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 = A_{L}×N^{2}/1,000,000

where L = inductance in microhenries, A_{L} = 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/A_{L})

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 (A L) 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 = A_{L}×N^{2}/10,000

where L = inductance in microhenries, A_{L} = 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/A_{L})

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 (A L) value of 40 microhenrys/100 turns. (E6D12)

N = 100 x sqrt (5/40) = 100 x .353 = 35.3 turns

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