There are two common types of toroidal inductors- Powdered-iron toroidal inductors and ferrite toroidal inductors. Here these two types of toroidal inductors are explained.
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Powdered-iron toroidal inductors
Powdered-iron toroidal inductors are made by mixing powered iron(carbonyl irons and hydrogen-reduced irons) with insulating material binder so that the individual iron particle are insulated from each other. This compound are formed into circular or doughnut shaped structure having magnetic property and such structure are called powdered iron toroidal inductors. The iron is powered and mixed with binder material to reduce iron loss which appears when iron core inductors are excited by DC and AC sources. Permeability of powered iron core inductors ranges from 1 to 40 for various micro-metal mixes. The core size is varying, from outer diameter 0.125 inch to 1.06 inch and even reaching outer diameter size of 5 inch for certain mix. Iron powered toroidal inductors can have Q well above 100 and can even reach 200.
Powdered-iron toroidal inductors come in different colors and number system(mix number) depending upon the core material composition and intended frequency of operation. The following table shows the color and mix number relating to the frequency of operation.
Different color code or mix number have different material composition and hence different magnetic permeability. The following table shows the different color and mix number and their corresponding permeability value and material composition.
The following table shows the application area for the different color and mix number.
Powered Iron core inductor are specified as T-xx-yy where xx is the toroid outer diameter in inch and yy is the mix type. The following table shows powered iron core inductor with dimensions.
The different dimension size of the toroid have corresponding inductance index(AL) value which is shown in the table below.
Ferrite Toroidal Inductors
Ferrite material are made by chemically combining of iron oxide(rust) and metal elements like nickel, zinc, manganese, aluminum, barium, cobalt, copper etc. They are poly-crystalline structurally, hard and brittle physically and generally black and gray in color. These material have magnetic property and when such material are formed into circular ring or doughnut shaped they are called ferrite toroidal inductors. The manganese- zinc ferrites inductors have higher permeability ranging from from 20 to above 10000. The nickel-zinc ferrites inductors which are used commonly as RF chokes and wideband transformers have lower permeability ranging from 20 to 800. The inductance factor or inductance index, A
L, are higher than that of iron powered toroidal inductors. The inductance factor is specified by manufacturer.
Ferrite core toroidal inductors dimension is specified as FT-xx-yy where xx is the rounded off outer diameter in inch and yy is the material mix type.The type is often specified with letters. The following table shows material type identified with letters, their correspond initial permeability and material composition.
The following table shows other characteristics of ferrite core such as magnetic properties, resistance, temperature, frequency etc.
The following table shows ferrite core material number, the corresponding permeability and application area.
The following table shows inductance index(AL) in terms of mH per 1,000 turns for various ferrite core inductor dimension and mixes.
Below is another ferrite core inductor inductance index table for common size.
See also How to derive Inductance Formula of a Solenoid.
