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Where to start studying inductance? I will understand after reading


A choke is a circuit element that will generate an electromotive force due to a change in passing current, thereby counteracting change in current.

The structure of an inductor is similar to that of a transformer, but there is only one winding, which is usually composed of a frame, winding, shield, packaging material, magnetic core or iron core, etc. .

If an inductor is in a no-current state, it will attempt to prevent current from flowing through it when circuit is on; if inductor is in a state of current flow, it will attempt to maintain current when circuit is off.

1. Definition of inductance

Inductance is ratio of magnetic flux of a wire to current that produces magnetic flux.

When a direct current is passed through an inductor, only fixed magnetic lines of force appear around it, which do not change with time, and when an alternating current is passed through a coil, magnetic lines of force vary in time. appear around him.

According to Faraday's law of electromagnetic induction - magnetism generates electricity, changing magnetic lines of force will generate an induced potential at both ends of coil, which is equivalent to a "new power source".

When a closed loop is formed, induced potential will generate an induced current. According to Lenz's law, it is known that total amount of magnetic field lines generated by induced current should prevent original magnetic field lines from changing.

Because change in magnetic field line is initially due to a change in external AC power supply, due to objective effect, inductor has characteristic of preventing change in current in AC circuit. .

An inductor has characteristics similar to inertia in mechanics and is called "self-induction" in electricity. Typically, sparks occur at moment switch is pulled or turned on, which is caused by high induced potential created by phenomenon of self-induction.

In short, when an inductor is connected to an AC power source, magnetic field lines inside coil will alternate with current and change constantly, causing coil to continuously generate electromagnetic fields. induction. This electromotive force, produced by changing current of coil itself, is called "self-induced electromotive force".

It can be seen that inductance is only a parameter related to number of turns, size, shape and material of coil. This is a measure of inertia of an inductor, nothing to do with applied current.

2. Symbol and unit of inductance

Inductance symbol: L

Where to start studying inductance? I will understand after reading

Inductance unit: Henry (H), millihenry (mH), microhenry (µH), 1H=103mH=106µH.

Rated inductance: straight label type, color ring label type, no label type

Inductor directivity: no direction

How to check if an inductance is good or bad: use an inductance meter to measure its inductance; use a multimeter to measure it on-off, ideal inductance resistance is very small, almost zero.

3. Classification of inductance

Classification by shape of inductance: fixed inductance, variable inductance.

Classification by magnetic properties: air core coil, ferrite core coil, iron core coil, copper core coil.

Classification according to operation principle: antenna coil, oscillation coil, choke coil, trap coil, deflection coil.

Classification by winding structure: single layer coil, multilayer coil, honeycomb coil.

Classification by operating frequency: high frequency coils, low frequency coils.

Classification by design features: magnetic core coils, variable inductors, color-coded inductors, coreless coils, etc.

4. Inductor and transformer

Inductor: When there is current in a wire, a magnetic field is created around it. We usually wind wire into a coil to increase magnetic field inside coil.

An inductor is made by winding wires (enamelled wire, bag of yarn or bare wire) turn by turn (wires insulated from each other) around an insulating tube (insulator, iron core or magnetic circuit).

As a rule, an inductor has only one winding.

Where to start studying inductance? I will understand after reading

Transformer: When alternating current flows through an inductor, an induced voltage is generated not only at both ends of inductor, but also in adjacent coil. This phenomenon is called mutual inductance. Two coils that are not connected to each other, but are close to each other and have electromagnetic induction between them, are usually called transformers.

Where to start studying inductance? I will understand after reading

5. The role of inductance

1. The role of inductors is basically to allow direct current to pass and block alternating current. In circuit, they mainly play role of filtering, generating, delaying and notching.

The inductor blocks alternating current. The amount of blocking effect is called inductive reactance XL and is measured in ohms. Its relation to inductance L and AC frequency f:


Chokes can be divided into high frequency choke coils and low frequency choke coils.

Tuning and frequency selection function: The inductor and capacitor are connected in parallel to form an LC tuning circuit. That is, the natural oscillation frequency f0 of circuit is equal to frequency f of non-variable signal, then inductive and capacitive reactances of circuit are also equal, so electromagnetic energy oscillates back and forth in inductance and capacitance, and resonance of LC circuit is a phenomenon.

At resonance, inductive reactance and capacitive reactance of circuit are equivalent and reversed, inductive reactance of total current of circuit is smallest, and current is largest (refers to AC signal with f="f0" ). The LC resonant circuit has a frequency selection function. It can select an AC signal of a certain frequency f.

2. The inductors also have functions of signal shielding, noise filtering, current stabilization, and electromagnetic wave interference suppression.

In electronic equipment, magnetic rings are often found, which, together with connecting cable, form an inductor (a wire in cable winds several turns of an inductor around a magnetic ring).

This is an anti-interference component commonly used in electronic circuits. High-frequency noise has a good shielding effect, so it is called an absorbing magnetic ring.

It is usually made of a ferrite material, also known as a ferrite magnetic ring (magnetic ring for short). The magnetic ring has different impedance characteristics at different frequencies. The impedance is very low at low frequencies, and impedance of magnetic ring rises sharply as frequency of signal increases.

6. Main characteristic parameters of inductance

1. Inductance L

The inductance L is an inherent characteristic of coil itself, which has nothing to do with magnitude of current. With exception of special inductors (colour-coded inductors), inductance is usually not marked on coil, but is marked with a specific name.

2. Inductive XL

The resistance of an inductor to AC current is called inductive reactance XL, and unit is ohm. Its connection with inductance L and frequency of alternating current f is XL=2πfL

3. Quality factor Q

The quality factor Q is a physical quantity representing quality of coil, and Q is ratio of inductive reactance XL to its equivalent resistance, namely: Q=XL/R. The higher quality factor of coil, lower losses in loop. The Q value of a coil is related to DC resistance of wire, dielectric loss of frame, loss caused by screen or iron core, influence of high frequency skin effect, and other factors. The value of quality factor of coil is usually from tens to hundreds. By using magnetic core coils, multi-strand thick coils can increase quality factor of coil.

4. Distributed Capacity

The capacitance that exists between turns of coil, between coil and shield, and between coil and bottom plate is called distributed capacitance. The presence of distributed capacitance reduces quality factor of coil and worsens stability, so smaller distributed capacitance of coil, better. The distributed capacitance can be reduced by segment winding.

5. Tolerance

Percentage difference between actual inductance value and nominal value, divided by nominal value.

6. Rated current

refers to amount of current that coil can carry, usually denoted by the letters A, B, C, D and E respectively. Current ratings are 50mA, 150mA, 300mA, 700mA, 1600mA.