An inductor is a passive two-terminal component in electronic systems. It works like a quiet energy station. The inductor stores energy by converting electrical energy into magnetic field energy.
Its basic principle is electromagnetic induction — when current flows through a wire, a magnetic field forms around it. A typical inductor is a coil made by winding insulated wire (like enameled wire or Litz wire) around a magnetic core (such as ferrite, iron powder, metal, or air). This structure gives the inductor its key feature: it resists sudden changes in current.
How It Works
An inductor works based on two famous electromagnetic laws:
Faraday’s Law: When current changes in the coil, the magnetic field around it also changes.
Lenz’s Law: This changing magnetic field produces a voltage (called induced EMF) that tries to stop the change in current.
Energy Storage: When current increases, the inductor stores magnetic energy. When current decreases, it releases this energy back into the circuit.
Key Characteristics
To understand inductors, you need to know these important features:
Inductive Reactance (XL): It shows how much the inductor resists AC (alternating current). XL = 2πfL. It increases with frequency (f) and inductance (L).
DC Resistance (DCR): The coil’s own resistance. It causes energy loss (copper loss).
Rated Current: Includes temperature rise current (for heat safety) and saturation current (when the core can’t hold more magnetic energy).
Self-Resonant Frequency (SRF): A natural limit set by parasitic capacitance. Above SRF, the inductor acts like a capacitor.
Temperature Coefficient: Shows how inductance changes with temperature.
Main Parameters
Parameter
Description and Importance
Units / Examples
Inductance (L)
The basic value. Controls energy storage and impedance.
H, mH (millihenry), µH (microhenry)
Tolerance
How much the inductance can vary in production.
±5%, ±10%, ±20%
Rated Current
Includes I_sat (saturation current) and I_RMS (thermal current).
Ampere (A)
DC Resistance (DCR)
Lower DCR is better for efficiency and heat.
Ohm (Ω)
Self-Resonant Frequency (SRF)
Must be much higher than the working frequency.
Hz, MHz
Working Temperature Range
Must fit the real use condition.
°C
Package/Size
Must fit PCB space.
mm, SMD code (e.g., 0805)
Core Material
Affects loss, saturation, and frequency. (Ferrite, iron powder, amorphous, etc.)
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Basic Functions
Energy Storage: Main part in DC-DC converters.
Filtering: Works with capacitors in LC filters to block high-frequency noise.
Current Smoothing: Helps stop sudden changes in current (di/dt).
Impedance Matching: Used in RF circuits to match different parts.
Resonance: Works with capacitors to create filters and oscillators.
EMI Protection: Used as chokes (e.g., EMI chokes) to block unwanted signals.
Applications in Real Life
Core of Power Management (DC-DC Converters)
In Buck, Boost, and Buck-Boost converters, the power inductor is the energy engine. For example, in a Buck converter:
When the switch is on, the inductor stores energy (current goes up).
When the switch turns off, it releases energy (current goes down).
The inductor and output capacitor work together to give stable DC voltage with low ripple. Using inductors with low DCR and high saturation current helps improve efficiency (>95%) and reduce size.
Heart of RF Circuits
In high-frequency circuits (like GHz level), inductors play key roles:
Matching Network: Work with capacitors to match antenna and amplifier impedance, improving signal transfer and reducing reflection.
Resonators: Create LC circuits with high Q (>50), key to oscillator and filter accuracy.
RF Chokes (RFC): Allow DC power to pass but block RF signals from going to the power supply.
Basics of Analog and Digital Circuits
Analog: LC filters shape signal response; LC oscillators create signal frequency.
Digital:
Power Decoupling: Removes high-frequency noise from power rails.
EMI/RFI Suppression: Use common-mode/differential-mode chokes to block interference.
Signal Line Filtering: Stop noise on USB, HDMI, and other fast digital lines.
Symbols in Circuit Diagrams
Standard Symbols
You can recognize inductors in schematics by their coil shape:
ANSI/IEEE (US standard): Zigzag coil (~~~ or ╋╋╋).
IEC (European standard): Semi-circle line (︶︶︶).
Core Line: A solid or dashed line under the coil shows the core type (iron, ferrite, etc.).
Common Variations
Inductor Type
Symbol
With Magnetic Core
Coil + thick solid (iron) or dashed line (ferrite) below
Adjustable Inductor
Arrow on coil or core
Tapped Inductor
Extra wire from the middle
Common Mode Choke
Two identical coils side by side, often with arrows
Product Highlight 1: Our ECW Series Common Mode Chokes use symmetrical winding and nanocrystalline core. Their impedance (Z_cm) is about 40% higher than ferrite chokes in 1–100 MHz range. This gives strong EMI filtering and helps pass Class B EMC tests easily.
Types of Inductors
By Structure
Wound Inductor: Most common. Wire wound on core. Wide power range.
Multilayer Chip Inductor: Printed coils in ceramic layers. Good for high-frequency SMD use (>100 MHz).
Thin-Film Inductor: Precise coil on substrate. High accuracy and stability.
Toroidal Inductor: Wire on a ring core. Closed magnetic path, low leakage.
Air-Core Inductor: No core. No magnetic loss or saturation. High Q, but small inductance.
Core Materials
Core material strongly affects performance:
Ferrite: Good for high-frequency (NiZn >1 MHz, MnZn <2 MHz). Common in power and RF circuits.
Iron Powder Core: Cheap, high saturation, but more high-frequency loss.
Sendust / Moly Permalloy: Balanced between loss and saturation.
Amorphous / Nanocrystalline: High-end choice. Very low loss and high saturation (>1.2T). Best for compact, high-efficiency, high-current use.
By Application
Power Inductor: Handles high current. Low DCR, high I_sat. Used in converters.
RF / High-Frequency Inductor: High Q, high SRF. Uses air-core, ferrite, or thin-film.
Choke: For noise blocking. Common-mode (two coils) or differential-mode (one coil).
Color Code Inductor: Axial leads with color bands. Seen in older devices.
Product Highlight 2: Our PMH Series Flat Wound Power Inductors use amorphous cores. With 2.2μH and 3A saturation, they are only 1.5mm tall (vs. >2.0mm for ferrite), and DCR is 15% lower (~35mΩ). Perfect for compact devices like TWS earbud cases.
Design Practice: How to Choose
Why Is an Inductor So Important in Switching Power?
In SMPS, the inductor has key roles:
Energy Storage: Charges when the switch is on, discharges when off.
Voltage Control: Buck/Boost uses changing inductor current to control output voltage.
Current Smoothing: Turns pulses into steady output current.
di/dt Limiter: Slows down current change to protect components.
Filter Partner: Works with capacitor to block switching noise.
Inductors in RF Circuits
RF systems need high quality and accuracy:
Impedance Matching: Needed to keep antennas and amplifiers working well.
Resonance: High Q LC circuits are key to filters and oscillators.
Signal Isolation: RFCs block RF from the DC power system.
Frequency Tuning: Adjustable inductors fine-tune circuits.
7 Steps to Select the Right Inductor
Set Inductance (L): Use formulas or simulations.
Check Current:
RMS current < Thermal rating.
Peak current < Saturation current (I_sat), keep 20–50% margin!
Check Frequency:
SRF > 5 × working frequency.
For RF, Q > 30 or 50 is often needed.
Estimate Loss:
Copper Loss: DCR × I_RMS².
Core Loss: Check material loss curves. Amorphous cores work better above 20kHz.
Check Other Specs: Tolerance, size, temperature range, cost.
Special Needs:
Need shielding
Need adjustability
Using a common-mode choke
Use Tools: Look at chip datasheets, online selector tools, and simulation software.
Product Highlight 3: Our FML Series Thin-Film Inductors offer high Q (>70) at 2.4GHz, better than multilayer (Q<45) or air-core (Q~60 but large). SRF >10GHz and TC ±30ppm/°C make them ideal for Wi-Fi 6/6E, Bluetooth, etc.
Quickly Finding Inductors in Schematics
How to Read
Look for these signs:
Coil Symbol: Wavy or loop shape (~~~ or ︶︶︶) means inductor!
Label: Look for values like L = 10µH or names like L1.
Prefix "L": Inductors are marked with "L" (L1, L2, LFB1).
Core Line: A line under the coil shows the core.
Arrow: Shows it’s adjustable.
Tapped Wire: A wire from the middle means it has a tap.
Practice Tips
Memorize Symbols: Review all types from Part 3.
Compare: Learn how they differ from resistors (zigzag), capacitors (||), transformers (multiple coils).
View Real Circuits: Study PCB diagrams of phones, adapters, and RF modules.
Notes
Sometimes symbols or prefixes may vary slightly (ANSI vs. IEC). But by focusing on coil shape + L prefix + inductance label, you can correctly spot inductors in most schematics. Know the symbols and functions well, and you can easily find these magnetic components in any circuit.
Frequently Asked Questions
What is the symbol for an inductor?
An inductor is represented by the letter L in electrical schematics. Its circuit diagram symbol features coiled loops signifying conductor windings, typically connected by straight terminal lines.
What is the function of an inductor?
As fundamental passive components, inductors store magnetic energy from current flow, resist changes in current, and interact with magnetic fields.
What are the three types of inductors?
Inductors fall into three primary types: air core, ferrite core, and iron core (also known as magnetic core). These types differ significantly in their performance, inductance value, and application.
