What is a Fixed Inductor?

Abstract: Fixed inductors are a kind of inductors according to their structure, and fixed inductors are commonly used inductive devices. Inductors can be divided into wire-wound inductors and non-wire-wound inductors (multilayer chip, printed inductors, etc.) according to their structure, and can also be divided into fixed inductors and adjustable inductors. An inductor with a fixed inductance is called a fixed inductor (or a fixed coil), and it can be a coil with a single layer, multiple layers, honeycomb type, or a magnetic core. These coils are winded on the magnetic core using a wire of corresponded diameter and they are packaged with epoxy resin or plastic according to the inductance and the maximum DC operating current. This kind of fixed inductor has the advantages of small size, light weight, stable structure, convenient use, and installation, etc. It is mainly used in circuits such as filtering, oscillation, trapping, and delay. It has two types of packaging, sealed and non-sealed, both of them have vertical and horizontal appearance structures.

Figure 1: Structure diagram of a fixed inductor

1)Vertical sealed fixed inductor: The vertical sealed fixed inductor adopts the same-directional pin. The inductance range of domestic ones is 0.1~2200μH (directly marked on the shell), the rated working current is 0.05~1.6A, and the error range is ±5%~±10%.  The imported ones have a larger current range and a smaller error range. TDK series color code inductors are imported, and their inductance is marked on the surface of the inductor with color dots.

2) Horizontal sealed fixed inductors: Horizontal sealed fixed inductors use axial pins, and domestic ones include LG1, LGA, LGX, and other series. The inductance range of the LG1 series is 0.1~22000μH (directly marked on the shell), the rated working current is 0.05~1.6A, and the error range is ±5%~±10%.

The LGA series adopts an ultra-small structure, and the appearance is similar to 1/2W color ring resistors. Its inductance range is 0.22~100μH (marked on the shell by the color ring) and the rated current is 0.09~0.4A.

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

Catalog:

I Constitution and main functions

A fixed inductor is actually a fixed coil, which can be a single-layer coil, a multi-layer coil, a honeycomb coil, and a coil with a magnetic core. In order to reduce the size of a fixed inductor, a wire of the corresponding diameter is usually selected to wind on the magnetic core according to the inductance and the maximum DC operating current, and then it is put into a plastic case and encapsulated with epoxy resin.

The main function of the fixed inductor is to form a resonant circuit with other electronic parts in the electronic circuit. Utilizing the inductance characteristic of "passing direct current and resisting alternating current" of inductors, it is widely used in electronic circuits. In electronic circuits, the inductance coil acts on the AC current limitation. It can form a high-pass or low-pass filter, phase-shifting circuit, and resonant circuit with resistors or capacitors. The transformer can perform AC coupling, voltage transformation, current conversion, and impedance transformation. Knowing from the inductive reactance XL=2πfL, the larger the inductance L and the higher the frequency f, the greater inductive reactance. The magnitude of the voltage across the inductor is proportional to the inductance L. The inductance coil is an energy storage element. It stores electric energy in the form of magnetism. The amount of stored electric energy can be expressed by the following formula: WL=1/2 Li2.

It can be seen that the greater the coil inductance, the more electric energy stored. The most common role of inductance in a circuit is to form an LC filter circuit together with a capacitor. If the direct current accompanied by many interference signals is passed through the LC filter circuit, the AC interference signal will be consumed by the capacitor as heat. When the pure DC current passes through the inductor, the AC interference signal in it will also become magnetic induction and heat energy, and the higher frequency is most easily impedance by the inductance, which can suppress the higher frequency interference signal.

The main functions are as follows:

Filtering: Filtering out specific band frequencies in the signal to suppress and prevent interference. This operation can make sure that signal waveforms are smooth and regular and reduce signal noise.

Oscillation: The oscillating current is a combination of an inductor and a capacitor to form a current electronic circuit whose size and direction change with the period. The circuit that can generate an oscillating current is called an oscillating circuit. The simplest oscillating circuit is called the LC loop.

Delay: In order to prevent the parts in the electronic circuit from going wrong due to the fast change of current. Lenz's law is used to add an inductor to the electronic circuit to produce a delay.

Notch: Notch is the periodic voltage disturbance generated when the AC input current is switched from one phase to another phase (commutation) under normal working conditions of power electronic devices. In the electronic circuit, the inductor has a limiting effect on the AC current. It can form a high-pass or low-pass filter with a resistor or capacitor to reduce the impact of periodic voltage disturbances.

II Test method

Use an ordinary multimeter to test the inductor. When it is suspected that the inductor is open or short-circuited on the printed circuit board, the RX1Ω block of the multimeter can be used to test the resistance at both ends of the inductor LX in the state of power failure. Generally, the DC internal resistance of high-frequency inductors is between a few tenths of ohms and several ohms; the internal resistances of low-frequency inductors are between several hundred ohms and several thousand ohms, and the internal resistance of medium-frequency inductors is between several ohms and tens of ohms. When testing, pay attention to the fact that some inductors have few turns of thick wire diameters, and have low DC resistance. Even if the RX1Ω block is used for testing, the resistance may also be zero. This is a normal phenomenon. If the resistance is large or infinite, This indicates that the inductor has been opened. (This method can only judge whether the fixed inductor is good or bad)

Figure 2: Method to test a fixed inductor

III Detection of inductance and replacement of inductor

To detect the inductance of inductors, special electronic instruments (such as Q meters) must be used. For repairers, they can use a multimeter to measure the resistance of the inductor to roughly judge whether it is good or bad. Generally, the resistance value of the inductor coil is relatively small. If the resistance value of the coil is measured to be infinite, it means that the internal or leading end has been opened. Of course, during testing, the coil must be disconnected from the external circuit.

The replacement of inductors should generally meet the requirements of the inductance value, especially for the inductors used in the tuning loop. The inductance value should correspond as accurately as possible and the wire diameter should also be basically the same, otherwise, it will affect the tuning loop parameters and cause the whole machine parameters to be re-adjusted.

IV Development and application

Fixed inductors are widely used in networks, telecommunications, computers, AC power supplies, and peripheral equipment.

In terms of mobile communications, the rapid technological revolution has led to the emergence of personal networks in the world, thus, equipment is reduced in size,  frequency increases, and progress is made in digitization. As a result, the demand for fixed inductors has expanded to the low inductance range, requiring reduction of inductance value error range and reduction of component size.

In Europe, the United States, and Japan, technological innovations are rapidly emerging. Mobile communication devices, such as portable phones, car phones, and cordless phones, have become smaller while gaining the characteristics of high-frequency operation capability and low power consumption. Major functional improvements are expected in the future, including innovations in communication systems.

Fixed inductors have been widely used in communication equipment, especially in antenna filter circuits, voltage-controlled oscillators, and power circuits, it is used as LC filter impedance matching coils, oscillation coils, and chokes. In recent years it used as EMI noise suppression components to facilitate digitization.

V Performance factors and Precautions 

Performance factors:

1. Number of turns: the number of turns of the enameled wire, the more turns, the greater the inductance.

2. Cross-sectional area: the thicker the enameled wire, the greater the inductance.

3. Whether it has a core or not

Precautions:

1. The iron core and winding of inductors are prone to change in inductance due to the effect of temperature rise, so it should be noted that the temperature must be within the use range.

2. The winding of the inductance easily forms an electromagnetic field after the current passes. When placing the components, pay attention to keeping the adjacent inductors away from each other, or the windings are at right angles to each other to reduce the amount of mutual induction.

3. Between the windings of each layer of the inductor, especially the multi-turn thin wire, there will also be gap capacitance, which will cause high-frequency signal bypass and reduce the actual filtering effect between the inductors.

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