Air Coil Inductors and Encapsulated Transformers in Modern Electronics

Author : Cet Technology | Published On : 12 Aug 2026

Magnetic components play an important role in electronic and electrical systems. Inductors can store energy and oppose changes in current, while transformers transfer electrical energy between circuits and can provide voltage transformation or isolation. The physical construction of these components varies considerably depending on frequency, power level, environmental conditions, and application requirements.

Two examples are air coil inductors and encapsulated transformers.

Air coil inductor do not use a conventional magnetic core. Instead, their inductance is produced primarily by the geometry of the winding and the magnetic field surrounding it. This makes them particularly useful in applications where high-frequency performance and low core-related losses are important.

Encapsulated transformers take a different approach. Their core and windings are surrounded by an insulating encapsulation material, commonly an epoxy-based resin system. This construction can protect internal components from moisture, dust, contaminants, and mechanical stresses while also providing electrical insulation.

Understanding how these two component types work helps engineers select an appropriate magnetic structure for a particular circuit.

What Is an Air Core Inductor?

An air core inductor is an inductor that does not use a ferromagnetic or ferrite core to concentrate its magnetic field.

The simplest example is a coil of conductive wire. When current passes through the coil, a magnetic field develops around the conductor. The coil's geometry, number of turns, diameter, spacing, and conductor characteristics influence its inductance.

Although the term "air core" suggests that the coil is literally unsupported, some designs use a nonmagnetic former or mechanical structure to hold the winding in position. Electronics Notes points out that formers may be used to maintain winding stability, provided the material does not introduce unwanted conductive effects.

The absence of a magnetic core is one of the most important characteristics of this type of inductor.

Why Use an Air Core Inductor?

The primary reason to use an air core inductor is often related to high-frequency behavior.

Magnetic-core inductors can experience core losses associated with the magnetic material. An air core eliminates those particular core losses because there is no magnetic material in the flux path. This can contribute to high Q performance when winding resistance and other losses are kept low.

Air coil inductors are therefore commonly associated with RF circuits, tuning networks, matching circuits, filters, and other applications where frequency-dependent performance is important.

For example, Coilcraft's air core inductor families are specified for RF applications and include characteristics such as Q factor, self-resonant frequency, DC resistance, inductance tolerance, and current capability.

However, an air core design is not automatically better than a magnetic-core design. It is a trade-off based on the electrical requirements.

Important Characteristics of Air Coil Inductors

Several parameters should be evaluated when selecting an air core inductor.

Inductance

Inductance describes an inductor's ability to oppose changes in current and store energy in its magnetic field.

For an air core design, inductance is strongly influenced by the physical geometry of the winding. Increasing the number of turns generally increases inductance, but additional turns also affect resistance, parasitic capacitance, physical size, and self-resonant behavior.

Q Factor

The quality factor, or Q, is an important parameter in many RF applications.

A higher Q generally indicates lower losses relative to the component's reactive behavior at the measurement frequency. Air core construction can support high Q because it avoids magnetic-core losses, although conductor resistance, proximity effects, radiation, and other parasitic effects still influence real-world performance.

Self-Resonant Frequency

An inductor is not an ideal component at all frequencies.

The winding contains parasitic capacitance, and eventually the inductor can reach a self-resonant frequency where its behavior changes significantly. Consequently, engineers should evaluate inductance and Q at the actual operating frequency rather than assuming that a nominal inductance value remains constant indefinitely.

DC Resistance

The resistance of the winding contributes to power loss and heating.

A larger conductor can reduce DC resistance, but it also occupies more physical space. The design therefore requires a balance between electrical performance, mechanical dimensions, and thermal requirements.

Current Capability

Current rating depends on factors such as conductor size, winding geometry, thermal conditions, and the manufacturer's test method.

An inductor intended for higher current may require a larger conductor or a different physical construction.

Where Are Air Coil Inductors Used?

Air coil inductors are commonly associated with applications where magnetic-core behavior is undesirable or where high-frequency characteristics are important.

Typical applications include:

• RF amplifier matching
• Tuning circuits
• Oscillators
• Radio-frequency filters
• Antenna circuits
• Communication equipment
• High-frequency signal processing
• Wireless electronics
• Specialized power and frequency-control circuits

Coilcraft, for example, describes air coil inductors for narrowband RF amplifier matching and tuning applications and publishes specifications for Q, self-resonant frequency, resistance, and current.

The appropriate design depends on the operating frequency and electrical requirements rather than the component name alone.

What Is an Encapsulated Transformer?

An encapsulated transformer is a transformer whose core and coil assembly is enclosed in an insulating encapsulation material.

The process is often called potting or resin encapsulation. The encapsulating compound surrounds the internal transformer components and cures into a solid protective structure.

Schneider Electric describes sealed resin epoxy encapsulated transformers as having the core and coil surrounded by epoxy resin and sand encapsulation material. The resulting structure helps seal the core and coil from moisture and contaminants.

Eaton similarly describes general-purpose encapsulated dry-type transformers as transformers in which the windings and core are encapsulated in an epoxy-coated aggregate mixture.

The exact resin formulation and manufacturing process vary according to the transformer design and application.

Why Are Transformers Encapsulated?

Encapsulation provides several potential advantages beyond the basic electrical function of the transformer.

Protection From Moisture and Contaminants

One of the major purposes is environmental protection.

A suitable encapsulation system can reduce the ability of moisture, dust, and airborne contaminants to reach the core and windings. This can make encapsulated construction useful in environments where a conventional ventilated transformer may require additional protection.

Electrical Insulation

The encapsulating material can contribute to the transformer's insulation system.

However, the resin should not be considered the only insulation feature. Transformer design normally involves several insulation elements, including wire insulation, barriers, bobbins, tapes, clearances, creepage distances, and the encapsulation system.

Mechanical Protection

The cured encapsulation material can provide mechanical support to the internal windings.

This can help protect the assembly against movement and vibration, although the effectiveness depends on the material, manufacturing process, mechanical design, and operating environment.

Reduced Exposure to Corrosive Environments

Encapsulation can reduce direct exposure of the core and winding system to environmental contaminants.

This can be particularly relevant in industrial, outdoor, marine, mining, or other demanding environments. Delta Transformers, for example, identifies encapsulated transformers for applications where moisture and airborne contaminants are concerns.

How Are Encapsulated Transformers Manufactured?

Manufacturing an encapsulated transformer involves more than simply covering an ordinary transformer with resin.

1. Electrical Design

The process begins with electrical requirements.

These can include:

• Input voltage
• Output voltage
• Frequency
• Power rating
• Current
• Isolation requirements
• Temperature limits
• Physical dimensions
• Mounting requirements
• Environmental conditions

These requirements determine the core, winding, conductor, and insulation system.

2. Core and Winding Construction

The transformer core is assembled according to the selected magnetic design.

The primary and secondary windings are then produced with the required number of turns, conductor dimensions, insulation, and winding arrangement.

3. Insulation and Assembly

The windings and core are assembled while maintaining the required electrical clearances and insulation barriers.

The design must also allow the encapsulation material to reach the intended areas without creating unwanted voids or defects.

4. Encapsulation

The assembly is placed into an appropriate mold, enclosure, or casting arrangement, and the resin system is introduced.

Depending on the process, manufacturers may use controlled filling, vacuum processing, curing cycles, or other techniques to achieve consistent encapsulation.

The goal is to create a mechanically stable and electrically reliable structure around the core and windings.

5. Curing

The encapsulating compound must cure according to the material manufacturer's requirements.

Temperature, curing time, resin formulation, and process control can affect the final properties of the encapsulated component.

6. Electrical Testing

After manufacturing, the transformer can be tested against its design requirements.

Depending on the application, tests may include winding resistance, turns ratio, insulation-related testing, dielectric withstand, no-load characteristics, and other electrical or mechanical checks.

The specific test program should be established according to the component design and applicable standards.

Air Core Inductor vs. Encapsulated Transformer

These two components have fundamentally different purposes.

Characteristic	Air Core Inductor	Encapsulated Transformer
Primary function	Energy storage/current-change control	Energy transfer and voltage transformation
Magnetic core	No conventional magnetic core	Magnetic core normally present
Windings	Usually one primary winding structure	Normally two or more electrically coupled windings
Common frequency focus	Often high-frequency/RF	Depends on transformer design
Environmental protection	Depends on package	Encapsulation provides additional protection
Key parameters	Inductance, Q, SRF, DCR, current	Voltage ratio, power, isolation, temperature, insulation
Typical applications	RF, tuning, matching, filtering	Power supplies, isolation, industrial electronics

They should therefore be selected according to the circuit function rather than compared as interchangeable components.

Design Considerations for Air Coil Inductors

Engineers designing or selecting an air core inductor should consider the complete operating environment.

Frequency

Frequency has a major influence on Q, parasitic capacitance, skin effect, proximity effect, and self-resonance.

Physical Geometry

Small changes in coil diameter, spacing, number of turns, and conductor position can change inductance and high-frequency behavior.

Mechanical Stability

Because the magnetic field is determined by the coil geometry, movement of the turns can change the electrical characteristics. A mechanically stable winding is therefore important in applications where vibration or physical movement is expected.

Electromagnetic Coupling

Air coil inductors can interact with nearby conductors and magnetic components. PCB layout and component placement should therefore be considered during the design process.

Design Considerations for Encapsulated Transformers

Encapsulated transformers involve a different set of considerations.

Thermal Management

Encapsulation changes how heat moves away from the transformer.

Schneider Electric notes that, for its sealed resin epoxy encapsulated transformers, heat removal is through the surface of the enclosure.

Consequently, the thermal characteristics of the resin, enclosure, mounting arrangement, ambient temperature, and transformer losses all matter.

Insulation System

The insulation system must be designed for the required voltage and operating conditions.

Encapsulation can contribute to insulation, but it does not eliminate the need for appropriate winding insulation and electrical clearances.

Resin Selection

The encapsulation material needs to be compatible with the operating temperature, mechanical environment, electrical requirements, and manufacturing process.

Voids and Manufacturing Consistency

The quality of the encapsulation process matters.

Voids, incomplete filling, contamination, or improper curing can affect mechanical and electrical performance. Process control is therefore an important part of encapsulated transformer manufacturing.

Applications of Encapsulated Transformers

Encapsulated transformers can be considered when environmental protection is an important design requirement.

Potential applications include:

• Industrial control systems
• Power supplies
• Outdoor electrical equipment
• HVAC systems
• Marine equipment
• Mining equipment
• Petrochemical installations
• Commercial electrical systems
• Equipment exposed to dust or moisture

Schneider Electric and Eaton both describe encapsulated transformer designs for applications where environmental protection and specific installation requirements are important.

However, encapsulation does not automatically make a transformer suitable for every harsh environment. The enclosure rating, insulation system, temperature rating, applicable standards, and complete installation design should all be evaluated.

How to Select Between Different Magnetic Components

Choosing a magnetic component should begin with the electrical specification rather than the component category.

For an air core inductor, engineers may start by defining:

1. Required inductance
2. Operating frequency
3. Required Q
4. Self-resonant frequency
5. Current
6. Maximum DCR
7. Physical dimensions
8. Mounting configuration
9. Temperature range

For an encapsulated transformer, the starting specifications may include:

1. Primary voltage
2. Secondary voltage
3. Frequency
4. Power or VA rating
5. Current
6. Isolation requirements
7. Insulation system
8. Temperature rise
9. Environmental conditions
10. Mounting and enclosure requirements

Providing accurate specifications early in the design process helps reduce the risk of selecting a component that meets one electrical parameter but fails another.

The Importance of Testing

Both air coil inductors and encapsulated transformers require appropriate testing.

For air coil inductors, engineers may evaluate inductance, Q factor, self-resonant frequency, DCR, current capability, and frequency response. Manufacturer data sheets commonly specify the test frequency and conditions because parameters such as inductance and Q are frequency-dependent.

For encapsulated transformers, testing can include electrical, insulation, dimensional, and thermal evaluations depending on the design.

The important principle is that test results should be interpreted under the conditions for which the component was designed and specified.

Frequently Asked Questions

What is an air core inductor?

An air core inductor is an inductor that does not use a conventional magnetic core. Its inductance is produced primarily by the geometry of its winding and surrounding magnetic field. Air core designs are commonly used in high-frequency and RF applications.

Why are air coil inductors used in RF circuits?

Air coil inductors can offer high Q and low core-related losses because they do not contain a magnetic core. These characteristics can make them useful for RF matching, tuning, and filtering applications.

What is an encapsulated transformer?

An encapsulated transformer is a transformer whose core and windings are surrounded by an insulating encapsulation material, often an epoxy-based compound. The encapsulation can protect the internal components from moisture and contaminants while contributing to mechanical and electrical insulation.

Are encapsulated transformers waterproof?

Encapsulation can provide substantial protection against moisture reaching the core and windings, but an encapsulated transformer should not automatically be described as completely waterproof. The overall enclosure, construction, ratings, cable entries, and installation determine environmental protection.

Are air coil inductors suitable for high-frequency applications?

They can be. Air coil inductors are frequently used in RF and high-frequency circuits because they avoid magnetic-core losses and can provide high Q. Their actual suitability depends on frequency, inductance, Q, self-resonant frequency, current, and physical design.

What is the main advantage of an encapsulated transformer?

One major advantage is environmental and mechanical protection. Encapsulation surrounds the core and coil, helping reduce exposure to moisture, dust, and contaminants. It can also contribute to insulation and mechanical stability.

Does encapsulation affect transformer cooling?

Yes. Encapsulation changes the transformer's thermal path. Heat must move through the encapsulation and surrounding structure, so thermal design is important when determining temperature rise and allowable loading.

What information is needed when specifying an air core inductor?

Important specifications typically include inductance, operating frequency, Q, self-resonant frequency, current, DCR, tolerance, dimensions, and mounting requirements.

What information is needed when specifying an encapsulated transformer?

Engineers should generally specify primary and secondary voltage, frequency, power rating, current, isolation requirements, dimensions, temperature conditions, environmental requirements, and applicable safety or regulatory requirements.

Conclusion

Air coil inductors and encapsulated transformers address very different engineering requirements, but both demonstrate how physical construction can significantly influence electrical performance.

Air coil inductors are particularly useful when high-frequency characteristics, Q factor, low core-related losses, and predictable RF behavior are important. Their performance depends heavily on winding geometry, frequency, parasitic capacitance, resistance, and mechanical stability.

Encapsulated transformer, on the other hand, focus on transferring electrical energy while providing additional protection around the core and windings. Encapsulation can help protect against moisture, dust, contaminants, and mechanical stress, although thermal management and insulation design remain essential.

For engineers selecting either component, the best starting point is a clear electrical and environmental specification. Looking beyond nominal inductance, voltage, or power ratings to factors such as frequency, losses, temperature, insulation, mechanical construction, and testing can lead to a more appropriate component choice for the final application.