Making a coil for a pulse metal detector with your own hands. How to check the ignition coil (reel) on a car What we came up with

In the manufacture of metal detectors of any type, special attention should be paid to the quality of the search coil (coils) and its fine tuning to the operating frequency of the search. The detection range and the stability of the generation frequency strongly depend on this. It often happens that with a correct and fully functional circuit, the frequency “floats”, which, of course, can also be explained by the temperature instability of the elements used (mainly capacitors). I personally assembled more than a dozen different metal detectors, and in practice, the temperature stability of passive elements still does not provide guaranteed frequency stability if the search coil itself is made carelessly and its fine tuning to the operating frequency is not ensured. Next will be given practical advice for the production of high-quality coil-sensors and their adjustment for single-coil metal detectors.

Making a Good Coil

Usually, metal detector coils are wound in bulk on some kind of mandrel - a saucepan, a can, etc. suitable diameter. Then wrap with electrical tape, shielding foil and again with electrical tape. Such coils do not have the necessary structural rigidity and stability, are very sensitive to the slightest deformation and change the frequency greatly even with simple finger pressure! A metal detector with such a coil will have to be adjusted every now and then, and from the knob-regulator your fingers will constantly be in big sore corns :). It is often recommended to “fill in epoxy” with such a coil, but where should you fill it, epoxy, if the coil is frameless? .. I can offer a simple and easy way to make a quality coil, sealed and resistant to all kinds external influences, which has sufficient structural rigidity and, moreover, provides simple fastening to a rod-rod without any brackets.

For the coil frame, you can make it using a plastic box (cable channel) of a suitable section. For example, for 80 - 100 turns of wire with a cross section of 0.3 ... 0.5 mm, a box with a cross section of 15 X 10 or less is quite suitable, depending on the cross section of your particular wire for winding. A single-core copper wire for low-current electrical circuits is suitable as a winding wire; it is sold in coils, such as CQR, KSPV, etc. This is a bare copper wire in PVC insulation. The cable can contain from 2 or more single-core wires with a cross section of 0.3 ... 0.5 mm in insulation different colors. We remove the outer sheath of the cable and get a few necessary wires. Such a wire is convenient in that it excludes the possibility of a short circuit of the turns in case of poor-quality insulation (as in the case of a wire with varnish insulation of the PEL or PEV brands, where minor damage to it is not visible to the eye). To determine how long the wire should be to wind the coil, you need to multiply the circumference of the coil by the number of its turns and leave a small margin for the conclusions. If there is no piece of wire of the required length, you can make a winding of several pieces of wire, the ends of which are well soldered to each other and carefully insulated with tape or heat shrink tubing.

Remove the cover from cable channel and cut the side walls with a sharp knife after 1 ... 2 cm:

After that, the cable channel can easily go around the cylindrical surface of the required diameter (jar, pan, etc.), corresponding to the diameter of the metal detector coil. The ends of the cable channel are glued together and a cylindrical frame with sides is obtained. It is easy to wind the required number of turns of wire on such a frame and coat them, for example, with varnish, epoxy, or fill everything with sealant.

From above, the frame with the wire is closed with a cable channel cover. If the sides of this cover are low (this depends on the size and type of the box), then side cuts on it can be omitted, because it already bends quite well. The output ends of the coil are brought out next to each other.

This results in a sealed coil with good structural rigidity. All sharp edges, protrusions and irregularities of the cable channel should be leveled with sandpaper or wrapped with a layer of electrical tape.

After checking the coil for operability (this can be done by connecting the coil even without a screen to your metal detector by the presence of generation), filling it with glue or sealant and machining the irregularities, you should make a screen. To do this, take foil from electrolytic capacitors or food foil from the store, which is cut into strips 1.5 ... 2 cm wide. The foil is wound around the coil tightly, without gaps, overlapping. Between the ends of the foil in the place of the coil leads must be left gap 1 ... 1.5 cm , otherwise it will form short-circuited coil and the coil will not work. The ends of the foil should be secured with glue. Then, from above, the foil is wrapped along the entire length with any tinned wire (without insulation) in a spiral, in increments of about 1 cm. The wire must be tinned, otherwise incompatible metal contact (aluminum-copper) may occur. One of the ends of this wire will be the common wire of the coil (GND).

Then the whole coil is wrapped with two or three layers of electrical tape to protect the foil screen from mechanical damage.

Tuning the coil to the desired frequency consists in the selection of capacitors, which, together with the coil, form an oscillatory circuit:

The actual inductance of the coil, as a rule, does not correspond to its calculated value, so the desired circuit frequency can be achieved by selecting the appropriate capacitors. To facilitate the selection of these capacitors, it is convenient to make the so-called "capacity store". To do this, you can take a suitable switch, for example, type P2K for 5 ... 10 buttons (or several such switches with fewer buttons), with dependent or independent fixation (anyway, the main thing is to be able to turn on several buttons at the same time). The more buttons on your switch, the correspondingly large quantity containers can be included in the "shop". The scheme is simple and is shown below. The entire installation is hinged, the capacitors are soldered directly to the button terminals.

Here is an example for selecting capacitors series resonant circuit (two capacitors + coil) with capacitances of about 5600 pF. By switching the buttons, you can use different capacities indicated on the corresponding button. In addition, by turning on several buttons at the same time, you can get the total capacity. For example, if you simultaneously press buttons 3 and 4, we get total capacitances of 5610 pF (5100 + 510), and when you press 3 and 5 - 5950 pF (5100 + 850). Thus, it is possible to create the necessary set of capacitances for the exact selection of the desired loop tuning frequency. You need to choose the capacitor capacities in the "capacitance store" based on the values ​​\u200b\u200bthat are given in your metal detector circuit. In the example given here, the capacitances of the capacitors according to the circuit are 5600pF. Therefore, the first thing included in the "shop" is, of course, these containers. Well, then take containers with lower ratings (4700, 4300, 3900 pF for example), and very small ones (100, 300, 470, 1000 pF) for a more accurate selection. Thus, by simply switching the buttons and their combination, you can get a very wide range of capacitances and tune the coil to the required frequency. Well, then it remains only to pick up capacitors with a capacity equal to the one you got as a result on the “capacity store”. Capacitors with such a capacity should be placed in the working circuit. It should be borne in mind that when selecting containers, the “store” itself must be connected to a metal detector exactly with the wire / cable that will be used in the future, and the wires connecting the “store” to the coil must be made as short as possible! Because all wires also have their own capacitance.

For a parallel circuit (one capacitor + coil) it will be enough to use in the “store”, respectively, one capacitor for each rating. After selecting them, it is better to solder the capacitors directly to the coil leads, for which it is convenient to make a small mounting plate from foil textolite and fix it on the rod next to the coil or on the coil itself:

Discuss the article METAL DETECTORS: ABOUT COILS

The standard design of an inductor consists of an insulated wire with one or more strands wound in a spiral around a dielectric frame, having a rectangular, cylindrical or shape. Sometimes, coil designs are frameless. The wire is wound in one or more layers.

In order to increase the inductance, ferromagnet cores are used. They also allow you to change the inductance within certain limits. Not everyone fully understands why an inductor is needed. It is used in electrical circuits as a good conductor. direct current. However, when self-induction occurs, resistance arises that prevents the passage of alternating current.

Varieties of inductors

There are several design options for inductors, the properties of which determine the scope of their use. For example, the use of loop inductors together with capacitors makes it possible to obtain resonant circuits. They are characterized by high stability, quality and precision.

Coupling coils provide inductive coupling of individual circuits and cascades. Thus, it becomes possible to divide the base and circuits in direct current. High precision is not required here, therefore, thin wire is used for these coils, wound in two small windings. The parameters of these devices are determined in accordance with the inductance and coupling coefficient.

Some coils are used as variometers. During operation, their inductance can change, which allows you to successfully rebuild oscillatory circuits. The whole device includes two coils connected in series. The moving coil rotates inside the fixed coil, thereby creating a change in inductance. In fact, they are a stator and a rotor. If their position changes, then the value of self-induction will also change. As a result, the inductance of the device can change by 4-5 times.

In the form of chokes, those devices are used that have high resistance with alternating current, and very low resistance with direct current. Due to this property, they are used in radio engineering devices as filter elements. At a frequency of 50-60 hertz, transformer steel is used to make their cores. If the frequency is higher, then the cores are made of ferrite or permalloy. Separate varieties of chokes can be observed in the form of so-called barrels that suppress interference on wires.

Where are inductors used?

The scope of each such device is closely related to the features of its design. Therefore, it is necessary to take into account its individual properties and technical characteristics.

Together with resistors or, the coils are involved in various circuits that have frequency-dependent properties. First of all, these are filters, oscillatory circuits, circuits feedback And so on. All types of these devices contribute to the accumulation of energy, the conversion of voltage levels in a switching regulator.

When two or more coils are inductively coupled together, a transformer is formed. These devices can be used as electromagnets, as well as an energy source that excites an inductively coupled plasma.

Inductive coils are successfully used in radio engineering, as a transmitter and receiver in annular structures and those working with electromagnetic waves.

One of the advantages of pulsed metal detectors is the ease of manufacture for them. search coils . At the same time, with a simple coil, pulse metal detectors have a good detection depth. This article will describe the most simple and available ways making search coils for pulsed metal detectors with your own hands.

Coils made by the manufacturing methods described below, suitable for almost all popular schemes of impulse metal detectors (Koschei, Clone, Tracker, Pirate, etc.).

1. Twisted Pair Pulse Metal Detector Coil

From a twisted pair wire, you can get an excellent sensor for pulse metal detectors. Such a coil will have a search depth of more than 1.5 meters and have good sensitivity to small objects (Coins, rings, etc.). To make it, you need a twisted pair wire (such a wire is used for an Internet connection and is on sale in any market and computer store). The wire consists of 4 twisted pairs of wires without screen!

The sequence of manufacturing a coil for a pulsed metal detector, from a twisted pair wire:

• Cut off 2.7 meters of wire.
• We find the middle of our piece (135 cm) and mark it. Then we measure 41 cm from it and also put marks.
• We connect the wire along the marks into a ring, as shown in the figure below, and fix it with tape or electrical tape.

• Now we begin to wrap the ends around the ring. We do this simultaneously on both sides, and make sure that the turns fit tightly, without gaps. As a result, you get a ring of 3 turns. This is how you should do it:

• The resulting ring is fixed with adhesive tape. And we bend the ends of our coil inward.
• Then we clean the insulation of the wires, and solder our wires, in the following sequence:

• We isolate the soldering points with the help of thermotubes or electrical tape.

• To output the coil, we take a wire 2 * 0.5 or 2 * 0.75 mm in rubber insulation, 1.2 meters long, and solder it to the remaining ends of the coil and also isolate it.
• Then you need to choose a suitable housing for the coil, you can buy it ready-made, or choose a plastic plate of a suitable diameter, etc.
• We put the coil into the case and fix it there with hot glue, we also fix our solder joints and wires to the leads. You should get something like this:

• Then the body is sealed, or if you used a plastic plate or pallet, then it is better to fill it with epoxy, this will give your structure additional rigidity. Before sealing the body, or filling it with epoxy, it is better to conduct intermediate performance tests! Since after gluing, there is nothing to fix!
• To attach the coil to the metal detector rod, you can use such a bracket (it is quite inexpensive), or you can make its likeness yourself.

• We solder the connector to the second end of the wire, and our coil is ready for use.

When testing such a coil with Koschey 5I metal detectors, the following data were obtained:

• Iron gates - 190 cm
• Helmet - 85 cm
• Coin 5 braids of the USSR - 30 cm.

1. Large coil for a pulse metal detector with your own hands.

Here we describe the method production of a deep coil 50 * 70 cm, for impulse metal detectors. Such a coil is well suited for finding large metal targets at great depths, but it is not suitable for finding small metal.

So, the process of manufacturing a coil for pulsed metal detectors:

• We make patterns. For this, in any graphics program, draw our template, and print it in 1:1 size.

• With the help of a template, we draw the outline of our coil on a sheet of plywood or chipboard.
• We drive in nails around the perimeter, or screw in screws (screws must be wrapped with electrical tape so that they do not scratch the wire), in increments of 5 - 10 cm.
• Then we wind on them a winding (for a Clone metal detector 18-19 turns) of a winding enamel wire 0.7-0.8mm, you can also use a stranded insulated wire, but then the weight of the coil will turn out a little more.
• Between the nails, we tighten the winding with cable ties, or with adhesive tape. And we coat the free areas with epoxy.

• After the epoxy has hardened, remove the nails and remove the coil. We remove our screeds. We solder the leads from a stranded wire 1.5 meters long to the ends of the coil. And we wrap the coil with fiberglass, with epoxy resin.

• For the manufacture of a cross, you can use a polypropylene pipe with a diameter of 20 mm. Such pipes are sold under the name "Heat-sealed pipes".

• You can work with polypropylene using an industrial hair dryer. It must be heated very carefully, because. at 280 degrees the material decomposes. So, we take two pieces of pipe, heat the middle of one of them, dig a hole through, expand it so that the second pipe crawls into it, heat the middle of this very second pipe (continuing to keep the middle of the first hot) and insert one into the other. Despite the complex description, it does not require special dexterity - I did it the first time. Two heated pieces of polypropylene stick together “to death”, you don’t have to worry about their strength.
• We heat up the ends of the cross and cut them with scissors (heated polypropylene cuts well) in order to obtain “notches” for winding. Then we insert the crosspiece inside the winding and, alternately heating the ends of the crosspiece with recesses, “seal” the winding in the latter. When putting on the winding on the cross, you can pass the cable through one of the pipes of the cross.
• We make a plate from a segment of the same pipe (using the hot flattening method), bend it with the letter "P" and weld it (again hot) to the middle of the cross. We drill holes for everyone's favorite bolts from the toilet lid.
• In order to give additional strength and tightness, we close the remaining gaps with all kinds of sealants, we wrap dubious places with fiberglass with epoxy, and finally, we wrap everything with electrical tape.

More than half a century of evolution of carburetor gasoline engines with a contact ignition system, the coil (or, as drivers of past years often called it, a “reel”) practically did not change its design and appearance, representing a high-voltage transformer in a metal sealed glass filled with transformer oil to improve insulation between winding turns and cooling.

An integral partner of the coil was a distributor - a low voltage mechanical switch and a high voltage distributor. The spark should have appeared in the respective cylinders at the end of the compression stroke of the air-fuel mixture - strictly at a certain moment. The distributor carried out both the generation of a spark, and its synchronization with the cycles of the engine, and distribution by candles.

The classic oil-filled ignition coil - "reel" (which in French meant "coil") - was extremely reliable. From mechanical influences it was protected by a steel glass of the case, from overheating - an effective heat sink through the oil filling the glass. However, according to the little-censored rhyme in the original version, “It was not about the reel - the idiot was sitting in the cab ...”, it turns out that a reliable reel sometimes failed, even if the driver was not such an idiot ...

If you look at the diagram of the contact ignition system, you can find that the muffled engine could stop in any position of the crankshaft, as with closed contacts low voltage breaker in the distributor, and open. If, during the previous shutdown, the engine stopped in the crankshaft position, in which the distributor cam closed the contacts of the breaker that supplied low voltage to the primary winding of the ignition coil, then when the driver, for some reason, turned on the ignition without starting the engine, and left the key in this position for a long time, the primary winding of the coil could overheat and burn out ... For a direct current of 8-10 amperes began to pass through it instead of an intermittent pulse.

Officially, the coil of the classic oil-filled type is not repairable: after the winding burned out, it was sent to the scrap. However, once upon a time at car depots, electricians managed to repair reels - they flared the case, drained the oil, rewound the windings and reassembled ... Yes, there were times!

And only after the mass introduction of contactless ignition, in which the distributor contacts were replaced by electronic switches, the problem of coil combustion almost disappeared. Most switches have automatic shutdown current through the ignition coil with the ignition on, but the engine not running. In other words, after the ignition was turned on, a small time interval began to count down, and if the driver did not start the engine during this time, the switch automatically turned off, protecting both the coil and itself from overheating.

dry coils

The next stage in the development of the classic ignition coil was the rejection of the oil-filled housing. "Wet" coils were replaced by "dry". Structurally, it was practically the same coil, but without metal case and oils, covered with a layer of epoxy compound on top to protect against dust and moisture. She worked in conjunction with the same distributor, and often on sale one could find both old “wet” coils and new “dry” coils for the same car model. They were completely interchangeable, even the “ears” of the mounts matched.

For the average car owner, there were essentially no advantages or disadvantages in changing the technology from wet to dry. If the latter, of course, was made with high quality. "Profit" was received only by manufacturers, since it is somewhat easier and cheaper to make a "dry" coil. However, if the "dry" coils of foreign car manufacturers were initially thought out and manufactured quite carefully and served almost as long as the "wet" ones, the Soviet and Russian "dry" coils gained notoriety, because they had a lot of quality problems and failed quite often without any reason.

One way or another, today “wet” ignition coils have completely given way to “dry”, and the quality of the latter, even of domestic production, is practically not satisfactory.

There were also hybrid coils: an ordinary “dry” coil and a conventional contactless ignition switch were sometimes combined into a single module. Such designs were found, for example, on single-injection Fords, Audis and a number of others. On the one hand, it looked to some extent technologically advanced, on the other hand, reliability decreased and the price increased. After all, two fairly heating nodes were combined into one, while individually they cooled better, and if one or another failed, the replacement was cheaper ...

Oh yes, even in the piggy bank of specific hybrids: on old Toyotas, there was often a variant of a coil integrated directly into the distributor of the distributor! It was integrated, of course, not tightly, and in the event of a failure, the “reel” could be easily removed and purchased separately.

Ignition module - failure of the distributor

A noticeable evolution in the coil world occurred during the development of injection motors. The first injectors included a “partial distributor” - the low-voltage circuit of the coil was already switched by the electronic engine control unit, but the classic runner distributor, driven by the camshaft, was still distributing the spark through the cylinders. It became possible to completely abandon this mechanical unit by using a combined coil, in the common body of which individual coils were hidden in an amount corresponding to the number of cylinders. Such nodes began to be called "ignition modules".

The electronic engine control unit (ECU) contained 4 transistor switches, which alternately applied 12 volts to primary windings all four coils of the ignition module, and they in turn sent a spark pulse high voltage each for its own candle. Simplified versions of combined coils are even more common, more technologically advanced and cheaper to manufacture. In them, in one housing of the ignition module of a four-cylinder engine, not four coils are placed, but two, but working, nevertheless, for four candles. In such a scheme, a spark is supplied to the candles in pairs - that is, it comes to one candle from a pair at the moment necessary to ignite the mixture, and to the other - idle, at the moment the exhaust gases are released from this cylinder.

The next stage in the development of combined coils was the transfer of electronic switching keys (transistors) from the engine control unit to the ignition module housing. The removal of powerful and heating transistors “to the wild” improved the temperature regime of the computer, and if any electronic switch-key failed, it was enough to replace the coil, and not change or solder the complex and expensive control unit. In which immobilizer passwords, individual for each car, and similar information are often registered.

Each cylinder - on the coil!

Another typical ignition solution for modern gasoline cars, which exists in parallel with modular coils, is individual coils for each cylinder, which are installed in the spark plug well and contact the spark plug directly, without a high-voltage wire.

The first "personal coils" were just coils, but then switching electronics moved into them - just as it happened with ignition modules. Of the advantages of this form factor is the rejection of high-voltage wires, as well as the ability to replace only one coil, rather than the entire module, if it fails.

True, it is worth saying that in this format (coils without high-voltage wires mounted on a candle) there are also coils in the form of a single block, united by a common base. Such, for example, like to use GM and PSA. This is truly a nightmare technical solution: the coils seem to be separate, but if one “bobbin” fails, you have to change the assembly of a large and very expensive unit ...

What have we come to?

The classic oil-filled bobbin was one of the most reliable and indestructible units in carbureted and early injection cars. Sudden failure of it was considered a rarity. True, its reliability, unfortunately, was "compensated" by an integral partner - a distributor, and later - an electronic switch (the latter, however, only applied to domestic products). The “dry” coils that replaced the “oil” coils were comparable in terms of reliability, but still failed somewhat more often for no apparent reason.

Injection evolution forced to get rid of the distributor. This is how various designs appeared that did not need a mechanical high-voltage distributor - modules and individual coils according to the number of cylinders. The reliability of such structures has decreased even more due to the complication and miniaturization of their "offal", as well as the extremely difficult conditions of their work. After several years of operation with constant heating from the engine on which the coils were mounted, cracks formed on the protective layer of the compound, through which moisture and oil entered the high-voltage winding, causing breakdowns inside the windings and misfiring. With individual coils that are installed in candle wells, working conditions are even more hellish. Also, gentle modern coils do not like washing the engine compartment and the increased gap in the electrodes of the spark plugs, which is formed as a result of the long-term operation of the latter. The spark is always looking for the shortest path, and often finds it inside the winding of the bobbin.

As a result, today the most reliable and correct design of the existing and used ones can be called an ignition module with built-in switching electronics, mounted on an engine with an air gap and connected to spark plugs by high-voltage wires. Less reliable are separate coils installed in the candle wells of the head of the block, and, from my point of view, the solution in the form of combined coils on a single ramp is completely unsuccessful.

I welcome everyone to our site!

We continue to study electronics from the very beginning, that is, from the very basics and the topic of today's article will be principle of operation and main characteristics of inductors. Looking ahead, I will say that first we will discuss the theoretical aspects, and we will devote several future articles entirely to the consideration of various electrical circuits, which use inductors, as well as elements that we studied earlier in our course - and.

The device and principle of operation of the inductor.

As is already clear from the name of the element - the inductor, first of all, is just a coil :), that is a large number of turns of insulated conductor. Moreover, the presence of insulation is the most important condition - the turns of the coil should not close with each other. Most often, the turns are wound on a cylindrical or toroidal frame:

The most important characteristic inductors is, of course, inductance, otherwise why would it be given such a name 🙂 Inductance is the ability to convert the energy of an electric field into the energy of a magnetic field. This property of the coil is due to the fact that when current flows through the conductor, a magnetic field arises around it:

And here is what the magnetic field that occurs when current passes through the coil looks like:

In general, strictly speaking, any element in electrical circuit has inductance, even an ordinary piece of wire. But the fact is that the value of such an inductance is very small, in contrast to the inductance of the coils. Actually, in order to characterize this value, the Henry unit (H) is used. 1 Henry is actually a very large value, so the most commonly used are µH (microhenry) and mH (milihenry). the value inductance coils can be calculated using the following formula:

Let's see what the value is included in this expression:

It follows from the formula that with an increase in the number of turns or, for example, the diameter (and, accordingly, the cross-sectional area) of the coil, the inductance will increase. And as the length increases, it decreases. Thus, the turns on the coil should be placed as close as possible to each other, as this will reduce the length of the coil.

FROM inductor device we figured it out, it's time to consider the physical processes that occur in this element during the passage electric current. To do this, we will consider two circuits - in one we will pass a direct current through the coil, and in the other - an alternating current 🙂

So, first of all, let's figure out what happens in the coil itself when current flows. If the current does not change its magnitude, then the coil has no effect on it. Does this mean that in the case of direct current, the use of inductors is not worth considering? But no 🙂 After all, direct current can be turned on / off, and just at the moments of switching, all the most interesting happens. Let's take a look at the circuit:

In this case, the resistor plays the role of a load, in its place could be, for example, a lamp. In addition to the resistor and inductance, the circuit includes a constant current source and a switch, with which we will close and open the circuit.

What happens when we close the switch?

Current through the coil will begin to change, since at the previous time it was equal to 0. A change in current will lead to a change in the magnetic flux inside the coil, which, in turn, will cause an emf ( electromotive force) self-induction, which can be expressed as follows:

The occurrence of EMF will lead to the appearance of an induction current in the coil, which will flow in the direction, opposite direction power supply current. Thus, the self-induction EMF will prevent the current from flowing through the coil (the inductive current will cancel the circuit current due to their opposite directions). And this means that at the initial moment of time (immediately after the switch is closed), the current through the coil will be equal to 0. At this moment of time, the self-induction EMF is maximum. And what will happen next? Since the magnitude of the EMF is directly proportional to the rate of change of the current, it will gradually weaken, and the current, respectively, on the contrary, will increase. Let's look at graphs illustrating what we have discussed:

On the first graph we see circuit input voltage- initially the circuit is open, and when the switch is closed, constant value. In the second graph, we see change in the amount of current through the coil inductance. Immediately after the key is closed, the current is absent due to the occurrence of self-induction EMF, and then it begins to increase smoothly. The voltage on the coil, on the contrary, at the initial moment of time is maximum, and then decreases. The voltage graph on the load will coincide in shape (but not in magnitude) with the graph of the current through the coil (since in a series connection, the current flowing through different elements of the circuit is the same). Thus, if we use a lamp as a load, then they will not light up immediately after the switch is closed, but with a slight delay (in accordance with the current graph).

A similar transient process in the circuit will also be observed when the key is opened. An EMF of self-induction will appear in the inductor, but the inductive current in the event of an opening will be directed in the same direction as the current in the circuit, and not in the opposite direction, so the stored energy of the inductor will go to maintain the current in the circuit:

After opening the key, an EMF of self-induction occurs, which prevents the current from decreasing through the coil, so the current reaches zero value not immediately, but after some time. The voltage in the coil is identical in form to the case of closing the switch, but opposite in sign. This is due to the fact that the change in current, and, accordingly, the EMF of self-induction in the first and second cases are opposite in sign (in the first case, the current increases, and in the second it decreases).

By the way, I mentioned that the value of the EMF of self-induction is directly proportional to the rate of change in the current strength, and so, the proportionality factor is nothing more than the inductance of the coil:

This concludes with inductors in DC circuits and moves on to AC circuits.

Consider a circuit in which an alternating current is applied to the inductor:

Let's look at the dependences of the current and EMF of self-induction on time, and then we'll figure out why they look like this:

As we have already found out EMF self-induction we have directly proportional and opposite in sign to the rate of change of current:

Actually, the graph demonstrates this dependence to us 🙂 See for yourself - between points 1 and 2, the current changes, and the closer to point 2, the less changes, and at point 2, for some short period of time, the current does not change at all its meaning. Accordingly, the rate of current change is maximum at point 1 and gradually decreases when approaching point 2, and at point 2 it is equal to 0, which we see on EMF diagram of self-induction. Moreover, on the entire interval 1-2, the current increases, which means that the rate of its change is positive, in connection with this, on the EMF, on the whole this interval, on the contrary, it takes negative values.

Similarly, between points 2 and 3 - the current decreases - the rate of current change is negative and increases - the self-induction EMF increases and is positive. I won’t describe the rest of the graph – all processes follow the same principle there 🙂

In addition, a very important point can be seen on the graph - with an increase in current (sections 1-2 and 3-4), the self-induction EMF and current have different signs (section 1-2: , title="(!LANG:Rendered by QuickLaTeX.com" height="12" width="39" style="vertical-align: 0px;">, участок 3-4: title="Rendered by QuickLaTeX.com" height="12" width="41" style="vertical-align: 0px;">, ). Таким образом, ЭДС самоиндукции препятствует возрастанию тока (индукционные токи направлены “навстречу” току источника). А на участках 2-3 и 4-5 все наоборот – ток убывает, а ЭДС препятствует убыванию тока (поскольку индукционные токи будут направлены в ту же сторону, что и ток источника и будут частично компенсировать уменьшение тока). И в итоге мы приходим к очень !} interesting fact- The inductor resists the alternating current flowing through the circuit. So it has resistance, which is called inductive or reactive and is calculated as follows:

Where is the circular frequency: . - this is .

Thus, the higher the frequency of the current, the more resistance the inductor will provide to it. And if the current is constant (= 0), then the reactance of the coil is 0, respectively, it does not affect the flowing current.

Let's go back to our graphs that we built for the case of using an inductor in an AC circuit. We have determined the EMF of the self-induction of the coil, but what will be the voltage? Everything is really simple here 🙂 According to the 2nd Kirchhoff law:

And consequently:

Let's build on one graph the dependences of current and voltage in the circuit on time:

As you can see, current and voltage are phase-shifted () relative to each other, and this is one of the most important properties of AC circuits that use an inductor:

When an inductor is connected to an alternating current circuit, a phase shift appears in the circuit between voltage and current, while the current lags behind the voltage by a quarter of the period.

So we figured out the inclusion of the coil in the AC circuit 🙂

On this, perhaps, we will finish today's article, it turned out to be quite voluminous, so we will talk further about inductors next time. So see you soon, we will be glad to see you on our website!