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Everything You Need To Know About Coated & Uncoated Toroidal Cores

Coated & Uncoated Toroidal Cores

Toroidal cores are used as power shapes in electric circuits such as inductors and transformers. They are an ideal material to use at low frequencies where large inductances are required. The cores offer the advantage of material permeability and have a uniform cross-sectional area. They do not have any gaps, so the leakage is very low. This is why the toroidal core windings are commonly used in current and instrument transformers

In modern technology, toroids are available as coated and uncoated to serve a range of applications. In this blog, we’re going to learn about coated and uncoated ferrite toroids, and how one stands out from the other. So, let’s look further to know. 

Benefits of using Ferrite Toroids

Before we jump on to the two categories of Ferrite Toroids, it’s important to understand more information on what toroids are. Toroids are widely used as power shapes in electric circuits for the following benefits:

  • Cooler operating temperature 
  • Lower magnetizing current
  • Lesser noise 
  • Lower operating temperature
  • Higher efficiency

Applications of Toroidal Cores

  • Security system 
  • Power distribution equipment
  • Telecommunication system 
  • Pulse transformer 
  • Energy meters for current sensing  
  • EMI and EMC 
  • Medical equipment 

Coated Toroids

coated Toroidscoated Toroids

Ferrite Toroid cores are coated to provide not only a smoother corner radius but also a winding surface. In these ferrite cores, a coating is beneficial because it provides additional edge coverage. Not only protection for the edges, but the coating also provides an insulation function. 

Epoxy paint is used for coating the toroid cores. They are usually available in green, blue & gray colors with CFR. Epoxy coating is UL approved and the UL file number is E483791.

The coating material for the product is in compliance with RoHS directive 2011/65/EU.

Parylene coating can be used for small ferrite rings that have a low coating thickness and high dielectric strength. 

Coating of the cores causes the initial permeability (μi) to drop depending on the size of the cores. This may also happen when the cores are subjected to high winding forces or have high permeability. 

Benefits of Coated Toroids

  • Ferrite toroids are compatible with different types of coatings including parylene, epoxy and powder coatings; these increase winding ease and improve voltage breakdown.
  • Epoxy coatings can work up to 200 degrees Celsius.
  • The coating not only offers protection for the edges, but also provides an insulation function to the cores.
  • The coating of ferrite toroids is necessary to create an insulation barrier between wire and ferrite cores (direct winding on the cores) to avoid a short circuit.
  • Coating doesn’t influence the AL value of toroids 

A toroidal core coated with epoxy finishing offers the benefits of:

  • Moisture resistance
  • Chemical resistance
  • Durability
  • Strength
  • Strong dielectric properties

Applications of Coated Toroid Cores

  • EMI Filters
  • Energy Meters
  • Inverters
  • Amplifiers
  • Common mode chokes
  • Wide Bands
  • Symmetrical transducers or as chokes.

Dielectric Strength of Coated Cores

Equipment Required: High voltage breakdown tester, test fixture, and test toroid.

The dielectric strength of coated toroids can be tested as follows:

1. Toroidal cores are to be placed in between plates A and B (test fixtures as per geometry size) as shown in fig. 1

2. Ground connection to be given in terminal B.

3. A plate to be connected to high voltage.

4. Start increasing the voltage from zero to the maximum specified voltage given in Table 1. The specified voltage to be applied for 2 seconds.

Coated & Uncoated Toroids

5. Specified RMS voltage for different toroids is as below:

Coated & Uncoated Toroids

Uncoated Toroids

Uncoated toroids are subject to surface oxidation. These cores must be protected from high humidity and moisture. They have to be handled with gloves to avoid the formation of surface blemishes in the hands. However, surface oxidation or discoloring of the cores is artificial, so it doesn’t affect their performance. 

Coated & Uncoated Toroidal Cores

Differences between Coated & Uncoated Toroids

Coated Cores

Un-Coated Cores

 Coating provides ready & safe insulation to the cores. 

 Taping is required for insulation in uncoated cores which is an extra operation before winding.

Coated toroids increase winding ease. 

Winding needs to be done with a lot of precautions so that wire breaks on the edges. 

Coating improves the high voltage breakdown of the core. 

No protection of high voltage breakdown. 

Coated cores can work at temperatures up to 200 degrees Celsius.

Un-coated cores can work at temperatures up to 150 degrees Celsius. 

Coated cores are chemical & moisture resistant. 

Chemical and moisture resistance is bad in uncoated cores.


Ferrite Toroids are the right choice of material for your electrical needs. Cosmo Ferrites, the leading manufacturer and supplier of soft ferrites, produces both coated and uncoated toroidal cores. If you have any queries, you can contact our experts to learn about the right toroids for your applications. You can also request a free sample. Click here to connect with us.