Do-it-yourself ferrite rings for the cable. Ferrite filter - what is it for? How to make a voltage converter with your own hands

We will talk about “lazy winding”. This is when you are too lazy to count the turns. https://site/

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Selecting the type of magnetic circuit.

The most universal magnetic cores are W-shaped and cup-shaped armor cores. They can be used in any switching power supply, thanks to the ability to set a gap between the parts of the core. But, we are going to wind a pulse transformer for a push-pull half-bridge converter, the core of which does not need a gap and therefore a ring magnetic circuit is quite suitable. https://site/

For a ring core there is no need to make a frame and make a winding device. The only thing you have to do is make a simple shuttle.

The picture shows a ferrite magnetic core M2000NM.

The standard size of the ring magnetic core can be identified by the following parameters.

D is the outer diameter of the ring.

d – internal diameter of the ring.

Obtaining initial data for simple calculation of a pulse transformer.

Supply voltage.

I remember when our power grids had not yet been privatized by foreigners, I built a switching power supply. The work dragged on until night. During the last tests, it suddenly turned out that the key transistors began to get very hot. It turned out that the network voltage jumped to 256 Volts at night!

Of course, 256 Volts is too much, but you shouldn’t rely on GOST 220 +5% –10% either. If you choose 220 Volts +10% as the maximum network voltage, then:

242 * 1.41 = 341.22V(we count the amplitude value).

341.22 – 0.8 * 2 ≈ 340V(subtract the drop on the rectifier).

Induction.

We determine the approximate value of induction from the table.

Example: M2000NM – 0.39T.

Frequency.

The generation frequency of a self-excited converter depends on many factors, including the size of the load. If you choose 20-30 kHz, you are unlikely to make a big mistake.

Limit frequencies and induction values ​​of widespread ferrites.

Manganese-zinc ferrites.

Nickel-zinc ferrites.

How to choose ferrite ring core?

You can select the approximate size of a ferrite ring using a calculator for calculating pulse transformers and a guide to ferrite magnetic cores. You can find both of them in.

We enter the data of the proposed magnetic core and the data obtained in the previous paragraph into the calculator form to determine the overall power of the core.

You should not choose ring dimensions close to the maximum load power. It is not so convenient to wind small rings, and you will have to wind a lot more turns.

If there is enough free space in the body of the future design, then you can choose a ring with a obviously larger overall power.

I had at my disposal an M2000NM ring of standard size K28x16x9mm. I entered the input data into the calculator form and received an overall power of 87 watts. This is more than enough for my 50 Watt power supply.

Launch the program. Select “Calculation of a half-bridge transformer with a master oscillator.”

To prevent the calculator from “swearing”, fill in the windows not used for calculating the secondary windings with zeros.

How to calculate the number of turns of the primary winding?

We enter the initial data obtained in the previous paragraphs into the calculator form and obtain the number of turns of the primary winding. By changing the size of the ring, the grade of ferrite and the generation frequency of the converter, you can change the number of turns of the primary winding.

It should be noted that this is a very, very simplified calculation of a pulse transformer.

But, the properties of our wonderful self-excited power supply are such that the converter itself adapts to the parameters of the transformer and the load size by changing the generation frequency. So, as the load increases and the transformer tries to enter saturation, the generation frequency increases and the operation returns to normal. Minor errors in our calculations are compensated in the same way. I tried to change the number of turns of the same transformer by more than one and a half times, which is reflected in the examples below, but I could not detect any significant changes in the operation of the power supply, except for a change in the generation frequency.

How to calculate the wire diameter for the primary and secondary windings?

The wire diameter of the primary and secondary windings depends on the power supply parameters entered in the form. The higher the winding current, the larger the wire diameter required. The primary winding current is proportional to the "Transformer Power Used".

Features of winding pulse transformers.

Winding pulse transformers, and especially transformers on ring and toroidal magnetic cores, has some features.

The fact is that if any winding of the transformer is not distributed evenly enough around the perimeter of the magnetic circuit, then individual sections of the magnetic circuit may become saturated, which can lead to a significant reduction in the power of the power supply and even lead to its failure.

We are trying to wind a “lazy winding”. And in this case, the easiest way is to wind a single-layer winding “turn to turn”.

What is needed for this?

It is necessary to select a wire of such a diameter that it fits “turn to turn”, in one layer, into the window of the existing ring core, and even so that the number of turns of the primary winding does not differ much from the calculated one.

If the number of turns obtained in the calculator does not differ by more than 10-20% from the number obtained in the formula for calculating the laying, then you can safely wind the winding without counting the turns.

True, for such winding, most likely, you will need to choose a magnetic circuit with a slightly higher overall power, which I already advised above.

1 – ring core.

2 - gasket.

3 – winding turns.

The picture shows that when winding “turn to turn”, the calculated perimeter will be much smaller than the internal diameter of the ferrite ring. This is due to both the diameter of the wire itself and the thickness of the gasket.

In fact, the actual perimeter that will be filled with wire will be even smaller. This is due to the fact that the winding wire does not adhere to the inner surface of the ring, forming some gap. Moreover, there is a direct relationship between the diameter of the wire and the size of this gap.

You should not increase the tension of the wire when winding in order to reduce this gap, since this can damage the insulation and the wire itself.

Using the empirical formula below, you can calculate the number of turns based on the diameter of the existing wire and the diameter of the core window.

The maximum calculation error is approximately –5% + 10% and depends on the density of the wire.

w = π(D – 10S – 4d) / d, Where:

w– number of turns of the primary winding,

π – 3,1416,

D– internal diameter of the ring magnetic core,

S– thickness of the insulating gasket,

d– diameter of wire with insulation,

/ - fractional line.

How to measure the diameter of a wire and determine the thickness of the insulation - described.

To make calculations easier, check out this link:

Several examples of calculations of real transformers.

● Power – 50 Watt.

Magnetic core – K28 x 16 x 9.

Wire – Ø0.35mm.

w= π (16 – 10*0.1 – 4*0.39) / 0.39 ≈ 108 (turns).

It actually fit - 114 turns.

● Power – 20 Watt.

Magnetic core – K28 x 16 x 9.

Wire – Ø0.23mm.

w = π (16 – 10*0.1 – 4*0.25) / 0.25 ≈ 176 (turns).

It actually fit - 176 turns.

● Power – 200 Watt.

Magnetic core – two rings K38 x 24 x 7.

Wire – Ø1.0mm.

w = π (24 – 10*0.1 – 4*1.07) / 1.07 ≈ 55 (turns).

In reality, 58 turns fit.

In the practice of a radio amateur, it is not often possible to select the diameter of the winding wire with the required accuracy.

If the wire turns out to be too thin for winding “turn to turn”, and this often happens when winding secondary windings, then you can always slightly stretch the winding by moving the turns apart. And if there is not enough wire cross-section, then the winding can be wound into several wires at once.

How to wind a pulse transformer?

First you need to prepare the ferrite ring.

To prevent the wire from cutting through the insulating gasket and damaging itself, it is advisable to dull the sharp edges of the ferrite core. But, this is not necessary, especially if the wire is thin or a reliable gasket is used. True, for some reason I always do this.

Using sandpaper, round the outer sharp edges.

We do the same with the inner faces of the ring.

To prevent breakdown between the primary winding and the core, an insulating gasket should be wound around the ring.

As an insulating material, you can choose varnished cloth, fiberglass cloth, keeper tape, Mylar film or even paper.

When winding large rings using wire thicker than 1-2mm, it is convenient to use keeper tape.

Sometimes, when making homemade pulse transformers, radio amateurs use fluoroplastic tape - FUM, which is used in plumbing.

It is convenient to work with this tape, but fluoroplastic has cold fluidity, and the pressure of the wire in the area of ​​the sharp edges of the ring can be significant.

In any case, if you are going to use FUM tape, then lay a strip of electrical cardboard or plain paper along the edge of the ring.

When winding gaskets onto small rings, it is very convenient to use a mounting hook.

The mounting hook can be made from a piece of steel wire or a bicycle spoke.

Carefully wrap the insulating tape around the ring so that each turn overlaps the previous one on the outside of the ring. Thus, the insulation on the outside of the ring becomes two-layer, and on the inside - four or five layers.

To wind the primary winding we need a shuttle. It can be easily made from two pieces of thick copper wire.

The required winding wire length is quite easy to determine. It is enough to measure the length of one turn and multiply this value by the required number of turns. A small allowance for conclusions and calculation errors will also not hurt.

34 (mm) * 120 (turns) * 1,1 (times) = 4488 (mm)

If a wire thinner than 0.1 mm is used for the winding, then stripping the insulation with a scalpel can reduce the reliability of the transformer. It is better to remove the insulation of such a wire using a soldering iron and an aspirin tablet (acetylsalicylic acid).

Be careful! When acetylsalicylic acid melts, toxic fumes are released!

If a wire with a diameter of less than 0.5 mm is used for any winding, then it is better to make the terminals from stranded wire. We solder a piece of stranded insulated wire to the beginning of the primary winding.

We insulate the soldering area with a small piece of electrical cardboard or ordinary paper with a thickness of 0.05 ... 0.1 mm.

We wind the beginning of the winding so as to securely secure the junction.

We perform the same operations with the output of the end of the winding, only this time we secure the junction with cotton threads. To prevent the tension of the thread from weakening while tying a knot, we secure the ends of the thread with a drop of melted rosin.

If a wire thicker than 0.5 mm is used for the winding, then the conclusions can be made with the same wire. At the ends you need to put pieces of polyvinyl chloride or other tube (cambric).

Then the leads together with the tube need to be secured with cotton thread.

We wrap two layers of varnished cloth or other insulating tape over the primary winding. This interwinding gasket is necessary for reliable isolation of the secondary circuits of the power supply from the lighting network. If you use a wire with a diameter of more than 1 millimeter, then it is a good idea to use keeper tape as a gasket.

If you intend to use it, then you can wind the secondary winding in two wires. This will ensure complete symmetry of the windings. The turns of the secondary windings must also be evenly distributed around the perimeter of the core. This is especially true for the most powerful windings in terms of power take-off. The secondary windings, which take away a small amount of power compared to the total, can be wound at random.

If you don’t have a wire of sufficient cross-section at hand, you can wind the winding with several wires connected in parallel.

The picture shows a secondary winding wound in four wires.

Let's look at how to make a converter circuit to power a super-bright LED. Such a circuit can be a good start for the practical study of electronics. Based on this converter, we will later assemble with our own hands several interesting and useful electronic homemade products.

How to make a voltage converter with your own hands

The first difficulty in assembling the circuit is purchasing a ferrite ring. Ferrite rings are an integral part of devices with switching power supplies (computers, televisions, monitors, VCRs, etc.) Finding such old or broken equipment is not difficult. For example, several rings can be found in a computer power supply in the power filter chokes. The chokes are removed from the board, the windings are dismantled, freeing the ferrite ring.

Computer power supply

Mined chokes

The second difficulty in assembling the circuit is finding the winding wire. The wire is also easily accessible, two pieces of insulated wire can be easily obtained from a UTP-type Internet cable; two wires 0.5-1 m long are enough.

A piece of UTP cable

Conductors for winding

Radio components are also removed from outdated or faulty equipment. You need one resistance with a nominal value of 300 Ohms - 10 kOhms, any n-p-n transistor and, of course, an LED. We determine the pinout of the transistor by entering the query “ transistor marking datashit." It is permissible to install p-n-p structure transistors into the circuit, but for this it will be necessary to change the polarity of the power supply to the circuit and the LED.

The assembly of a toroidal transformer is shown in the video. The windings are wound with your own hands into two wires at once. The midpoint is formed by connecting the beginning of one winding to the end of another. Look at the photo. The number of turns is 10-30 turns.

Winding wires

Transformer windings

Forming the midpoint

A correctly assembled circuit starts working immediately. The use of a toroidal transformer, in comparison with the circuit, sharply increases the efficiency and economy of the converter circuit. The converter will start even when a voltage of 0.3 volts is applied (!) and will produce a voltage for operating the LED of 2.5-3 volts. If you have questions, ask!

I had almost no dealings with ferrite rings before, what can I do with faceless components. There are no markings on them, I haven’t seen them. The main source of their appearance is “parsing”. However, I bought it once when I was assembling a transistor tester; I needed it for the circuit. I bought it - the store served the same faceless product as the ones lying at home, I was not impressed with the purchase. Trust, of course, is a necessary thing and the seller’s assurances were accepted, but the device assembled on this ring did not work. I don't buy anymore. Today I know for sure that the ring from an “energy-saving” light bulb definitely works in low-voltage converters. But what about the others - play for luck? I tried it a couple of times, it didn’t burn out, so now I think it’s better to throw it away. However, necessity forced us to learn something, even if this determination method provides magnetic permeability parameters only for “estimating” the possible use of the ferrite ring of interest, nevertheless, this is already information.

Six ferrite rings were selected for the test with the intention of selecting those that can be tried for use in low-voltage step-up voltage converters. The following is necessary: ​​measure each ferrite ring with a caliper, the outer and inner diameter, its height (thickness) in mm, then evenly wind 10 - 20 turns of wire with a diameter of 0.3 - 0.4 mm on it and measure the inductance in microhenry (µH).

1. No. 1 is covered with a plastic shell (and lo and behold! it is marked “G.N.T. 1203”), dimensions (D x d x h) 14.6 x 6.7 x 5.5 mm
2. No. 2 in a green shell, 13 x 7.5 x 6.7 mm
3. No. 3 in a yellow shell, 13 x 7.5 x 5.3 mm
4. No. 4 small in a green shell, 10 x 5.5 x 5.5 mm
5. No. 5 from an “energy saving” light bulb, 10 x 5 x 5 mm
6. No. 6 ferrite without shell, 9.2 x 5 x 5.2 mm

Each of the rings was wound with 10 turns of copper wire in insulation with a core diameter of 0.4 mm. You can wind it. The inductance of ring No. 1 was 2.81 μH; no inductance was detected in No. 2 and No. 3 and they “left the race.”

The inductance of ring No. 4 turned out to be 0.48 µH, No. 5 - 0.47 µH, No. 6 - 0.30 µH

The obtained data, overall dimensions and inductance value, were inserted into a calculator for calculating the magnetic permeability of ferrite materials (enter fractional numbers through a dot). It is also necessary to indicate the type of magnetic circuit (put a dot in the “window”), in this case it is “Torus” and the number of actually wound turns of wire (W). Click calculate and get the result - effective magnetic permeability.

• For No. 1 it is 34.43792, for No. 4 it is 7.515167

• Magnetic permeability of ferrite ring No. 5 - 7.050014, No. 6 - 4.876385

As a result of the above actions, previously faceless ferrite rings, what to do with which it was completely unclear, received personal information and became practically suitable for further use, because by correlating the now available data with the data of ferrite rings tested in operation (that is, exemplary ones, which in this particular case the ring from the “energy saving” light bulb has protruded) you can choose what you need. For example, of those tested, ring No. 4 has data similar to the “model” one No. 5; you can safely try it in a boost low-voltage voltage converter (I’m already starting to assemble 2.4 - 9 V). No. 6 should also work. I can’t say anything about No. 1 yet - there is no such “sample”.

Using this formula, you can do without a special software calculator; an ordinary one will be enough. I tried it.

Formula for calculating magnetic permeability

Magnetic permeability is a physical quantity, a coefficient (depending on the properties of the medium) characterizing the relationship between magnetic induction B and magnetic field strength H in matter. Material prepared by Babay iz Barnaula.

Discuss the article SELECTION OF A FERRITE RING

Why are ferrite rings needed on computer cables and what is their effect?

Internal and external computer cables can act as miniature antennas as they convert voltage and current noise into electromagnetic radiation.

Ferrite rings for flat and round cables provide effective suppression of noise currents before they are emitted as electromagnetic interference.

Unshielded cables emit noise due to common-mode noise flowing through their copper conductors, that is, high-frequency current flowing in the same direction throughout all cable conductors.
These currents create a magnetic field of a certain magnitude and direction.

Cable ferrites attenuate noise currents by “capturing” the magnetic field and dissipating some of its energy in the form of heat, i.e., a ferrite element placed on the cable conductors creates a high active impedance for common-mode currents.
Ferrites can be used on internal AC or DC power cables and on conductors carrying analog and digital signals.

Manufacturers of electronic equipment use ferrites to suppress electromagnetic radiation from external power and signal cables of computer system units, monitors, keyboards, printers and other peripheral devices.

Long external power and signal cables act as antennas, effectively radiating interference generated inside the device to the outside environment.
The use of ferrite products reduces the shielding requirements for external cables and, in many cases, makes it possible to reduce their cost.

Cable ferrites for EMI suppression should be selected based on the specific application; the cable ferrite should produce the maximum series impedance for the noise signal frequencies.

Once the core material and approximate dimensions have been selected, the series impedance it produces and the noise reduction performance can be optimized by:

1. Increasing the length of the part of the conductor covered by ferrite;
2. Increasing the cross-section of the ferrite core (especially for power circuits);
3. Selecting a core with an internal diameter closest to the external diameter of the conductor or cable;

In general, the best ferrite core is the longest and thickest that can be placed on the cable, with an internal diameter that matches the external diameter of the cable.
When installed on flexible cables, solid ferrite cores must be encased in heat shrink tubing or otherwise protected and secured in place.

The series impedance introduced by the high-frequency ferrite core can be increased by making several turns of conductor on it.
According to the theory, impedance increases in proportion to the square of the number of turns.
However, due to the nonlinearity of ferrites and losses in them, two turns on the core will increase the impedance not by four times, but somewhat less.

In most cases, the ferrite should be located as close as possible to the source of interference, which will prevent the transmission of interference through other elements of the device design, where it is much more difficult to filter out.

But for data cables where conductors enter or exit a shielded housing, the ferrite cores should be placed as close as possible to where they pass through the shield.
This will prevent the conductors inside the housing after the filter from emitting noise.

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Each of us has seen small cylinders on power cords or coordination cables for electronic devices. They can be found on the most common computer systems, both in the office and at home, at the ends of the wires that connect the system unit with the keyboard, mouse, monitor, printer, scanner, etc. This element is called a “ferrite ring” (or ferrite filter). In this article we will look at the purpose for which manufacturers of computer and high-frequency equipment equip their cable products with the mentioned elements.

Main purpose

Physical properties

Ferrite is a ferromagnetic material that does not conduct electric current, that is, it is essentially a magnetic insulator. They are not created in this material and therefore it very quickly remagnetizes - in time with the frequency of external electromagnetic fields. This material property is the basis for effective protection of electronic devices. A ferrite ring placed on a cable can create a high active impedance for common-mode currents.

This material is formed from a chemical combination of iron oxides with oxides of other metals. It has unique magnetic characteristics and low electrical conductivity. Thanks to this, ferrites have practically no competitors among other magnetic materials in high-frequency technology. 2000nm ferrite rings significantly increase the cable inductance (several hundred or thousand times), which ensures suppression of high-frequency interference. This element is installed on the cord during its production or, cut into two semicircles, is put on the wire immediately after its manufacture. The ferrite filter is packaged in a plastic case. If you cut it open, you can see a piece of metal inside.

Do you need a ferrite filter? Or is this another deception?

Computers are very “noisy” (in electromagnetic terms) devices. Thus, the motherboard inside the system unit is capable of oscillating at a frequency of one kilohertz. The keyboard has a microchip that also operates at high frequencies. All this leads to the so-called generation of radio noise near the system. In most cases, they are eliminated by shielding the board from electromagnetic fields with a metal case. However, another source of noise is the copper wires that connect various devices. In essence, they act as long antennas that pick up signals from the cables of other radio and television equipment, and affect the operation of “their” device. The ferrite filter eliminates electromagnetic noise and broadcast signals. These elements convert electromagnetic high-frequency vibrations into thermal energy. That's why they are installed at the ends of most cables.

How to choose the right ferrite filter

To install a ferrite ring on a cable with your own hands, you need to understand the types of these products. After all, it depends on the type of wire and its thickness which filter (from what material) will need to be used. For example, a ring installed on a multi-core cable (power cord, data cable, video or USB interface) creates a so-called common-mode transformer in this section, transmitting anti-phase signals carrying useful information, and also reflects common-mode interference. In this case, one should not use absorbing ferrite to avoid disruption of information transmission, but a higher-frequency ferromaterial. But it is preferable to choose ferrite rings from a material that will dissipate high-frequency interference rather than reflecting it back into the wire. As you can see, an incorrectly selected product can worsen the performance of your device.

Ferrite cylinders

Thick ferrite cylinders cope most effectively with interference. However, it should be borne in mind that too bulky filters are very inconvenient to use, and the results of their work are unlikely to differ much in practice from slightly smaller ones. You should always use filters of optimal dimensions: the internal diameter should ideally match the wire, and its width should correspond to the width of the cable connector.

Don’t forget that it’s not only ferrite filters that help combat noise. For example, for better conductivity it is recommended to use cables with a larger cross-section. When choosing the length of the cord, you should not leave a large margin of length between the connected devices. In addition, poor quality of the connection between the wire and the connector can also be a source of interference.

Video: How to properly wind a long wire around a ferrite ring or toroidal core of a transformer

Ferrite ring markings

The most widespread type of record for marking ferrite rings is as follows: K D d N, where:

K is short for “ring”;

D – outer diameter of the product;

D – internal diameter of the ferrite ring;

H – filter height.

Video: How to properly impregnate a transformer wound on a ferrite ring with an epoxy resin compound

In addition to the overall dimensions of the product, the type of ferromagnetic material is encrypted in the marking. An example entry may look like this: M20VN-1 K 4x2.5x1.6. The second half corresponds to the overall dimensions of the ring, and the first half is encrypted with the initial one (20 &mu-i). In addition to the specified parameters, in the reference description, each manufacturer indicates the critical frequency, resistivity and Curie temperature parameters for a specific product.

How else are ferrite rings used?

In addition to the well-known application as high-frequency protection, ferromagnetic materials are used for the manufacture of transformers. They can often be seen in technology. It is well known that a ferrite ring transformer is very effective in balanced mixers. However, not everyone knows that it is possible to “stretch” balancing. This modification of the transformer is capable of performing the balancing operation more accurately. In addition, transformers on ferrite rings are widely used to match the output and input resistances of cascades of transistor devices. In this case, the active and are transformed. Thanks to the latter, this device can be used to change the range of capacitance tuning. "Stretch" transformers work well at frequencies below 10 MHz.

Conclusion

Those who are interested in how to wind a ferrite ring themselves should keep in mind that the series impedance introduced by a high-frequency ferrite core can easily be increased by making several turns of conductor on it. As electrical engineering theory suggests, the impedance of such a system will increase in proportion to the square of the number of turns. But this is in theory, but in practice the picture is somewhat different due to the nonlinearity of ferromagnetic materials and losses in them.

A couple of turns on the core does not increase the impedance by four times as it should be, but a little less. As a result, in order for several turns to fit in a cable filter, you should choose a ring of a obviously larger standard size. If this is unacceptable and the wire must remain the same length, it is better to use several filters.