Toroidal inductors are very popular these days. A primary advantage of using a toroidal core instead of a solenoidal core in an inductor is that toroidal cores confine most of the magnetic field within the core material. (E6D10)
Another reason for their popularity is the frequency range over which you can use them. The usable frequency range of inductors that use toroidal cores, assuming a correct selection of core material for the frequency being used is from less than 20 Hz to approximately 300 MHz. (E6D07) Ferrite beads are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of transistorized HF amplifiers. (E6D09)
An important characteristic of a toroid core is its permeability. Permeability is the core material property that determines the inductance of a toroidal inductor. (E6D06)
One important reason for using powdered-iron toroids rather than ferrite toroids in an inductor is that powdered-iron toroids generally maintain their characteristics at higher currents. (E6D08) One reason for using ferrite toroids rather than powdered-iron toroids in an inductor is that ferrite toroids generally require fewer turns to produce a given inductance value. (E6D16)
To calculate the inductance of a ferrite-core toroid, we need the inductance index of the core material. The formula that we use to calculate the inductance of a ferrite-core toroid inductor is:
L = AL×N2/1,000,000
where L = inductance in microhenries, AL = inductance index in µH per 1000 turns, and N = number of turns
We can solve for N to get the following formula:
N = 1000 x sqrt (L/AL)
Using that equation, we see that 43 turns will be required to produce a 1-mH inductor using a ferrite toroidal core that has an inductance index (AL) value of 523 millihenrys/1000 turns. (E6D11)
N = 1000 x sqrt (1/523) = 1000 x .0437 = 43.7 turns
The formula for calculating the inductance of a powdered-iron core toroid inductor is:
L = AL×N2/10,000
where L = inductance in microhenries, AL = inductance index in µH per 1000 turns, and N = number of turns
We can solve for N to get the following formula:
N = 100 x sqrt (L/AL)
Using that equation, we calculate that 35 turns will be required to produce a 5-microhenry inductor using a powdered-iron toroidal core that has an inductance index (AL) value of 40 microhenrys/100 turns. (E6D12)
N = 100 x sqrt (5/40) = 100 x .353 = 35.3 turns
I think you have a typo on this page about the toroids and inductance index.
In particular, the line “N = 1000 x sqrt (5/40) = 100 x .353 = 35.3 turns”
should read: N = 100 x sqrt (5/40) = 100 x .353 = 35.3 turns
Ooops. Thanks for finding that. In addition to that typo, I noticed that in the paragraph above, I repeated the word “turns,” so I fixed that, too.
Shouldn’t this also say “AL = inductance index in µH per 100 turns”?
The formula for calculating the inductance of a powdered-iron core toroid inductor is:
L = AL×N2/10,000
where L = inductance in microhenries, AL = inductance index in µH per 1000 turns, and N = number of turns
Yes. You’re right! I’ll this to the errata and correct it in the next version. Thanks.
Is there an easy way of identifying what a core is? the mix etc? Or should I just buy from Amidon or other known source what I need? The reason I ask is that I have many cores laying around that might fit the boat but at this time they are unknown.
I think that there’s some kind of color code, but I’m not sure it’s an industry-standard kind of thing. One thing you might do is to wind ten or 20 turns on the core and then measure the inductance. You could perhaps work backwards from that to figure out what you have.
Thanks for explaining toroid characteristics. I’ve spent hours searching for this info and today, here it is!
L = AL×N2/1,000,000 should be L = AL×N2/1,000 ?