INDUCTORS AND TRANSFORMERS
In this lesson, we will consider such radio components as inductors and transformers. You will see later that almost no modern radio engineering device is complete without them. I have tried to combine the explanation of inductors and AC transformers in one lesson simply because these components have a lot in common, and therefore it is easier to consider them by analogy with each other. The practical work will be very interesting and very useful, namely, building a simple laboratory power supply (PSU) using an adjustable parametric regulator. From previous lessons, you've probably noticed that using batteries is not entirely convenient, and a battery with the right voltage is not always at hand. That is why we will build a universal power supply with an adjustable voltage from 0.5 to 12V. For a start, this will be quite enough for us.
Inductors of oscillatory circuits
Inductors (coils) have the property of providing reactance to alternating current with little resistance to direct current.
Together with capacitors, they are used to create filters that perform frequency selection (the ability to isolate and filter out) of electrical signals, as well as to create signal delay elements and memory elements, provide coupling between circuits via magnetic flux, etc.
Unlike resistors and capacitors, they are not standardized products but are manufactured for specific purposes and have parameters necessary to implement certain conversions of electrical signals, currents, and voltages. The operation of inductors is based on the interaction of current and magnetic flux. It is known that when the magnetic flux changes in a conductor located in a magnetic field, an electromotive force (EMF) is generated, determined by the rate of change of the magnetic flux.
In the oscillatory circuits of receivers, radio amateurs usually use both factory-made and homemade coils of various designs. For winding coils, in addition to standard enameled magnet wire (like PEV or PEL), winding wires with various types of insulation are used: PVO (wire in a single cotton braid); SHLO (wire in a single silk braid); PSHD (the same in a double braid); PELSHO (wire with enamel varnish-resistant insulation and a single silk braid). Many coils in industrial devices are wound with the so-called litz wire — individually enameled wires twisted into a bundle and all together having a single or double silk braid. Such a wire, if necessary, can be twisted by yourself using a drill.
Practically any brand of wire is suitable for oscillatory circuit coils of homemade receivers, as long as its insulation is reliable, but it shouldn't be too thick, otherwise the coil will turn out bulky. Coils intended for receiving broadcast stations in the medium-wave and long-wave bands are usually wound with wire with a diameter of 0.1 to 0.3 mm, short-wave — with 0.8-1 mm wire, and ultra-short-wave — with wire up to 3 mm.
There is a rule that must be remembered: the shorter the wavelength for which the coil is designed, the thicker the wire it should be wound with.
If there is a wire whose diameter is unknown, it can be approximately determined as follows: wind the wire turn to turn on a pencil, and then divide the winding length by the number of turns. The accuracy of determining the wire diameter using this method will be higher the more turns are wound. If there is no wire of the recommended diameter, but there is another one of a close diameter, it can usually be used. For example, instead of a wire with a diameter of 0.18 mm, you can use a wire with a diameter of 0.15 or 0.2 mm. Depending on the size of the formers and the range of received radio waves, the coils contain from a few turns to several hundred turns.
The longer the radio waves and the smaller the coil diameter, the more turns it must contain.
For crystal receivers (the structure and principle of operation of a crystal receiver will be discussed later), single-layer coils wound on large formers with relatively thick wire are sometimes recommended. And this is no coincidence; such coils have lower losses of high-frequency energy. And the lower these losses, the better the receiver works.
Coils for transistor and tube receivers are most often wound on relatively small formers and with thinner wire than coils for crystal receivers. At the same time, the wire in long-wave coils is laid in several layers. These are multi-layer coils. They are more compact than single-layer ones. High-frequency energy losses in such coils are somewhat greater than in large coils, but they are compensated by the introduction of high-frequency cores into the coils, and by the amplifying properties of transistors and radio tubes.
Multi-layer coils of many industrial receivers' circuits are wound in a special way called universal winding. With such a winding, having the proper mutual intersection of turns, the internal (inter-turn) capacitance of the coil decreases, which increases the frequency range coverage of the circuit. Radio amateurs wind similar coils on paper or cardboard spools randomly (bulk winding), intentionally not laying the wire in even rows. With such winding, the internal capacitance of the coil is also relatively small.
As an example, I will tell you how to make an oscillatory circuit coil of a similar design, which can be used for the simplest transistor or tube radio receiver (Fig. 1). The former is a cardboard tube 18 - 20 mm in diameter, glued from thick paper. The coil itself consists of two sections: L2 — the main one and L1 — the tuning one. The side walls of section L2 are cardboard circles put on the former and glued to it. The outer diameter of the circles is 32 - 35 mm, the inner one fits the former's diameter, and the distance between them is 4 - 5 mm. Section L1 is wound on a spool that can move along the former with slight friction.
The spool for it is made like this. Wrap the former with a strip of thick paper 6 - 8 mm wide. Put cardboard circles on top of the strip on the former, placing them at a distance of 2 - 3 mm from each other. Without shifting the circles, glue them to the paper ring. When the glue dries, carefully cut off the outward-protruding edges of the paper ring — you will get a spool.
Wire with a diameter of 0.2 - 0.3 mm with any insulation is suitable for the coil sections. Section L1 should contain 40 - 50 turns, bulk-wound, and section L2 — 250 - 260 turns, wound in the same way, but with taps from the 50th and 150th turns. The taps are needed for rough tuning of the circuit in which the coils will work. Bring the leads and taps out through punctures in the cardboard sides. Connect the end of section L1 to the beginning of section L2.
The inductance of such a coil depends on the mutual arrangement of its sections. If the turns of both sections are directed in one direction and section L1 is pushed closely to section L2, the inductance of the coil is at its maximum. In this case, the circuit will be tuned to the lowest frequency (the longest wavelength). As section L1 moves away from L2, the total inductance of the coil will begin to decrease, and the receiver will be tuned to a higher frequency (a shorter wave).
Section L1 can be removed from the former, turned over, and put back on the former with the other side. Now the turns of the coil sections will be directed in opposite directions, and if you bring them closer, the coil's inductance will smoothly decrease, and the circuit will tune to stations operating on shorter wavelengths. Thus, this design is a simple variometer — a variable inductance coil. Rough tuning of the circuit is done by switching the taps of the section, and fine tuning is done by changing the distance and position of the turns of section L1 relative to the turns of section L2. After tuning the circuit to a radio station, you can glue the spool of section L1 to the former — you will get a receiver with a fixed tuning to one radio station.
Coils of such designs are good because they are simple. However, coils with high-frequency cores are preferable. A core that increases the quality factor (Q-factor) of the coil and thereby reduces losses in it, allows you to significantly reduce the number of turns and the dimensions of the coil. And if the core is tunable, i.e., can move inside the coil, then it additionally allows you to change the coil's inductance within certain limits and, thus, tune the circuit to the desired frequency. The most common magnetic high-frequency cores are ferrite and carbonyl cores. They are made in the form of rods, rings, and cups.
One of the possible designs of a homemade sectioned coil with a tuning core 9 mm in diameter is shown in (Fig. 2).
An increase in the coil's inductance is achieved by screwing the core into its former, and a decrease — by unscrewing it.
The former for such a coil is glued from a strip of thick paper 40 mm wide on a round blank, glass tube, or test tube 9.5 - 10 mm in diameter. At a distance of 6 - 7 mm from the upper edge of the finished and well-dried former, rectangular holes are cut into it with a sharp knife on two opposite sides. In the places of the cutouts, the former is wrapped in one layer with thick thread; its turns will serve as a thread for screwing in the core. The coil cheeks are cut out of thin getinax, textolite, or thick cardboard 0.3 - 0.5 mm thick, placed on the former, and glued to it.
The coil is bulk-wound with standard enameled wire 0.12 - 0.18 mm in diameter. If the coil is for medium-wave, it should contain a total of 135 turns (three sections of 45 turns), and for long-wave — 450 turns (three sections of 150 turns).
Fig. 3 Medium-wave (a) and long-wave (b) coils with a ferrite rod.
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First, the first section is wound between the two upper cheeks, the wire is moved to the section between the middle cheeks, and the second section is wound, then the third section is wound between the lower cheeks. The coil leads are passed through punctures in the cheeks. You can mount such a coil on the receiver panel using a plywood ring glued to the panel, or by gluing the lower end of the former into a hole in the panel.
The oscillatory circuit coil can be wound on a paper sleeve and slipped onto a piece of a 400NN or 600NN ferrite rod 8 mm in diameter and 25 - 30 mm long (Fig. 3). For receiving medium-wave radio stations, it should contain 70 - 80 turns of wire 0.12 - 0.2 mm in diameter wound in a single layer, and for long-wave radio stations — 225 - 250 turns of the same wire, but wound in four to five sections of 45 - 50 turns in each section.
The maximum inductance of such a coil will occur when it is in the middle of the ferrite rod.
As it moves towards one of the ends of the rod, the coil's inductance decreases. Thus, by moving the coil along the rod, you can tune the circuit to the required frequency of the most long-wave part of the band.
Many industrial receivers use coils wound on unified (standard) plastic sectioned formers with ferrite rings and rod tuning cores (Fig. 4, a). A coil wound on such a former sits between two ferrite rings that increase its inductance. The rod core, fastened to a threaded cylinder, can be screwed with a screwdriver (the screwdriver must be made of non-magnetic material) to different depths inside the former, thereby tuning the coil's inductance.
A similar homemade former that can be used for coils for various purposes is shown in (Fig. 4, b). To make it, you need two rings made of 600NN brand ferrite with an outer diameter of 8 - 9 mm and an inner diameter of 3 - 3.5 mm, and a tuning rod core of the same brand with a diameter of 2.7 mm and a length of 15 mm. The base of the former is a paper sleeve 12 mm long with a diameter equal to the inner diameter of the rings. The rings are glued to the sleeve at a distance of 6 mm. The lower protruding end of the sleeve will be inserted into a hole in the circuit board (or chassis) and glued to it. The tuning core is held inside the former by a paper or cloth gasket. The number of turns and wire for a coil wound on such a former depend on its purpose.
Transformers — AC transformation
Alternating current compares favorably with direct current in that it is easily transformed, i.e., converting a relatively high voltage current into a lower voltage current, or vice versa. Transformers allow transmitting alternating current over wires over long distances with small energy losses. To do this, the alternating voltage generated at power plants by generators is stepped up using transformers to a voltage of several hundred thousand volts and sent along power transmission lines (PTL) in various directions. By increasing the voltage, the current in the PTL decreases for the same transmitted power, which leads to reduced losses and allows the use of smaller cross-section wires. In cities and villages hundreds and thousands of kilometers away from power plants, this voltage is stepped down by transformers to a lower voltage, which is used to power lighting bulbs, electric motors, and other electrical appliances.
Transformers are widely used in radio engineering.
The operation of a transformer is based on the phenomenon of electromagnetic induction. An alternating current flowing through one of the transformer windings creates an alternating magnetic field around it and in the magnetic core. This field intersects the turns of the other transformer winding, inducing an alternating voltage of the same frequency in it.
If a load, such as an incandescent lamp, is connected to this winding, an alternating current will flow in the resulting closed circuit — the lamp will light up.
The winding to which the AC voltage intended for transformation is applied is called the primary winding, and the winding in which the AC voltage is induced is called the secondary winding.
The schematic structure of the simplest transformer is shown in (Fig. 5). It consists of two coils of insulated wire, called windings, mounted on a magnetic core assembled from plates of special, so-called transformer steel. Transformer windings are depicted on diagrams in the same way as inductors, and the magnetic core is depicted as a straight line between them.
The voltage obtained at the ends of the secondary winding depends on the ratio of the number of turns in the windings.
With an equal number of turns, the voltage on the secondary winding is approximately equal to the voltage applied to the primary winding. If the secondary winding contains fewer turns than the primary, then its voltage is less than the voltage applied to the primary winding. Conversely, if the secondary winding contains more turns than the primary, the voltage developed in it will be greater than the voltage applied to the primary winding.
In the first case, the transformer will step down the AC voltage, and in the second case, it will step it up.
The voltage induced in the secondary winding can be fairly accurately calculated from the ratio of the number of turns of the transformer windings: by however many times it has a larger (or smaller) number of turns compared to the number of turns of the primary winding, by that same amount the voltage across it will be greater (or smaller) compared to the voltage applied to the primary winding.
So, for example, if one transformer winding has 1000 turns and the second has 2000 turns, then by connecting the first winding to an AC network with a voltage of 220 V, we will get a voltage of 440 V in the second winding — this is a step-up transformer. If the 220 V voltage is applied to the winding with 2000 turns, then in the winding containing 1000 turns, we will get a voltage of 110 V — this is a step-down transformer. The winding with 2000 turns will be the secondary in the first case, and the primary in the second case.
But when using a transformer, you must not forget that the current power (P = UI) that can be obtained in the secondary winding circuit never exceeds the power of the primary winding. This means that to obtain the same power from the secondary winding, you can either step up the voltage and decrease the current, or consume a stepped-down voltage from it at an increased current. Consequently, by increasing the voltage we lose in current value, and by winning in current value, we necessarily lose in voltage.
For powering radio equipment from the AC mains, transformers with several secondary windings with a different number of turns are often used (Fig. 6).
With the help of such transformers, called mains or power transformers, several voltages are obtained to power different circuits. The maximum current power that can be transformed depends on the size of the transformer's magnetic core and the diameter of the wire from which the windings are made. The larger the volume of the magnetic core, the greater the power that can be transformed. In practice, however, a portion of the power is always uselessly lost in a transformer. Therefore, the power in the secondary winding circuit (or the sum of powers obtained from all secondary windings) is always slightly less than the power consumed by the primary winding.
Remember: transformers do not transform direct current (DC).
If, however, a pulsating current flows in the primary winding of a transformer, an alternating voltage will be induced in the secondary winding, the frequency of which is equal to the frequency of current pulsations in the primary winding. This property of a transformer is used for inductive coupling between different circuits, dividing a pulsating current into its components, and several other purposes, which we will discuss later.
All transformers with steel magnetic cores and magnetic cores made of iron-nickel alloys (permalloy) are called low-frequency (audio-frequency) transformers, as they are only suitable for converting AC voltage in the low-frequency range. On diagrams, low-frequency transformers are denoted by the letter T, and their windings by Roman numerals.
The operating principle of high-frequency (radio-frequency) transformers, intended for the transformation of high-frequency oscillations, is also based on electromagnetic induction.
They can be with or without cores. Their windings (coils) are placed on the same or different formers, but necessarily close to one another (Fig. 7). When a high-frequency current appears in one of the coils, an alternating magnetic field arises around it, which induces a voltage of the same frequency in the second coil. As in low-frequency transformers, the voltage in the secondary coil depends on the ratio of the number of turns in the coils.
| Fig. 7 High-frequency transformers without cores (left - transformer coils with a common former; right - transformer coils on separate formers; center - schematic symbol). | Fig. 8 High-frequency transformers with magneto-dielectric cores (left - with a rod core, right - with a ring (toroidal) core). |
To strengthen the coupling between coils in high-frequency transformers, cores in the form of rods or rings (Fig. 8) are used, which are a pressed mass of non-metallic materials. They are called magneto-dielectric or high-frequency cores. Ferrite cores are the most common. A ferrite core not only strengthens the coupling between coils but also increases their inductance, so they can have fewer turns compared to the coils of a coreless transformer. A magneto-dielectric core of a high-frequency transformer, regardless of its design and shape, is denoted on diagrams in the same way as the magnetic core of a low-frequency transformer — by a straight line between the coils, and the windings, like inductors, by the Latin letter (L).
Moving on to the next lesson!
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