuit, including UHF memory circuit (DDR SDRAM, RAMBUS, etc.). Inductors are energy storage elements used in LC oscillation circuits and low and medium frequency(rarely exceeds 50MHZ) filter circuits;

5. The Inductor is generally used for circuit matching and signal quality control in ground connection and power supply connection. Magnetic beads are usually used in the combination place of AGND(analog ground) and DGND(digital ground). They are also used for signal lines.

The size of the bead (or the characteristic curve of the bead) depends on the frequency of the interference wave that the bead needs to absorb.

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