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Why put this ferrite bead on motor — Parallax Forums

Why put this ferrite bead on motor

T ChapT Chap Posts: 4,083
edited 2022-08-09 14:23 in General Discussion

The red and black are 24vdc to the motor. The white wires are to a “temp sensor” inside the motor that connects to a controller to alert to over heating. But why put both inside the device as shown? Seems like it just increases crosstalk from the motor wires to the unshielded temp sensor wires.


  • T ChapT Chap Posts: 4,083

  • bob_g4bbybob_g4bby Posts: 336
    edited 2022-08-09 15:01

    The ferrite bead may reduce RF emission from all four wires radiating as end-fed antennas driven by the commutator signal 'edges' against ground, with harmonics spreading all the way up to 100MHz. The ferrite is only effective at RF, not LF signals. In any case, since both sides of each circuit are passed through the bead an equal and opposite voltage will be induced in the 'go' and 'return' wire of the temp sensor, largely cancelling each other out. So spurious signals from the motor circuit are not coupled to any extent to the temp sensor circuit. If the sensor does happen to be disturbed more than you like, just double up on the ferrites, one per circuit. You quite often see a 0.01uF disc ceramic fitted across the motor for RF suppression too. Had the motor been disturbing a nearby radio system and the bead fitted as a fix? (Or more likely the system may have been EMC tested and found to fail an emission test - but passed with this 'quick fix' fitted. EMC engineers and radio hams love clip-on ferrites)
    Cheers, Bob (ex EMC engineer and ham radio operator G4BBY)

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