EMC Question of the Week: January 21, 2019
Ferrite beads have an impedance that is best described as
- an inductance
- a capacitance
- a frequency-dependent resistance
- an open circuit
Answer
The best answer is "c." Ferrite beads consist of a lossy ferrous material packed around a conducting wire. At DC they have very little impedance. At higher frequencies, the time-varying magnetic fields induced in the ferrous material cause power dissipation that results in an overall resistance.
The impedance of a ferrite bead consists of both a resistance and a reactance that are functions of frequency. An example of a ferrite bead impedance is plotted on the right. At the frequencies with the highest impedance, ferrite beads are designed to be primarily resistive.
At some frequencies, the magnitude of the reactance may exceed the magnitude of the resistance. At those frequencies, the impedance could be viewed as that of a lossy inductor or a lossy capacitor. Nevertheless, at the frequencies where the ferrite bead is designed to be most effective, its impedance is best described as a frequency-dependent resistance.
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