10 Common Types of Inductors and Their Functions
An inductor is a passive electronic component commonly referred to simply as an “inductor.” In essence, it is a coil of wire, which can be either an air-core coil or a solid-core coil. Solid-core inductors use materials such as iron or other magnetic substances as the core.
The unit of inductance is the Henry (H). Smaller units include millihenry (mH) and microhenry (μH):
1 H = 1000 mH = 1,000,000 μH
When alternating current (AC) flows through an inductor, it generates a changing magnetic flux around the conductor. The ratio between magnetic flux and current defines inductance.
With DC current, the magnetic field remains constant.
With AC current, the magnetic field continuously changes over time.
This difference is the fundamental reason inductors behave differently under AC and DC conditions.
Basic Functions of Inductors
One of the most fundamental functions of an inductor is “blocking AC and passing DC.” In practical terms, an inductor offers almost no resistance to direct current, behaving like a short circuit. However, for alternating current, it introduces impedance, and this impedance increases as the frequency rises. Therefore, the higher the frequency, the stronger the inductor’s blocking effect on the signal.
Inductors are also widely used in transformers. A transformer consists of two or more inductive coils, and it works based on the principle of electromagnetic induction. The voltage ratio between primary and secondary windings is proportional to the turn ratio, while the current ratio behaves inversely. This allows transformers to step voltage up or down without changing the total power, making them essential in power transmission and conversion systems.
Another important application is in RL filters, including low-pass and high-pass filters. In a low-pass RL filter, low-frequency signals can pass through easily while high-frequency signals are gradually attenuated due to increasing inductive impedance. The cutoff frequency is determined by the formula f = R / (2πL). In a high-pass RL filter, the circuit configuration is reversed, allowing high-frequency signals to pass while blocking low-frequency components, using the same cutoff frequency relationship.
Drum Core Inductor (I-shaped Inductor)
The drum core inductor, also known as the I-shaped inductor, is an improved version of traditional wound inductors. It typically includes a shielding structure that enhances energy storage capability while reducing DC resistance and improving overall efficiency. This structure also helps control electromagnetic interference (EMI) direction and magnitude, making it suitable for both signal and power applications.
These inductors are commonly used in switching power supplies operating in the hundreds of kilohertz to low megahertz range. Typical applications include LED drivers, DSL communication circuits, and low-power power conversion systems. With a moderate Q factor, they provide a balance between signal processing and power handling capability. However, since the magnetic circuit is not fully closed, EMI issues and audible noise can still be a concern in certain designs.
Color Ring Inductor
The color ring inductor is derived from the basic rod core inductor and is mainly used in signal processing applications. Its most notable feature is the use of color bands to indicate inductance values, making identification simple and cost-effective. Because of its through-hole structure, it is still used in some low-cost or traditional electronic products.
However, this type of inductor has largely been phased out in modern high-density electronic designs. Its relatively large size and limited integration capability make it unsuitable for compact or high-frequency circuits. As surface-mount technology continues to dominate, color ring inductors are gradually becoming obsolete.
Air Core Inductor
Air core inductors do not use any magnetic material, relying entirely on air as the core medium. This structure eliminates core losses and makes them highly suitable for high-frequency applications such as RF resonance circuits, signal transmission, and receiving systems.
Because there is no magnetic core, air core inductors exhibit excellent linearity and stable high-frequency performance. However, their inductance value is relatively small, and the physical structure is less stable compared to core-based inductors. As a result, they are mainly used in specialized communication and RF applications where precision and high-frequency behavior are more important than compact size.
Toroidal Inductor (Ring Core Inductor)
The toroidal inductor is widely considered one of the most efficient inductor geometries due to its closed magnetic loop structure. This design significantly reduces electromagnetic interference (EMI) and improves magnetic field utilization, resulting in high efficiency and predictable performance.
In practical applications, toroidal inductors are often used in power electronics, especially those based on iron powder cores. They can handle relatively high currents, often exceeding 20A in DC applications. Despite their excellent performance characteristics, manufacturing is challenging because winding the coil around a toroidal core is labor-intensive and often requires manual or semi-automated processes.
Multilayer High-Frequency SMD Inductor
Multilayer high-frequency SMD inductors are widely used in modern compact electronic devices. They are manufactured using multilayer ceramic processes, allowing them to achieve extremely small size while maintaining stable electrical characteristics. Their structure is similar to air-core inductors in terms of high-frequency behavior.
These inductors are commonly found in RF circuits and high-speed communication systems. They offer excellent consistency in mass production and are well suited for automated assembly. However, their Q factor is generally lower than that of wire-wound inductors, and material properties such as temperature stability play a critical role in performance.
Rod Core Inductor (Magnetic Bar Inductor)
Rod core inductors are among the simplest and most traditional types of inductors. By inserting a magnetic rod into a coil, the inductance and Q factor are significantly improved compared to air-core designs. This makes them suitable for basic filtering, tuning, and RF signal applications.
They have been widely used for decades in various electronic systems, and their structure remains simple and cost-effective. However, compared to modern SMD components, rod core inductors are relatively bulky and less suitable for high-density circuit integration.
SMD Power Inductor
SMD power inductors are designed specifically for energy storage in switching power supply systems. Their primary focus is on high current handling capability and low power loss, making them essential components in DC-DC converters and voltage regulation modules.
These inductors are widely used in smartphones, laptops, automotive electronics, and industrial power systems. Their compact size and high efficiency make them ideal for modern power management designs, where space and energy efficiency are critical factors.
Ferrite Bead (Through-Hole Type)
Ferrite beads are often considered noise suppression components rather than traditional inductors. They allow low-frequency signals to pass through while attenuating high-frequency noise. This makes them highly effective for electromagnetic interference (EMI) filtering in electronic circuits.
Through-hole ferrite beads are used in older or specialized designs where robustness and ease of replacement are important. Their main role is to suppress unwanted high-frequency signals and improve signal integrity in mixed-signal systems.
SMD Ferrite Bead
SMD ferrite beads are the surface-mount version of traditional ferrite beads and are widely used in modern compact electronic systems. They provide effective high-frequency noise suppression while occupying minimal PCB space.
These components are commonly used in high-speed digital circuits, communication interfaces, and power supply lines. Their small size and high integration capability make them a standard choice in modern PCB design for EMI control.
High-Frequency Transformers (SMD & Through-Hole)
High-frequency transformers are specialized inductive components used primarily in switching power supplies. They operate at high switching frequencies to efficiently transfer energy between circuits while providing electrical isolation.
These transformers are widely used in power adapters, industrial power systems, and communication equipment. Their ability to handle high-frequency energy conversion makes them essential in modern compact and efficient power supply designs.
Conclusion
Inductors are fundamental components in electronic systems, playing key roles in energy storage, filtering, signal processing, and power conversion. From simple air-core coils to advanced multilayer SMD structures, each type of inductor is optimized for specific frequency ranges, power levels, and application environments.
As electronic systems continue to evolve toward higher frequency operation, miniaturization, and greater efficiency, inductor technology is also advancing rapidly. Future developments will likely focus on higher integration, lower losses, improved EMI performance, and better suitability for high-density circuit design.