Understanding Inductance Coils: Principles, Types and Selection

inductance coil is a device that uses the principle of electromagnetic induction to work. When a current flows through a wire, a certain electromagnetic field will be generated around the wire, and the wire of this electromagnetic field will induce the wire within the range of this electromagnetic field. The effect on the wire itself that generates the electromagnetic field is called "self-inductance", that is, the changing current generated by the wire itself produces a changing magnetic field, which further affects the current in the wire; the effect on other wires in the electromagnetic field range, Called "mutual inductance".

The electrical characteristics of inductance coils are opposite to capacitors, "blocking high frequencies and passing low frequencies". When high-frequency signals pass through the inductance coil, they will encounter great resistance and it is difficult to pass; while the resistance presented by low-frequency signals passing through it is relatively small, that is, low-frequency signals can pass through it easily. The resistance of the inductor to direct current is almost zero.

Resistance, capacitance, and inductance, they all present a certain resistance to the flow of electrical signals in the circuit. This resistance is called "impedance". The impedance presented by the inductor to the current signal uses the self-inductance of the coil. Inductance coil is sometimes referred to as "inductance" or "coil", represented by the letter "L". When winding an inductance coil, the number of turns of the coil is generally called the "number of turns" of the coil.

The Main Performance Indicators

The performance index of the inductor coil is mainly the size of the inductance. In addition, the wire that winds the inductor coil generally has a certain resistance, usually this resistance is very small and can be ignored. But when the current flowing in some circuits is very large, the small resistance of the coil cannot be ignored, because a large current will consume power on the coil, causing the coil to heat up or even burn out, so sometimes it must be considered. The electric power that the coil can withstand.

Inductance

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 inductance coils (color code inductance), the inductance is generally not specifically marked on the coil, but marked with a specific name.

Inductive resistance

The hindering effect of the inductance coil on the alternating current is called the inductive reactance XL, and the unit is ohm. Its relationship with inductance L and alternating current frequency f is XL=2πfL.

Quality factor

The quality factor Q is a physical quantity that represents the quality of the coil, and Q is the ratio of the inductive reactance XL to its equivalent resistance, namely: Q=XL/R. The higher the Q value of the coil, the smaller the loss of the loop. The Q value of the coil is related to the DC resistance of the wire, the dielectric loss of the frame, the loss caused by the shield or the iron core, and the influence of the high-frequency skin effect. The Q value of the coil is usually tens to hundreds.

Distributed capacitance

For any inductance coil, there are certain capacitances between turns, between layers, between the coil and the reference ground, and between the coil and the magnetic shield. These capacitances are called the distributed capacitance of the inductance coil. If these distributed capacitances are integrated together, it becomes an equivalent capacitance C connected in parallel with the inductance coil. The existence of distributed capacitance reduces the Q value of the coil and the stability becomes worse. Therefore, the smaller the distributed capacitance of the coil, the better.

Classification

There are roughly the following categories of inductance coils commonly used in circuits:

Classified according to the form of inductance: fixed inductance, variable inductance.

According to the nature of the magnetic conductor: air core coil, ferrite coil, iron core coil, copper core coil.

Classified by work nature: antenna coil, oscillating coil, choke coil, trap coil, deflection coil.

Classified by winding structure: single-layer coils, multi-layer coils, honeycomb coils, densely wound coils, indirect wound coils, unborn coils, honeycomb coils, random winding coils.

Commonly used coils

1. Single layer coil

Single-layer coils are wound around a paper tube or bakelite frame with insulated wires. Such as the medium wave antenna coil of a transistor radio.

2. Honeycomb coil

If the plane of the coil being wound is not parallel to the rotating surface, but intersects at a certain angle, this kind of coil is called a honeycomb coil. The number of times the wire is bent back and forth after one rotation is often called the number of bending points. The advantages of the honeycomb winding method are small size, small distributed capacitance, and large inductance. The honeycomb coils are all wound by the honeycomb winding machine, the more the turning 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 the ferrite core in the air-cored coil can increase the inductance and improve the quality factor of the coil.

4. Copper core coil

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

5. Color code inductor

The color code inductor is an inductor with a fixed inductance, and its inductance marking method is the same as a resistance with a color ring.

6. Choke coil (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's scanning circuit. The deflection coil requires: high deflection sensitivity, uniform magnetic field, high Q value, small size, and low price.

effect

Choke effect

The self-induced electromotive force in the inductance coil always counterbalances the current change in the coil. The inductance coil has an obstructive effect on the alternating current, and the size of the obstructive effect is called the inductive reactance xl, and the unit is ohm. Its relationship with inductance l and alternating current frequency f is xl=2πfl, and inductors can be mainly divided into high-frequency choke coils and low-frequency choke coils.

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 back and forth between the inductance and the capacitance, which is the resonance phenomenon of the lc loop. At 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 is the largest (referring to the AC signal of f="f0"). The lc resonance circuit has the function of selecting the frequency and an AC signal of frequency f can be selected.

Detect

(1) When selecting and using inductance coils, we must first consider the inspection and measurement of the coils, and then judge the quality of the coils and whether they are good or bad. In order to accurately detect the inductance and quality factor Q of the inductor coil, a special instrument is 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 judgment of the size of the Q value are performed. The DC resistance of the coil can be measured with the resistance profile of a multimeter first, and then compared with the originally determined resistance or nominal resistance. If the measured resistance is much larger than the original determined resistance or nominal resistance, the pointer will not even move. (The resistance value tends to infinity X) It can be judged that the coil is broken; if the measured resistance value is extremely small, it is judged to be a serious short circuit. It is difficult to compare the two situations. It can be judged that the coil is broken. The coil does not work. If the detection resistance is not much different from the originally determined or nominal resistance value, it can be judged that the coil is good. In this case, we can judge the quality of the coil, that is, the size of the Q value based on 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 greater the Q value; if multi-stranded wire is used, the more strands of the wire, the higher 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 when an ordinary silicon steel sheet is used as an iron core; the smaller the coil distributed capacitance and magnetic flux leakage, the higher its Q value. For example, the Q value of the honeycomb winding method is higher than that of flat winding, and higher than that of random winding; when the coil has no shielding cover and there are no metal components around the installation position, its Q value is higher. On the contrary, the Q value is Lower. The closer the shielding cover or the metal member is to the coil, the more severe its Q value will be reduced; the position of the magnetic core should be properly arranged; the antenna coil and the oscillating coil should be perpendicular to each other, which avoids the influence of mutual coupling.

(2) The coil should be visually inspected before installation

Before use, check whether the structure of the coil is firm, whether the wire turns are loose or loose, whether the lead contact is loose, whether the magnetic core rotates flexibly, whether there is a sliding buckle, etc. After all these aspects are checked and qualified, the installation can be carried out.

(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 coil turns. Therefore, the method of fine-tuning should be considered when selecting. For example, a single-layer coil can be used to remove the number of coils close to the end point, that is, wind 3 to 4 turns on one end of the coil in advance, and move its position to change the inductance during fine-tuning. Practice has proved that this adjustment method can achieve fine-tuning ±2%-±3% inductance. For coils used in shortwave and ultrashortwave circuits, a half circle is often reserved for fine adjustment, and this half circle is removed or turned to change the inductance to achieve fine adjustment. The fine-tuning of the multi-layer segmented coil can be realized 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%. For a coil with a magnetic core, the inductance of the coil can be fine-tuned by adjusting the position of the magnetic core in the coil tube.

(4) When using the coil, pay attention to maintaining the inductance of the original coil

When the coil is in use, do not change the shape of the coil casually. The size and the distance between the coils, otherwise it will affect the original inductance of the coil. Especially the higher the frequency, the fewer the coils. Therefore, the high-frequency coils currently 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 not to cause detuning.

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

The adjustable coil should be installed in an easy-to-adjust position of the machine, so that the inductance of the coil can be adjusted to achieve the best working condition. [1]

principle

Inductance is the ratio of the magnetic flux of the wire to the current that produces this magnetic flux when alternating current is passed through the wire.

When a DC current passes through the inductor, there are only fixed magnetic lines of force around it, which do not change with time; but when an alternating current is passed through the coil, it will show magnetic lines of force around it that change with time. According to Faraday's law of electromagnetic induction-magnetism generates electricity, the changing magnetic field lines will generate an induced electric potential at both ends of the coil. This induced electric potential is equivalent to a "new power source". When a closed loop is formed, this induced potential will produce induced current. It is known from Lenz's law that the total amount of magnetic field lines produced by the induced current is to try to prevent the original magnetic field lines from changing. Since the original magnetic field line changes originate 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. Inductance coils have characteristics similar to inertia in mechanics. They are called "self-induction" in electricity. Usually, when the knife switch is opened or the knife switch is turned on, sparks will occur. This is the phenomenon of self-induction. Caused by a very high induced potential.

In short, when the inductance coil is connected to the AC power supply, the magnetic lines of force inside the coil will always change with the alternating current, causing the coil to continuously produce electromagnetic induction. This kind of electromotive force generated by the change of the coil's own current is called "self-induced 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 and has nothing to do with the applied current.