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Selection of turns for magnetic rings
Feb 26,2025
Passing the entire cable through a ferrite magnetic ring forms a common mode choke, and the cable can also be wound around the magnetic ring a few times as needed. The more turns there are, the better the suppression effect on interference at lower frequencies, while the suppression effect on noise at higher frequencies is weaker. In practical engineering, the number of turns of the magnetic ring should be adjusted according to the frequency characteristics of the interference current. Usually, when the frequency band of the interference signal is wide, two magnetic rings can be placed on the cable, each with a different number of turns, which can simultaneously suppress high-frequency interference and low-frequency interference. From the mechanism of the common mode choke coil, the larger its impedance, the more obvious its interference suppression effect. The impedance of the common mode choke coil comes from the common mode inductance Lcm=jwLcm. It is not difficult to see from the formula that for a certain frequency of noise, the larger the inductance of the magnetic ring, the better. But the actual situation is not like that, because there are parasitic capacitors on the actual magnetic ring, which exist in parallel with the inductor. When encountering high-frequency interference signals, the capacitance impedance of the capacitor is small, which short circuits the inductance of the magnetic ring, thereby rendering the common mode choke ineffective.
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