Ferrite Bead / Ring
A ferrite bead is a passive component slipped over a cable that creates a lossy impedance at high frequency. It has almost no effect on low frequency signal and supply current, while it damps common mode noise by turning it into heat. For a correct result, its position and the number of turns matter.
A ferrite's behaviour depends on frequency: at low frequency its impedance is negligible, while at high frequency it takes on a resistive character and absorbs energy. That is why it does not reflect like a filter, it damps.
- Position: it is fitted as close as possible to the noise source. A ferrite fitted in the middle of the cable does not stop the section between the source and the ferrite from radiating.
- Turns: passing the cable through the ring two or three times increases the impedance noticeably; it scales with the square of the number of turns.
- Coverage: both the outgoing and return conductors must pass through the same ring; if only one does, no common mode suppression is produced.
- Material: a mix suited to the target frequency range must be selected.
In wireless remote control installations a ferrite is typically added to the receiver supply cable, the motor cable and long sensor lines. Do not expect miracles from it on its own; applied together with cable separation and shielding, it gives a measurable gain.
Context where this term is used: Wireless remote control installation: pairing, relay, safety