Ferrite Bead / Clamp
A magnetic ring slipped over the cable that adds series impedance to high-frequency current and turns the noise into heat. It has almost no effect on the low-frequency operating current. For it to be effective the whole cable bundle, outgoing and return included, must pass through the bead.
Ferrite material behaves lossily within a certain frequency band. As high-frequency current passes through the bead it loses part of its energy as heat. The effect depends on the frequency-impedance curve of the bead and on the number of turns of the cable passing through it.
- Turns: passing the cable through the bead two or three times increases the impedance at low frequencies noticeably; however, as the number of turns rises, the effect at very high frequency falls because of the capacitance between the cable runs.
- Position: the bead is fitted as close as possible to the noise source; at the motor terminals or at the board input.
- Saturation: a ferrite can saturate with the magnetic field of the net current passing through it. If you pass only a single core through it, the operating current of the motor saturates the ferrite and the bead becomes useless.
In applications with brushed DC motors a ferrite bead alone is not enough; the result is much better when it forms an LC filter together with capacitors placed between the motor terminals and the body. Choose the ferrite so that it does not saturate at the normal operating current of the motor.
The context in which this term is used: Installation, redundancy and field continuity