The use of magnetic core

The use of magnetic core

The use of magnetic core
The magnetic ring that we usually see at one or both ends of the power line or signal line of an electronic device is a common mode choke. The common mode choke can form a large impedance to the common mode interference current, but has no effect on the differential mode signal (the working signal is a differential mode signal), so it is simple to use without considering the signal distortion. And the common mode choke does not need to be grounded and can be directly added to the cable. Selection of the number of turns of the magnetic ring Pass the entire bundle of cables through a ferrite magnetic ring to form a common mode choke coil. According to needs, the cable can also be wound several turns on the magnetic ring. The more the number of turns, the better the suppression effect on lower frequency interference, and the weaker the suppression effect on higher frequency noise. In actual 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 relatively wide, two magnetic rings can be put on the cable, and each magnetic ring has a different number of turns, so that both high-frequency interference and low-frequency interference can be suppressed. From the point of view of the mechanism of the common mode choke coil, the larger the impedance, the more obvious the 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 the case, because there is a parasitic capacitance on the actual magnetic ring, and its way of existence is in parallel with the inductance. When encountering high-frequency interference signals, the capacitive reactance of the capacitor is small, which short-circuits the inductance of the magnetic ring, thereby making the common mode choke coil useless. According to the frequency characteristics of the interference signal, nickel-zinc ferrite or manganese-zinc ferrite can be selected. The high frequency characteristics of the former are better than the latter. The permeability of manganese-zinc ferrite is in the thousands --- tens of thousands, while the permeability of nickel-zinc ferrite is in the hundreds ---thousands. The higher the permeability of ferrite, the greater the impedance at low frequencies and the smaller the impedance at high frequencies. Therefore, when suppressing high-frequency interference, nickel-zinc ferrite should be used; otherwise, manganese-zinc ferrite should be used. Or put manganese-zinc and nickel-zinc ferrite on the same cable at the same time, so that the interference frequency band that can be suppressed is wider. The greater the difference between the inner and outer diameters of the magnetic ring and the greater the longitudinal height, the greater the impedance, but the inner diameter of the magnetic ring must be tightly wrapped around the cable to avoid magnetic leakage. The installation position of the magnetic ring should be as close as possible to the source of interference, that is, close to the entrance and exit of the cable.
1 The longer the magnetic ring, the better
2 The tighter the aperture and the cable passing through are combined, the better.
3 When the low-frequency end is disturbed, it is recommended to wind the cable with 2 to 3 turns. When the high-frequency end is disturbed, it cannot be wound (because of the distributed capacitance), and a longer magnetic ring should be used.
 

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