How to measure the quality of inductance_How to judge the quality of the inductance with a multimeter
First, the definition of inductance
Inductance is the ratio of the magnetic flux of the wire to the current that produces this magnetic flux when an alternating current is passed through the wire, which generates an alternating magnetic flux in and around the wire.
When a DC current is passed through the inductor, there are only fixed magnetic lines of force around it, which do not change with time; however, when an AC current is passed through the coil, there will be magnetic lines of force around it that change with time. According to Faraday's Law of Electromagnetic Induction---Magnetic Electricity, the changing magnetic lines of force will generate an induced potential at both ends of the coil, which is equivalent to a "new power supply". When a closed loop is formed, this induced potential will generate an induced current. It is known from Lenz's law that the total amount of magnetic field lines generated by the induced current should try to prevent the change of the original magnetic field lines. Since the original magnetic field line change 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 alternating current circuit. The inductance coil has similar characteristics to the inertia in mechanics, and is named "self-induction" in electricity. Usually, sparks will occur at the moment when the knife switch is opened or the knife switch is turned on. This is the phenomenon of self-induction. caused by 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 change constantly with the alternating current, causing the coil to continuously generate electromagnetic induction. This electromotive force generated by the change of the current of the coil itself is called "self-induced electromotive force". It can be seen that the inductance is only a parameter related to the number of turns, size, shape and medium of the coil. It is a measure of the inertia of the inductive coil and has nothing to do with the applied current.
2. Inductance characteristics
The characteristics of inductors are the opposite of those of capacitors. They have the characteristics of preventing alternating current from passing through and allowing direct current to pass through smoothly. When the DC signal passes through the coil, the resistance is the resistance voltage drop of the wire itself. When the AC signal passes through the coil, a self-induced electromotive force will be generated at both ends of the coil. The direction of the self-induced electromotive force is opposite to the direction of the applied voltage, which hinders the passage of AC. , so the characteristics of the inductor are to pass DC and block 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 inductance to manufacture choke coils, transformers, relays, etc. Direct current: It means that the inductor is in a closed state to the direct current. If the resistance of the inductance coil is not considered, then the direct current can pass through the inductor "unimpeded". For direct current, the resistance of the coil itself has very little hindering effect on direct current, so Often ignored in circuit analysis.
Blocking alternating current: When alternating current passes through the inductive coil, the inductor hinders the alternating current, and it is the inductive reactance of the inductive coil that hinders the alternating current.
3. Inductance structure
Inductors are generally composed of skeletons, windings, shields, packaging materials, magnetic cores or iron cores.
1. Skeleton The skeleton generally refers to the bracket for winding the coil. Some larger fixed inductors or adjustable inductors (such as oscillating coils, choke coils, etc.), most of which are enameled wire (or yarn-covered wire) around the skeleton, and then the magnetic core or copper core, iron core, etc. Installed into the inner cavity of the skeleton to increase its inductance. The skeleton is usually made of plastic, bakelite, and ceramics, and can be made into different shapes according to actual needs. Small inductors (such as color-coded inductors) generally do not use a bobbin, but instead have the enameled wire wound directly around the core. Air-core inductors (also known as unwrapped coils or air-core coils, mostly used in high-frequency circuits) do not use magnetic cores, skeletons and shields, etc., but are first wound on the mold and then take off the mold, and the coil is pulled between each coil. Drive a certain distance.
2. Winding Winding refers to a group of coils with specified functions, which is the basic component of inductors. There are single-layer and multi-layer windings. There are two types of single-layer windings: dense winding (conductors are wound one turn after another) and intermediate winding (there is a certain distance between each turn of wires during winding); multi-layer windings have layered flat winding, random winding Winding, honeycomb winding, etc.
3. Magnetic cores and magnetic rods Magnetic cores and magnetic rods are generally made of nickel-zinc ferrite (NX series) or manganese-zinc ferrite (MX series) and other materials. Shape, can shape and other shapes.
4. Iron core The iron core material mainly includes silicon steel sheet, permalloy, etc., and its shape is mostly "E" type.
5. Shielding cover 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 oscillation coil of a semiconductor radio, etc.) is added to it. The use of shielded inductors will increase the loss of the coil and reduce the Q value.
6. Packaging materials After some inductors (such as color code inductors, color ring inductors, etc.) are wound, the coils and magnetic cores are sealed with packaging materials. The encapsulation material is plastic or epoxy resin.
Fourth, the main parameters of the inductor
1. Inductance
Inductance, also known as self-inductance coefficient, is a physical quantity that represents the ability of an inductor to generate self-induction. The size of the inductance of the inductor mainly depends on the number of turns (number of turns) of the coil, the winding method, the presence or absence of a magnetic core and the material of the magnetic core, etc. Generally, the more coil turns and the denser the coils are wound, 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 core permeability has a larger inductance.
The basic unit of inductance is Henry (referred to as Henry), which is represented by the letter "H". Commonly used units are millihenry (mH) and microhenry (μH). The relationship between them is:
1H=1000mH
1mH=1000μH
2. Allowable deviation
The allowable deviation refers to the allowable error value between the nominal inductance on the inductor and the actual inductance. Inductors generally used in circuits such as oscillation or filtering require high precision, and the allowable deviation is ±0.2%~±0.5%; while the accuracy requirements for coils such as coupling and high-frequency blocking current are not high; the allowable deviation is ±10 %~15%.
3. Quality factor
The quality factor, also known as the Q value or the figure of merit, is the main parameter to measure the quality of the inductor. It refers to the ratio of the inductive reactance presented by the inductor to its equivalent loss resistance when it operates under an AC voltage of a certain frequency. The higher the Q of the inductor, the lower its losses and the higher the efficiency. The quality factor of the inductor is related to the DC resistance of the coil wire, the dielectric loss of the coil skeleton, and the loss caused by the iron core and shield.
4. Distributed capacitance
Distributed capacitance refers to the capacitance that exists between 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. Distributed capacitance can make the equivalent energy dissipation resistance larger and the quality factor larger. To reduce distributed capacitance, wire-covered wire or multi-strand enameled wire are commonly used, and sometimes honeycomb winding method is 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 performance parameters of the inductor will change due to heat generation, and even burn out due to overcurrent.
Five, the function of the inductor
Inductors mainly play the functions of filtering, oscillation, delay and notch in the circuit, as well as filtering signals, filtering noise, stabilizing current and suppressing electromagnetic wave interference. The most common role of inductors in circuits is to form LC filter circuits together with capacitors. Capacitors have the characteristics of "blocking DC and passing AC", while inductors have the function of "passing DC and blocking AC". If the DC with many interference signals is passed through the LC filter circuit, the AC interference signal will be consumed by the inductance into heat energy; when the purer DC current passes through the inductor, the AC interference signal will also be turned into magnetic induction. And thermal energy, the higher frequency is most likely to be impedance by the inductor, which can suppress the higher frequency interference signal.
Inductors have the property of blocking the passage of alternating current and allowing direct current to pass smoothly. The higher the frequency, the greater the coil impedance. Therefore, the main function of the inductor is to isolate and filter the AC signal or form a resonant circuit with capacitors and resistors.
6. How to judge the quality of the inductance with a multimeter
1. Inductance measurement: turn the multimeter to the buzzer diode gear, put the test leads on the two pins, and see the reading of the multimeter.
2. Judgment of good or bad: The reading of the chip inductance should be zero at this time. If the multimeter reading is too large or infinite, it means that the inductance is damaged.
For inductive coils with a large number of turns and a thin wire diameter, the reading will reach tens to hundreds of times. Usually, the DC resistance of the coil is only a few ohms. The damage is manifested as hot or obvious damage to the inductance magnetic ring. If the inductance coil is not seriously damaged and cannot be determined, the inductance can be measured with an inductance meter or the replacement method can be used to judge.
For the inductor coil with a metal shield, it is also necessary to check whether there is a short circuit between the coil and the shield. If the resistance between each pin of the coil and the casing (shield) detected by the multimeter is not infinite, but has a certain resistance value or zero resistance, it means that the inductor is internally short-circuited.
Precautions:
1. For inductive components, the core and windings are prone to change in inductance due to the effect of temperature rise. It should be noted that the temperature of the body must be within the scope of use specifications. .
2. The winding of the inductor is easy to form an electromagnetic field after the current passes through. When placing the components, pay attention to keep the adjacent inductors away from each other, or make the windings at right angles to each other to reduce the mutual inductance.
3. Between the winding layers of the inductor, especially the multi-turn thin wires, the gap capacitance will also be generated, which will cause the bypass of the high-frequency signal and reduce the actual filtering effect of the inductor.
4. When testing the inductance value and Q value with the instrument, in order to obtain the correct data, the test lead should be as close to the component body as possible.
