What is a magnetic ring?
What is a magnetic ring?
A magnetic ring is a ring-shaped magnet. Magnetic ring is an anti-interference element commonly used in electronic circuits, which has a good inhibitory effect on high frequency noise. It is generally made of ferrite material (Mn-Zn). Magnetic ring has different impedance characteristics at different frequencies. Generally, the impedance is very small at low frequency. When the signal frequency increases, the impedance of magnetic ring increases sharply. As we all know, the higher the signal frequency, the easier it is to radiate (to buy a high-quality computer chassis is also to reduce electromagnetic leakage), and the general signal lines are not shielded, then these signal lines become very good antennas, receiving various messy high frequency signals from the surrounding environment, and the superposition of these signals on the originally transmitted signals will even change the original transmitted useful signals. Then, under the action of the magnetic ring, the normal and useful signal can pass through well, and the high frequency interference signal can pass through well, and the cost is low. Therefore, it is not surprising that everyone looks at the plastic pimple type integrated magnetic ring on the display signal cable, USB cable, and even high-end keyboard and mouse.
Selection of turns
Crossing the entire cable through a ferrite magnetic ring constitutes a common mode choke coil, and the cable can also be wound around several turns on the magnetic ring as needed. The more turns, the better the interference suppression effect for lower frequency, and the weaker the noise suppression effect for higher frequency. In practical engineering, the number of turns of magnetic ring should be adjusted according to the frequency characteristics of interference current. Usually, when the frequency band of the interference signal is wider, two magnetic rings can be set on the cable, and each magnetic ring is wound by a different number of turns, so that high frequency interference and low frequency interference can be suppressed at the same time. From the mechanism of the common mode choke, the greater its impedance, the more obvious the interference suppression effect. However, the impedance of common mode choke comes from common mode inductor Lcm = jwLcm. It is not difficult to see from the formula that for noise of certain frequency, the greater the inductance of magnetic ring, the better. However, this is not the case in fact, because the actual magnetic ring still has parasitic capacitance, which exists in parallel with inductance. When high frequency interference signals are encountered, the capacitance of the capacitor is small, and the inductance of the magnetic ring is short-circuited, so that the common mode choke coil loses its effect.
Test method for magnetic ring
The appearance of the magnetic ring can be examined by visual inspection under normal lighting conditions.
The outer dimensions of the magnetic ring shall be checked with a vernier caliper with the accuracy of 0.02mm under normal lighting conditions.
The combined factor of the magnetic ring is measured by “CF-3A magnetic ring sorting instrument” with an excitation current of 2.5A and A frequency of 40kHz.
The 4-coil inductance of the magnetic ring is measured by “YD2810D lcrdigital Bridge” at the frequency of 1 KHz.
The induced electromotive force of magnetic rings is measured with “UI100 type high frequency power source” and “UI9720 magnetic material dynamic analysis system” under specified conditions.
Use “YD2810D lcrdigital Bridge” and precision oven to measure the center of magnetic ring.
Selection of magnetic ring
Different magnetic materials have different permeability and different temperature characteristics. Among them, the temperature characteristic is the most important, because in the work of an energy-saving lamp, the magnetic ring must experience normal temperature, high temperature (up to 100℃) and low temperature, and then work constantly in high temperature. However, the temperature curves of different materials will be very different. The low permeability will appear to be flat at the front half, while the high permeability will appear to be steep; At different temperatures, the change of BS of saturated magnetic induction strength will also be different, assuming that the BS value of 3K material at normal temperature is 200, but the BS value will rise to 300 at 100℃. Similarly, at normal temperature, the BS value of 2.5k material is 200, but at 100℃, the BS value is only 250. The change of temperature will cause the change of BS value u, H, H C; The change of BS value will cause the change of working condition of energy saving lamp line; the increase of BS value will reduce the driving current of triode. Therefore, in the 110V line, if a magnetic ring with a large change in BS value at high temperature is selected, it will cause the lamp to be turned off and then turned on immediately, the lamp cannot be started; The filament at both ends of the lamp tube is red because the lamp tube cannot be started; The power will be twice the positive power. In addition, because the lamp tube cannot start normally, the temperature of the filament at both ends will rise very high (nearly 300℃ or above), which will burn the plastic parts. If we choose the magnetic ring whose BS value changes little with the temperature, that is the magnetic ring whose permeability is not high, the above problems can be solved. However, there is another problem to consider about the high and low permeability of magnetic permeability: it is its loss problem. Generally, the loss of magnetic permeability is very small like 5K and 10K magnetic rings. After it is made into a finished pulse transformer, because its magnetic circuit impedance is relatively small and the delay time is relatively small, its output wave type can be done well, but it is unable to adapt to the above temperature problems; When the permeability is low, although its surface performance is not as good as 5K and 10K, it will not appear that the lamp cannot be started when it is turned on. Jiangmen powder 2.5K magnetic ring is suitable for 110V directly driven lamp; Zhitong electronic 3K magnetic ring is suitable for 220V lamp. Why? The reason is that the 110V direct drive circuit is easy to cause a hot start problem; And the 220V circuit does not have a hot start problem. The magnetic ring of Jiangmen powder has little change in interference with temperature, while 220V energy-saving lamp needs to be properly reduced at high temperature, so it is necessary to properly reduce the driving current of triode to avoid the lamp at high temperature, burn out when it’s high. Assuming that the temperature rises, the magnification of triode rises, the current rises and the lamp power increases. At this time, it is necessary to adjust the power appropriately and select the Zhitong 3K magnetic ring, which is more sensitive to the increase of temperature. When the temperature rises, BS will follow the rise, the driving current of triode will decrease, and the lamp power will decrease, ensure the contradiction between temperature rise and lamp power.
What is the reason why the anti-interference effect of magnetic ring is not good or cannot resist interference?
Many people do not know why the product has no anti-interference effect because it is equipped with a magnetic ring?
In fact, the product is added with a magnetic ring, which cannot play an anti-interference effect. There are several reasons, as follows:
- One: after adding the magnetic ring, see if the product is not reflected at all. If so, then the frequency of the magnetic ring is wrong. We should also choose the appropriate magnetic ring according to the frequency of product interference, in order to eliminate the interference one by one.
- Two: if you don’t know the frequency of product interference, we will spend more time and test; For example, choose a low-frequency magnetic ring to test, if not, then choose a high-frequency magnetic ring or a high-power magnetic ring, try to test as much as possible, I believe there is always a magnetic ring suitable for your electronic products.
- Three: in the process of the test, if the low-frequency magnetic ring is not reflected, and the high-frequency magnetic ring is reflected, but the interference problem cannot be solved. This is that the interference signal is too strong, one magnetic ring can’t bear the disturbed attack, and one or two magnetic rings must be added until the electronic product can run normally.
- Four: many products have added a magnetic ring on the first day. It will not work the next morning. It shows that the magnetic ring has a saturation phenomenon. It must also add one to two magnetic rings, then start with the material of the magnetic ring, try to choose a magnetic ring made of high-characteristic materials, and try to wind a few more turns on the interference source line, and at the same time, add a magnetic ring at the outlet end to finish the interference signal.