Inductors
Inductors allow electrical energy to be stored in a magnetic field. Typical applications are smoothing filters and various selective circuits.
The electrical characteristics of inductive coils are determined by their design, the properties of the material of the magnetic core and its configuration, the number of coil turns.
Below are the main factors to consider when choosing an inductor:
a) the required value of inductance (H, mH, mkГ-n. nHn),
b) maximum coil current. High current is very dangerous due to excessive heating which damages the insulation of the coils. In addition, if the current is too large, saturation of the magnetic circuit with the magnetic flux may occur, which will lead to a significant reduction in inductance,
(c) the accuracy of the inductance,
d) temperature coefficient of inductance,
e) stability determined by the dependence of the inductance on external factors,
f) active resistance of the winding wire,
g) Q-factor of the coil. It is usually defined at the operating frequency as the ratio of inductive and active resistance,
h) frequency range of the coil.
RF inductors are currently being produced for fixed frequency values with inductances from 1 μH to 10 mH. For tuning resonant circuits, it is desirable to have coils with adjustable inductance.
Single layer inductors with an open magnetic circuit are used in instrument tuning circuits.
Multilayer open magnetic circuit windings are used in filters and high frequency transformers. Armored multilayer inductors with a ferrite core are used in low- and medium-pass filters and transformers, and similar windings, but with a steel core, are used in smoothing chokes and low-pass filters.
Inductor formulas
The main approximation relationships used in the design of inductors are as follows.
1. The parameters of single-layer inductors, where the ratio of length to diameter is greater than 5, are defined as
where L — inductance, μH, M — number of turns, d — coil diameter, cm, l — winding length, see
2. The parameters of multilayer inductors, where the ratio of diameter to length is greater than 1, are defined as
where L — inductance, μH, n — the number of turns, dm — the average diameter of the coil, cm, e — thickness of the coil, see
Single and multilayer coils with an open ferrite magnetic circuit will have an inductance of 1.5 - 3 times this, depending on the properties and configuration of the core. Brass core placed instead of ferrite core. will reduce the inductance by up to 60-90% compared to its coreless value.
A ferrite core can be used to reduce the number of turns while maintaining the same inductance.
When producing coils with an inductance of 100 μH to 100 mH for low and medium frequencies, it is recommended to use core ferrite armor cores of the KM series. In this case, the magnetic circuit consists of two cups mounted side by side, to which are attached a single-section coil, two fixing brackets and an adjusting rod.
The required inductance and the number of turns can be calculated from the formulas
where N is the number of turns, L — inductance, nH, Al — coefficient of inductance, nH/vit.
You should always remember that before calculating the inductance, you need to determine the number of turns that can fit on a given coil.
The smaller the diameter of the wire, the greater the number of turns, but the greater the resistance of the wire and, of course, its heating due to the released power equal to Az2R... The effective value of the coil current should not exceed 100 mA for a wire with a diameter of 0.2 mm. 750 mA — for 0.5 mm and 4 A — for 1 mm.
Small notes and tips
The inductance of steel core windings decreases very rapidly as the DC current in the winding increases. This should be considered especially when designing power supply smoothing filters.
The maximum current of the inductor depends on the ambient temperature and allows wives to decrease as it increases. Therefore, to ensure reliable operation of the device, a large current reserve must be provided.
Ferrite toroidal cores are effective for making filters and transformers above 30 MHz. In this case, the windings consist of only a few turns.
When any type of wire is used, part of the magnetic field lines are closed not along the magnetic circuit, but through the space around it. This effect is particularly pronounced in the case of open magnetic circuits. Note that these stray magnetic fields are sources of interference, so the cores must be placed in the equipment in such a way as to reduce this interference as much as possible.
Inductors have a certain parasitic capacitance which forms an oscillating circuit in combination with the inductance of the coil. The resonant frequency of such a circuit for different types of inductors can vary from 20 kHz to 100 MHz.
