Ferrite ring - what is it? How to make a ferrite ring with your own hands?

Each of us saw small cylinders on power cords or on cables for reconciling 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 to the keyboard, mouse, monitor, printer, scanner, etc. This element is called a "ferrite ring" (or Ferrite filter). In this article, we'll figure out why computers and high-frequency equipment manufacturers equip their cable products with these elements.

Main purpose

The ferrite ring is able to reduce the effect of radio frequency and electromagnetic interference on the signal that is transmitted over the wire. Long signal and power cables of both computer and other power equipment have parasitic properties, that is, they work as antennas. They very effectively radiate into the external environment various noises that are created inside the device, thereby creating interference on radio stations when receiving a radio signal and on other electronic equipment. Conversely, by taking interference from the air from radio transmitting devices, a computer or other electronic device may malfunction. Here to eliminate this phenomenon and use a ferrite ring, which is worn on the supply or matching cable.

Physical properties

Ferrite is a ferromagnet that does not conduct electric current, that is, in fact it is a magnetic insulator. In this material, eddy currents are not created , and therefore it reverses very rapidly - in time with the frequency of external electromagnetic fields. This property of the material is the basis for the effective protection of electronic devices. A ferrite ring, mounted on a cable, can create a large active impedance for in-phase currents.

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

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

Computers are very "noisy" (in the electromagnetic plan) devices. So, the motherboard inside the system unit can oscillate at the frequency of one kilohertz. The keyboard has a microchip that also works at a high frequency. 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 casing. However, another source of noise is copper wires that connect different devices. In fact, they act as long antennas that capture signals from cables of other radio and television equipment, and affect the operation of the "own" device. The ferrite filter eliminates electromagnetic noise and broadcast signals. These elements transform electromagnetic high-frequency oscillations into thermal energy. That's why they are installed on the ends of most cables.

How to choose the right ferrite filter

To install a ferrite ring on your cable with your own hands, you need to understand the types of these products. After all, depending on the type of wire and its thickness, it depends which filter (from what material) it will be necessary to use. For example, a ring mounted on a multicore cable (power cord, data transmission, video or USB interface) creates a so-called in-phase transformer on this section that passes antiphase signals that carry useful information, and also reflects in-phase noise. In this case, it is necessary to use non-absorbing ferrite in order to avoid infringement of information transmission, and higher-frequency ferromaterial. But ferrite rings on the antenna cable is preferable to choose from a material that will dissipate high-frequency noise, rather than reflect them again in the wire. As you can see, an incorrectly selected product can worsen the performance of your device.

Ferrite Cylinders

The thickest ferrite cylinders are most effective in dealing with interference. However, it should be borne in mind that too bulky filters are very inconvenient to use, and the results of their work will hardly differ in practice from those slightly smaller in size. Always use filters of optimum dimensions: the inner diameter should ideally match the wire, and its width should correspond to the width of the cable connector.

Do not forget that not only ferrite filters help to fight noise. For example, for better conductivity, it is recommended to use a cable with a large cross section. When choosing the length of the cord, do not make a large margin of length between the connected devices. In addition, the poor quality of the connection between the wire and the connector can also be a source of interference.

Marking of ferrite rings

The most widely used type of marking of ferrite rings is as follows: K D × d × H, where:

- K is an abbreviation for the word "ring";

- D - the outer diameter of the product;

D is the inner diameter of the ferrite ring;

- Н - height of the filter.

In addition to the overall dimensions of the product, the type of ferromagnetic material is encoded in the marking. An example of a record can have the following form: M20BH-1 K 4x2.5x1.6. The second half corresponds to the overall dimensions of the ring, and the initial magnetic permeability (20 μ _i ) is encrypted in the first one. In addition to these parameters, in the reference description, each manufacturer indicates the critical frequency, hysteresis loop parameters , resistivity and Curie temperature for a particular product.

How else to use ferrite rings

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 computer power supply units . It is well known that a transformer on a ferrite ring is very effective in balanced mixers. However, not everyone knows that there is a possibility of "stretching" the balancing. This modification of the transformer is able to perform the balancing operation more accurately. In addition, transformers on ferrite rings are widely used to match output and input impedances of cascades of transistor devices. At the same time, active and reactive resistances are transformed . Thanks to the latter, this device can be used to change the range of capacity adjustment. "Stretch" transformers work well at frequencies below 10 MHz.

Conclusion

For those who are interested in how to wind a ferrite ring on their own, it should be borne in mind that the serial impedance, which is introduced by the high-frequency ferrite core, can easily be increased by making several turns of the conductor on it. As suggested by the theory of electrical engineering, 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 the losses in them.

A pair of turns on the core increases the impedance not four times, as it should be, but slightly less. As a result, in order to allow several turns to fit in the cable filter, you should choose a ring of a significantly larger size. If this is unacceptable, and the wire should remain the same length, it is better to apply several filters.