An electric coil is a fundamental type of electronic component which provides inductance in a circuit. In other words, it is an electrical feature that opposes current flow. These coils are classified according to the coil diameter, wire gauges, number of turns and core materials.

Electric coils are commonly used in manufacturing, medical instruments, and other devices that generate electricity for homes and businesses. They are meticulously designed according to their intended applications. Various electric coils are made up of a core and wiring wrapped around it; one of these is the bobbin wound coil.

Definition Of A Bobbin Wound Coil

Bobbin wound coils are created by wrapping a wire around a bobbin to keep its shape and rigidity. It makes winding assembly into or onto the magnetic core easier. The bobbin diameter varies with voltage and current levels, rating, and operating frequency.

Bobbins are rigid, pre-formed components that typically act as insulators. As long as an adequate resistive distance is used, they provide an electrical connection between the coil and the connecting core material. They also specify the coil’s characteristics and the amount of wire used. Moreover, they aid in determining the coil’s temperature rating, the device’s dielectric properties, and other coil-related features.

Most bobbins are molded, while others are fabricated. Designs for these frequently include terminals, pins, and surface mount pads for wire termination and printed circuit board mounting.

Considerations When Designing A Bobbin Wound Coil

Bobbin wound coils are the most efficient and cost-effective method of producing coils. However, it is critical to choose the most suitable bobbin to achieve the best bobbin and wire combination. The following are some of the most important factors to consider:

Material Choice

A critical material consideration is the amount of heat a plastic molded bobbin must withstand; it should be strong enough to endure soldering temperatures for a short period during production. Two types of plastics are used in bobbin molding: thermoplastic and thermoset.

Thermoplastic materials are the most commonly used in bobbin molding. When mixed with mineral, quartz, or glass fillers, thermoplastic bobbins are economical and can satisfy many heat and flammability requirements. However, these materials are more costly. Still, they provide the highest strength and temperature performance.

Although DAP and epoxy plastics are suitable for toroid mounts and potting cups, their brittleness makes them unsuitable for bobbin applications. On the other hand, phenolic plastic is ideal as a bobbin because it is stable at high temperatures.

Cost And Shape

The square stack lamination sizes E187, E24/25, E375, E21, and E42/15 have the cheapest core and bobbin combination with the greatest availability. However, such leg shapes may need to be protected with copper foil to reduce electromagnetic interference (EMI), increasing the cost.

You can also consider the newer round center leg cores and bobbin combinations. Common examples of this core style include ETDs, PQs, EFDs, and EPs made of ferrite. These recent combinations are more expensive than the square ones, but they are easier to wind and provide good EMI shielding.

Orientation Of The Bobbin And Core

Vertical mount bobbins require less board space but are taller than horizontal mount bobbins. Wire termination is also more complex, and only a few multi-section types are available.

On the flip side, horizontal mount bobbins are less visible but take up more board space. These types also make wire termination easier, and many multi-section types are available.

Safety Requirements

The requirements of the safety agencies apply to the entire process of inductor construction. These regulations address insulation and spacing to prevent thermal aging and an electrical malfunction in the winding from developing over time.

UL 1446 is the primary specification in the United States that controls the insulation requirements of these products. This standard covers motor, transformer, and coil insulation. Meanwhile, UL 510 is another essential regulation for the flammability of electronic tape, frequently used in the design of a transformer or inductor.

Tips For Insulating Bobbin Wound Coil

Coils can be insulated with various materials after being wound on a bobbin to meet your specifications. Some methods for insulating a coil are as follows:

Tape or Thread Covered

The most cost-effective method of coil insulation is to use one of the many electrical tapes on the market. These tapes are widely available and come in various materials, adhesives, and temperature ratings. Despite not providing the same level of protection as other options, tapes frequently fulfill the requirements of less stringent applications at a lower cost.

Vacuum Impregnation

Impregnation can provide many benefits of molded and encapsulated parts without the expense of dedicated tooling. Impregnants form a bond between the wire and the filler, improving heat dissipation. You can also improve the dielectric properties by selecting the right impregnated coils.

Encapsulation or Potting

Encapsulation involves enclosing the electric coils in silicone rubber, polyester, liquid, or thermoformed epoxy casing. They provide better protection from harsh weather, oil, and vibration than molded coils because they are frequently dipped or thermoformed into sealing liquid epoxies.


In overmolding or insert molding, a coil is overmolded using a synthetic material in a subsequent process step. This method protects the winding materials from harsh environmental conditions.

Endicott Coil Company For High-Quality Custom Coils And Transformers

For over 65 years, Endicott Coil Company, Inc. has been the top supplier of magnetic components to a wide range of OEMs. We have built a reputation for service and quality, allowing us to meet and surpass our customer’s needs and requirements.

We have a highly skilled engineering team that specializes in custom electric coil winding assemblies. Our expertise and versatility allow us to design and build any coil type, including air core, injection molded, impregnated, bobbin wound, and sensing coils. Endicott Coil Company, Inc. can also help with material selection, test parameters, and redesigning an existing coil design to save money.

Contact us now for more information on our custom coil services. You can also request a quote and let us make your innovative ideas a reality!

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