Inductor Fundamentals.

In practice, inductors have some resistance and some 'distributed' capacitance. The capacitance means that inductors have a self-resonant frequency. Below this their reactance is inductive. At resonance the reactance is resistive. Above it the reactance is capacitive. The following diagrams illustrate the effect:

Real Inductor Equivalent Circuit

Real Inductor Frequency Response

Inductors can sometimes be referred to as chokes (when used in non value-critical applications as filtering \ signal-blocking components) or as coils (as precisely valued components for tuned circuits etc.). Coils tend to have low values of self-capacitance and high Q.

Inductance of a straight wire.

MathML formulaFormula ImageStraight text formula
L = 0.0002 b 2.303 Log 10 4 b d - 0.8 Straight Wire Inductance L = 0.0002 b [ ( 2.303 Log10 ( 4b / d ) ) - 0.8 ]


L = inductance, in µH

d = wire diameter, in mm

b = wire length, in mm

Powdered Iron Core Materials.

The following table lists various types of powdered iron material mixtures that are used for inductors:

Powdered Iron Core Materials:
Material ur Comments
0 1 Used up to 200MHz. Inductance varies with method of winding.
1 20 Made of Carbonyl C. Similar to mixture no. 3 but is more stable, and has a higher volume resistivity.
2 10 Made of Carbonyl E. High Q and good volume resistivity over range of 1 to 30MHz.
3 35 Made of Carbonyl HP. Very good stability and good Q over range of 50kHz to 500kHz.
6 8 Made of Carbonyl SF. Is similar to mixture no. 2, but has higher Q over range 20 to 50MHz.
10 6 Type W powdered iron. Good Q and high stability from 40 to 100MHz.
12 3 Made of a synthetic oxide material. Good Q but only moderate stability over the range 50 to 100MHz.
15 25 Made of Carbonyl GS6. Excellent stability and good Q over range 0.1 to 2MHz. Recommended for AM BCB and VLF applications.
17 3 Carbonyl material similar to mixture no. 12, but has greater temperature stability and lower Q.
26 75 Made of Hydrogen reduced iron. Has very high permeability. Used in EMI filters and DC chokes.

Ferrite Core Materials.

The following table lists various types of ferrite material mixtures that are used for inductors:

33850Manganese-Zinc. Used over 1kHz to 1MHz for loopstick antenna rods. Low volume resistivity.
43850Nickel-Zinc. Medium wave inductors and wideband transformers to 50MHz. High attenuation over 30 to 400MHz. High volume resistivity.
61125Nickel-Zinc. High Q over 0.2 to 15MHz. Moderate temperature stability. Used for wideband transforemers to 200MHz.
63 40High Q over 15 to 25MHz. Low permeability and high volume resistivity.
6740Nickel-Zinc. High Q operation over 10 to 80MHz. Relatively high flux density and good temperature stability. Is similar to type 63, but has lower volume resistivity. Used in wideband transformers to 200MHz.
6820Nickel-Zinc. Excellent temperature stability and high Q over 80 to 180MHz. High volume resistivity.
722000 High Q to 0.5MHz, but used in EMI filters from 0.5 to 50MHz. Low volume resistivity.
J/755000Used in pulse and wideband transformers from 1kHz to 1MHz, and in EMI filters from 0.5 to 20MHz. Low volume resistivity and low core losses.
7720000.001 to 1MHz. Used in wideband transformers and power converters, and in EMI and noise filters from 0.5 to 50MHz.
F3000Is similar to type 77 above, but offers a higher volume resistivity, higher initial permeability, and higher flux saturation density. Used for power converters and in EMI/noisefilters from 0.5 to 50MHz.

Inductor Color Code.

Some Radio Frequency chokes have their values indicated by a color code similar to that of resistors:

Inductor Color Codes