Inductive reactance Xl = 2(pi)fL
Q = 2(pi)fL / R
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:
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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.
Where
L=inductance, in uH
Calculations to give the inductance of a single-layer close-wound air-cored coil are extremely complex, but a good approximation is given by the following formula provided that these assumptions are adhered to:
The formula is often called Wheeler's Formula
Where
L=inductance, in uHThe following table lists various types of powdered iron material mixtures that are used for inductors:
| 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. |
The following table lists various types of ferrite material mixtures that are used for inductors:
| Material | ur | Comments |
|---|---|---|
| 33 | 850 | Manganese-Zinc. Used over 1kHz to 1MHz for loopstick antenna rods. Low volume resistivity. |
| 43 | 850 | Nickel-Zinc. Medium wave inductors and wideband transformers to 50MHz. High attenuation over 30 to 400MHz. High volume resistivity. |
| 61 | 125 | Nickel-Zinc. High Q over 0.2 to 15MHz. Moderate temperature stability. Used for wideband transforemers to 200MHz. |
| 63 | 40 | High Q over 15 to 25MHz. Low permeability and high volume resistivity. |
| 67 | 40 | Nickel-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. |
| 68 | 20 | Nickel-Zinc. Excellent temperature stability and high Q over 80 to 180MHz. High volume resistivity. |
| 72 | 2000 | High Q to 0.5MHz, but used in EMI filters from 0.5 to 50MHz. Low volume resistivity. |
| J/75 | 5000 | Used 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. |
| 77 | 2000 | 0.001 to 1MHz. Used in wideband transformers and power converters, and in EMI and noise filters from 0.5 to 50MHz. |
| F | 3000 | Is 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. |
Some Radio Frequency chokes have their values indicated by a color code similar to that of resistors:
