What Is an Inductor Coil?

The coil is wound around the insulation tube by a wire, and the wires are insulated from each other, and the insulation tube may be hollow, or it may include an iron core or a magnetic powder core. The inductance of the coil is represented by L, and the unit is Henry (H), Milli-Henry (mH), Micro-Henry (H), 1H = 10 ^ 3mH = 10 ^ 6H.

Chinese name: Inductance coil

Function: Pass low frequency, block high frequency
Main performance index: Inductance, inductance, quality factors, etc.

Introduction to Inductive Coils

Inductive coils are devices that work using the principle of electromagnetic induction. When a current flows through a wire, a certain electromagnetic field is generated around the wire, and the wire of the electromagnetic field itself induces an effect on the wire within the range of the electromagnetic field. The effect on the wire itself that generates the electromagnetic field is called "self-induction", that is, the changing current generated by the wire itself generates a changing magnetic field, and this magnetic field further affects the current in the wire; the effect on other wires in this electromagnetic field range , Called "mutual inductance".

The electrical characteristics of an inductor are the opposite of capacitors, "passing low frequencies and blocking high frequencies." High-frequency signals will encounter great resistance when passing through the inductive coil, and it will be difficult to pass; while resistance to low-frequency signals when passing through it will be relatively small, that is, low-frequency signals can pass through it more easily. The resistance of the inductor to direct current is almost zero.

Resistors, capacitors, and inductors all present a certain resistance to the flow of electrical signals in the circuit. This resistance is called "impedance". The impedance of the inductive coil to the current signal uses the self-inductance of the coil. Inductive coils are sometimes referred to as "inductance" or "coil" for short, and are represented by the letter "L". When winding an inductance coil, we generally call it the number of turns of the coil.

Main performance index of inductor coil

The performance index of the inductance coil is mainly the size of the inductance. In addition, the wire wound around the inductance coil generally has a certain resistance. Usually this resistance is very small and can be ignored. However, when the current flowing in some circuits is very large, this small resistance of the coil cannot be ignored, because a large current will consume power on this coil, causing the coil to heat up or even burn out, so sometimes it is necessary to consider The electrical power that the coil can withstand.

Inductance of inductance coil

The inductance L represents the inherent characteristics of the coil itself and has nothing to do with the magnitude of the current. Except for special inductor coils (color-coded inductors), the inductance is generally not specifically marked on the coil, but is labeled with a specific name. The inductance is also called the self-inductance coefficient, which is a physical quantity that indicates the self-inductance ability of the inductor. The amount of inductance of the inductor mainly depends on the number of turns (turns) of the coil, the winding method, the presence or absence of the core and the material of the core, and so on. Generally, the larger the number of 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 core; a coil with a larger magnetic permeability has a larger inductance.

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

1H = 1000mH

1mH = 1000H

Inductive coil inductance

The magnitude of the resistance of the inductor to the AC current is called the inductive reactance XL, and the unit is ohm, symbol . Its relationship with the inductance L and the AC frequency f is XL = 2fL

Inductive coil quality factor

The quality factor Q is a physical quantity representing the quality of the coil, Q is the ratio of the inductive reactance XL to its equivalent resistance, that is: Q = XL / R. It refers to the ratio of the inductive reactance and the equivalent loss resistance of the inductor when it is operated under an AC voltage of a certain frequency. The higher the Q of an inductor, the lower its loss and the higher its efficiency. The Q value of the coil is related to the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shielding cover or the iron core, and the influence of high-frequency skin effects. The Q value of the coil is usually tens to hundreds. The quality factor of an inductor is related to the DC resistance of the coil wire, the dielectric loss of the coil bobbin, and the loss caused by the core, shield, etc.

Inductive coil distributed capacitance

There are certain capacitances in any inductive coil between turns and turns, between layers and between layers, between the coil and reference ground, between the coil and the magnetic shield, and these capacitances are called the distributed capacitance of the inductance coil. If these distributed capacitances are integrated together, they become an equivalent capacitance C in parallel with the inductor coil. The presence of distributed capacitance reduces the Q value of the coil and worsens the stability, so the smaller the distributed capacitance of the coil, the better.

Rated current of inductor

Rated current refers to the maximum current value that the inductor is allowed to pass when it is working normally. If the working current exceeds the rated current, the inductor will change its performance parameters due to heat generation, and even burn out due to overcurrent.

Tolerance of inductance coil

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

Generally, inductors used in circuits such as oscillation or filtering require high accuracy, and the allowable deviation is ± 0.2 [%] ~ ± 0.5 [%]; while the accuracy requirements for coils such as coupling and high-frequency choke are not high; The deviation is ± 10 [%] ~ 15 [%].

Classification of inductors

The classification of inductors commonly used in circuits is roughly as follows:

Classification by inductance: fixed inductance, variable inductance.

Classified by the nature of the magnetizer: air-core coil, ferrite coil, iron-core coil, copper-core coil.

Classified by working nature: antenna coil, oscillating coil, choke coil, notch coil, deflection coil.

Classification according to the winding structure: single-layer coils, multilayer coils, hive coils, close-wound coils, space-wound coils, birth-type coils, hive-type coils, random-wound coils

Commonly used coils

1.Single layer coil

The single-layer coil is wound around the paper tube or bakelite skeleton with insulated wires one by one. Such as transistor radio medium wave antenna coil.

2.Hive coil

If the coil is wound, its plane is not parallel to the rotating surface, but intersects at a certain angle. This coil is called a hive coil. The number of times the wire is bent back and forth after one rotation is often called the number of folds. The advantages of the honeycomb winding method are small volume, small distributed capacitance, and large inductance. The hive coils are all wound with a hive winding machine. The more fold points, the smaller the distributed capacitance.

3.Ferrite core and iron powder core coil

The inductance of the coil is related to the presence or absence of a magnetic core. Inserting a ferrite core into an air-core coil can increase the inductance and improve the quality factor of the coil.

4.copper coil

Copper core coils are widely used in the ultra-short wave range. The position of the copper core in the coil is used to change the inductance. This adjustment is convenient and durable.

5.Color code inductor

Color-coded inductors are inductors with a fixed amount of inductance. The method of marking the inductance is marked with a color ring in the same way as a resistor.

6. choke coil

The coil that restricts the passage of alternating current is called a choke coil, which is divided into a high-frequency choke coil and a low-frequency choke coil.

7. Deflection coil

The deflection coil is the load of the output stage of the TV scanning circuit. The deflection coil requires high deflection sensitivity, uniform magnetic field, high Q value, small size and low price.

Inductive coil action

Choke blocking

The self-inductive electromotive force in an inductor coil always counterbalances the change in current in the coil. Inductive coils have a blocking effect on AC current. The size of the blocking effect is called inductive reactance xl, and the unit is ohm. Its relationship with the inductance l and the alternating current frequency f is xl = 2fl. The inductor can be mainly divided into a high-frequency choke coil and a low-frequency choke coil.

Tuning and frequency selection of inductors

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, the inductive reactance and capacitive reactance of the loop are also equal, so the electromagnetic energy oscillates back and forth in the inductance and capacitance, which is the resonance phenomenon of the lc loop. At the time of resonance, the inductive reactance and capacitive reactance of the circuit are reversed, the inductive reactance of the total loop current is the smallest, and the current amount is the largest (refer to the f = f0 AC signal). The lc resonant circuit has the function of selecting the frequency, An AC signal of a frequency f is selected.

Inductance detection

(1) When selecting and using an induction coil, first of all, think of the coil's inspection and measurement, and then judge the quality of the coil and its quality. To accurately detect the inductance and quality factor Q of an inductance coil, special instruments are generally required, and the test method is more complicated. In actual work, this kind of detection is generally not performed, and only the on-off check of the coil and the determination of the Q value are performed. ^[1] The DC resistance of the coil can be measured by using a multimeter resistance file, and then compared with the originally determined or nominal resistance. If the measured resistance is much larger than the originally determined or nominal resistance, even If the pointer does not move (the resistance value goes to infinity X), it can be judged that the coil is disconnected; if the measured resistance value is extremely small, it is judged that it is a severe short circuit or a local short circuit, which is difficult to compare. In both cases, it can be judged that the coil is broken and cannot be used. If the detection resistance is not significantly different from the originally determined or nominal resistance value, it can be judged that this coil is good. In this case, we can judge the quality of the coil, that is, the value of the Q value, according to the following situations. When the inductance of the coil is the same, the smaller the DC resistance, the higher the Q value; the larger the diameter of the wire used, the larger the Q value; if the multi-strand winding is used, the more the number of wires, the more the Q value High; the smaller the loss of the material used in the coil bobbin (or iron core), the higher its Q value. For example, when a high-silicon silicon steel sheet is used as an iron core, its Q value is higher than that when an ordinary silicon steel sheet is used as an iron core; the smaller the coil distributed capacitance and magnetic leakage, the higher its Q value. For example, the coil of the honeycomb winding method has a higher Q value than that of the flat winding, and a higher Q value than the random winding; the coil has no shielding cover, and there is no metal member around the installation position. Lower. The closer the shield or metal component is to the coil, the more serious its Q value decreases; the position with the magnetic core should be properly arranged and reasonable; the antenna coil and the oscillating coil should be perpendicular to each other, which avoids the influence of mutual coupling.

(2) Visual inspection of the coil before installation

Before use, check whether the structure of the coil is firm, whether the turns are loose and loose, whether the lead contacts are loose, whether the magnetic core rotates flexibly, and whether there are sliding buckles. After all these aspects have been checked, the installation can be performed.

(3) If the coil needs to be fine-tuned during use, the fine-tuning method should be considered

Some coils need to be fine-tuned during use, and it is inconvenient to change the number of coils. Therefore, the fine-tuning method should be considered when selecting. For example, the single-layer coil can be moved by removing the number of sleepy coils at the end points, that is, winding three to four turns on one end of the coil in advance. When fine-tuning, moving its position can change the inductance. Practice has proved that this adjustment method can achieve a fine adjustment of ± 2%-± 3% of the inductance. For coils used in short-wave and ultra-short-wave circuits, half a turn is often reserved for fine-tuning. Removing or turning this half-turn changes the inductance and achieves fine-tuning. The multi-layer segmented coil can be fine-tuned by moving the relative distance of a segment. The number of turns of the movable segment should be 20% -30% of the total number of turns. Practice has proved that this fine-tuning range can reach 10% -15%. A coil with a magnetic core can fine-tune the coil inductance by adjusting the position of the magnetic core in the coil tube.

(4) When using the coil, care should be taken to maintain the inductance of the original coil

When the coil is in use, do not change the shape of the coil at will. Size and the distance between the coils, otherwise the original inductance of the coil will be affected. Especially the higher the frequency, the less the number of turns. Therefore, high-frequency coils used in televisions are generally sealed and fixed with high-frequency wax or other dielectric materials. In addition, it should be noted that during maintenance, do not change or adjust the position of the original coil at will, so as to avoid detuning failure.

(5) The installation of adjustable coils should be easy to adjust

The adjustable coil should be installed in an easily adjustable position of the machine in order to adjust the inductance of the coil to achieve the best working condition. ^[2]

Inductive coil principle

Inductance is the ratio of the magnetic flux of a wire to the current that produces it when an alternating current flows through it.

When a DC current is passed through an inductor, only a fixed magnetic field line appears around it, and it does not change with time. However, when an AC current is passed through a coil, a magnetic field line around it appears over time. According to Faraday's law of electromagnetic induction, magnetoelectricity, the changing magnetic field lines will generate an induced potential at both ends of the coil. This induced potential is equivalent to a "new power source". When a closed loop is formed, this induced potential generates an induced current. According to Lenz's law, the total amount of magnetic field lines generated by the induced current should try to prevent the original magnetic field lines from changing. Because the original change of magnetic field lines comes from the change of the external alternating power supply, from the objective effect, the inductance coil has the characteristic of preventing the current change in the AC circuit. Inductive coils have characteristics similar to inertia in mechanics. They are called "self-inductive" in electricity. Usually, the moment the switch is opened or the switch is turned on, a spark will occur. Caused by high induced potential.

In short, when the inductive coil is connected to the AC power source, the magnetic lines of force inside the coil will change at all times with the change of the current, causing the coil to constantly generate electromagnetic induction. The electromotive force generated by the change in the coil's current is called "self-inductive electromotive force".

It can be seen that the inductance is only a parameter related to the number of turns, size and shape of the coil and the medium. It is a measure of the inertia of the inductor coil and has nothing to do with the applied current. ^[2]

Inductive Coil Dispensing

Inductive coil dispensing is mainly an automated dispensing operation performed for the purpose of bonding the gap between the inductor coil and the bottom plate to fix the inductor coil. Dispensing on both sides requires a four-axis dispensing operation based on the original three-axis dispensing manipulator to ensure the same amount of glue and the effect of the dispensing on both sides.