Published:2011/7/25 2:52:00 Author:Li xiao na From:SeekIC
By B. Kainka
Some practical examples
Suppose you want to wind a 330-/JH inductor for a medium-wave detector radio on a cardboard roll with a diameter of 42 mm. Further suppose that the wire diameter is 0.5 mm, so 100 turns will yield a coil length of 50 mm. Now you can simply try several different values, which ultimately yields a result of approximately 80 turns. For tuning the medium-wave (MW) band starting at 530 kHz, the variable capacitor must have a maximum capacitance of at least 45 pF.
For higher frequencies, you will need fewer turns. An inductor for a VHF FM receiver, for example, will have only five turns, with D = 8 mm and 1 = 10 mm. The calculated inductance is 0.16 μH. With a 20-pF capacitor, this inductor will resonate at 88.9 MHz, which is almost exactly the lower limit of the VHF FM broadcast band.
The above examples use air-core inductors. But how can you use a ferrite core? Usually, you won’t have any exact data for the core. You will thus have to estimate how much it will increase the inductance or reduce the frequency. An inductor for the short-wave band, for example, might have n = 18 turns, D = 8 mm and 1 = 12 mm. For a pure air-core inductor, this gives a calculated inductance of 1.7μH. But with a 275-pF variable capacitor, this inductor achieves a lower frequency limit of approximately 5 MHz with the core fully threaded in, which corresponds to an inductance of approximately 3.7μH.
The frequency can thus be reduced by a factor of two using a threaded core, and the inductance can be up to four times as large. A relatively long medium-wave ferrite rod can in turn increase the inductance by a factor of approximately ten. Roughly speaking, we can say that an inductor on a ferrite rod only needs to have approximately one third as many turns as a similarly dimensioned air-core inductor having the same inductance.
The resonant frequency of a resonant circuit can change considerably when it is built into a circuit. Particularly at relatively high frequencies, wring capacitances have a significant effect. This means that it is often necessary to make adjustments after assembly or build tuning capability into the circuit by means of a threaded core or trimmer capacitor.
For major modifications, it is often helpful to use a few rules of thumb that can be directly derived from the formulas give above and simulated using the LCFR program. For instance, doubling the number of turns quadruples the inductance and cuts the frequency in half if the capacitance remains the same. The frequency is thus inversely proportional to the number of turns and inversely proportional to square of the capacitance. This means that twice the frequency can be attained with one fourth of the capacitance. In order to tune over a frequency range of 1:3 using a variable capacitor, you need a capacitor with a capacity ratio of at least 1:9.
Inductors are not necessarily limited to RF circuits. They are also used in interference filters, low-frequency/audio filters and voltage converters. Schematic diagrams often show only the inductance value, without any other data for the inductor. Particularly in blocking-type voltage converters, the saturation current level and resistance of the inductor are also important factors.
It is also certainly possible to use a fixed inductor with the correct inductance but still not obtain the optimum result. Consequently, it is often worthwhile to wind your own inductors, even if only for initial testing. For instance, a 1.5-mH inductor for a voltage converter can be wound on a ferrite rod from an old medium-wave radio. If you have a relatively small ferrite rod that originally had 100 turns (which can be easily counted when you unwind the coil), it must have had an inductance of 300 /JH, since the commonly used variable capacitors have a maximum capacitance of approximately 300 pF. You can thus calculate an AL value of 30 nH/n2. From here it’s only a small step to the desired result: you will have to wind approximately 220 turns on the rod to obtain 1.5 mH.
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