EMC Question of the Week: May 28, 2018
When the DC saturation current of a ferrite bead is exceeded, its impedance at high frequencies
- decreases significantly
- increases significantly
- is mostly unaffected
- is unaffected in magnitude, but shifts in phase
Answer
The correct answer is "a." Ferrites beads provide a frequency-dependent resistance that is largely the result of losses incurred as magnetic domains in the material become more or less aligned due to the time varying current through the bead. A DC current strong enough to saturate the bead aligns all of the magnetic domains. In this state, the magnetic properties of the ferrite material are similar to those of air and the ferrite bead basically becomes a jumper.
Note that circuit boards and schematics often label ferrite beads with an "L". Nevertheless, they are primarily resistive (not inductive) at the frequencies where they are designed to be used.
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