Inductor Knowledge | Parameters, Coils, Functions, Models, Specifications, Naming, Applications, Relationship and Differences with Ferrite Beads, Calculation Formulas, Measurement, Precautions

1. Definition of Inductors

1.1 Definition of Inductance:

Inductance is the property of a conductor, where a varying current in the conductor generates a changing magnetic flux inside and around the conductor. The inductance value is the ratio of the magnetic flux generated to the current producing this flux.

When direct current flows through an inductor, it produces a steady magnetic field. However, when alternating current flows through the inductor, the magnetic field around it changes over time. According to Faraday’s law of electromagnetic induction, a varying magnetic field generates an induced electromotive force (EMF) across the inductor, which resists the change in current. This phenomenon is similar to inertia in mechanics, and in electrical terms, it is known as "self-induction."

1.2 Inductor vs. Transformer:

• Inductor: A coil of wire that, when current flows through it, generates a magnetic field. The magnetic flux changes with alternating current, leading to the phenomenon of self-induction.

• Transformer: Consists of two inductive coils that are magnetically coupled but not electrically connected. Transformers utilize mutual inductance to induce voltage in nearby coils.

1.3 Inductance Symbol and Unit:

• Symbol: L

• Units: Henry (H), millihenry (mH), microhenry (μH)

1.4 Classification of Inductance:

• By form: Fixed inductors, variable inductors.

• By magnetic core material: Air-core, ferrite-core, iron-core, copper-core inductors.

• By operating characteristics: Antenna coils, oscillation coils, chokes, resonant coils, deflection coils.

• By winding structure: Single-layer, multi-layer, honeycomb coils.

• By frequency: High-frequency, low-frequency inductors.

• By structural features: Magnetic core inductors, variable inductors, color-coded inductors, and non-magnetic inductors.

2. Main Characteristics and Parameters of Inductors

2.1 Inductance (L):

Inductance refers to the inherent property of the coil that is independent of the current passing through it. Inductance is typically not marked on the inductor itself but is represented by specific names.

2.2 Reactance (XL):

Reactance is the opposition that an inductor presents to alternating current. It is measured in ohms and is calculated as:
$XL = 2\pi f L$XL=2πfL
where $f$f is the frequency of the AC and $L$L is the inductance.

2.3 Quality Factor (Q):

The quality factor (Q) represents the efficiency of an inductor and is defined as the ratio of inductive reactance to the equivalent resistance of the coil:
$Q = \frac{X_L}{R}$Q=RXL​​
A higher Q value indicates lower losses. Q values typically range from tens to hundreds.

2.4 Distributed Capacitance:

The capacitance between turns of the coil, between the coil and its shield, or between the coil and the substrate, which reduces the Q value and stability of the inductor. A smaller distributed capacitance is preferable.

2.5 Tolerance:

Tolerance is the percentage difference between the actual inductance value and the nominal value.

2.6 Rated Current:

The maximum current that the inductor can carry, usually denoted by letters (A, B, C, etc.), with typical values like 50mA, 150mA, 300mA, etc.

3. Common Types of Inductors

3.1 Single-Layer Coil:

A coil made by winding insulated wire on a paper or plastic core. This is commonly used in AM radio antenna circuits.

3.2 Honeycomb Coil:

A coil where the winding plane intersects at an angle, reducing distributed capacitance and increasing inductance. These coils are compact and widely used for high-frequency applications.

3.3 Ferrite-Core Coil:

Inductors with ferrite cores to increase inductance and improve the quality factor. These are typically used in high-frequency circuits.

3.4 Copper-Core Coil:

Copper-core coils are used in ultra-shortwave frequencies. The position of the copper core can be adjusted to change the inductance.

3.5 Color-Coded Inductor:

A high-frequency inductor with a fixed inductance value, marked with color codes similar to resistors. It operates in the frequency range of 10kHz to 200MHz.

3.6 Choke (Inductive Filter):

An inductor designed to block AC signals while allowing DC to pass. It is used for noise suppression in power supplies and signal lines.

3.7 Deflection Coil:

Used in the output stage of television scanning circuits. It requires high sensitivity, uniform magnetic field, high Q value, small size, and low cost.

4. Role of Inductors in Circuits

• Basic Functions: Filtering, oscillation, delay, and notch filtering.

• Layman’s Terms: "Pass DC, block AC."

• Detailed Explanation: Inductors oppose changes in alternating current and can be used in combination with capacitors to form high-pass or low-pass filters, phase-shifting circuits, and resonant circuits. Transformers can perform AC coupling, voltage conversion, current conversion, and impedance matching.

The energy stored in an inductor is proportional to its inductance and the square of the current:
$W_L = \frac{1}{2} L i^2$WL​=21​Li2

5. Inductor Models, Specifications, and Naming

5.1 Chip Inductors:

• Inductance range: 10nH – 1mH

• Materials: Ferrite and ceramic.

• Accuracy: J = ±5%, K = ±10%, M = ±20%

• Sizes: 0402, 0603, etc.

5.2 Power Inductors:

• Inductance range: 1nH – 20mH

• Sizes: SMD43, SMD54, SMD73, SMD75, etc.

5.3 Chip Ferrite Beads:

• Impedance range: 5Ω – 3KΩ

• Sizes: 0402, 0603, etc.

5.4 Through-Hole Ferrite Beads:

• Specifications: RH3.5

5.5 Color-Coded Inductors:

• Inductance range: 0.1μH – 22mH

• Sizes: 0204, 0307, 0410, 0512

• Accuracy: J = ±5%, K = ±10%, M = ±20%

5.6 Vertical Inductors:

• Inductance range: 0.1μH – 3mH

• Specifications: PK0455, PK0608, PK0810, etc.

5.7 Axial Leaded Filter Inductors:

• Inductance range: 0.1μH – 10mH

• Rated current: 65mA to 10A

• High Q value and relatively low cost.

5.8 Magnetic Ring Inductors:

• Specifications: TC3026, TC3726, TC4426, etc.

• Sizes: 3.25mm to 15.88mm

5.9 Air-Core Inductors:

• Used for achieving large inductance values with lighter weight.

6. Applications of Inductors in Circuits

Inductors are commonly used in LC filter circuits, where they work with capacitors to filter out unwanted AC signals from DC signals.

• Power Filtering: LC circuits remove high-frequency interference in DC power supplies.

• Signal Filtering: Inductors block high-frequency noise and allow pure DC or lower-frequency signals to pass.

7. Common Magnetic Cores and Rings

• Iron Powder Cores: Widely used in computers, power supplies, UPS systems, and home appliance control circuits. Available in various sizes and materials.

• Ferrite Cores: Used for high-frequency attenuation and filtering in circuits.

8. Relationship and Differences Between Inductors and Ferrite Beads

1. Energy Storage vs. Energy Conversion:

   • Inductors store energy, while ferrite beads dissipate energy.

2. Applications:

   • Inductors are used in power supply filtering, while ferrite beads are used in signal circuits for EMC (electromagnetic compatibility) suppression.

3. Frequency Range:

   • Inductors are used in low to medium-frequency circuits, while ferrite beads are primarily used for high-frequency signal attenuation.

9. Inductor Calculation Formulas

9.1 Ring Core Inductance:

The inductance of a ring core can be calculated as:
$L = N^2 \times AL$L=N2×AL
where $N$N is the number of turns and $AL$AL is the inductance factor.

9.2 General Inductance Calculation Formula:

$L = \frac{k \times \mu_0 \times \mu_s \times N^2 \times S}{l}$L=lk×μ0​×μs​×N2×S​
where:

• $\mu_0$μ0​ is the permeability of free space (4π × 10⁻⁷ H/m),

• $\mu_s$μs​ is the relative permeability of the core material,

• $N$N is the number of turns,

• $S$S is the cross-sectional area of the core,

• $l$l is the length of the coil.

10. Measuring Inductance

Inductance can be measured using an RLC meter or a specialized inductance meter.
Steps to measure inductance:

1. Select the inductance (L) mode on the meter.

2. Connect the probes to the leads of the inductor.

3. Record the reading.

4. Take multiple measurements and average the results.

11. Precautions for Using Inductors

1. Operating Environment: Consider factors such as temperature, humidity, and physical stresses.

2. Frequency Characteristics: Inductors behave differently at high and low frequencies. For instance, inductance tends to increase with higher frequencies.

3. Maximum Current and Heat: Ensure the inductor does not exceed its rated current to prevent overheating.