What does a ferrite bead do and where is it fitted?
A ferrite bead damps the noise current travelling along a cable by creating impedance at high frequency. It has almost no effect on the low-frequency useful signal and the DC supply current. It is fitted at the point closest to the source of the noise, for example at the terminals of a brushed motor, and together with a capacitor going to chassis it forms an LC filter.
A ferrite adds a series impedance to the high-frequency components of the cable passing through it. At DC and low frequency it is almost invisible, so it does not restrict the supply current, but it attenuates noise in the megahertz region considerably.
- Where: always as close to the source as possible. On a DC motor producing brush arcing the ferrite is fitted at the motor terminals, close to the motor and not at the driver output. If you want to stop noise entering a device, put it at the entry point of the enclosure.
- How: instead of passing the cable through the ferrite once, winding it two or three turns increases the impedance noticeably; if there is room, increase the number of turns.
- For common mode: pass both the outgoing and the return conductor together through the same ferrite. This suppresses common-mode noise returning via the chassis and does not affect the operating current.
- Saturation: if a single conductor is passed through, the ferrite must be chosen so that it does not saturate at the operating current of the motor; a saturated ferrite does nothing at all.
A ferrite is not a solution on its own. On a brushed motor the most effective combination is an X capacitor between the terminals for differential mode, a Y capacitor between terminal and chassis for common mode, and a ferrite close to the motor.
The full topic: Installation, redundancy and field continuity