DC servos and ferrite rings: on the motor, on the encoder, or both?

Not open for further replies.


Active member
I've bought several little DC servos from eBay over the years, some have the motor wires wound once or twice through a ferrite ring, some arrive with nothing on the motor wires, but the encoder wires are wound through a ring, some have neither.

My Teensys seem to cope quite well with running big motors with high-frequency encoders even if I don't put rings on the wires, but what *should* I be doing?

(see the way I squeezed Teensys into a slightly off-topic question ;) )
You ask and I obey: here's a quick selection out of the drawer:


OK, I guess the tiny one gets by without any ferrite rings, but the bigger ones often have a ring on one or t'other of their sets of wires. But there's no consistency to it.

If it helps, I tend to use higher frequency PWM (25kHz) to drive the smaller motors to keep the audible noise down, lower frequency (< 1kHz) for the bigger ones as it seems to slightly help the motors' fight against friction at low speeds.

Guess the general question I'm asking is: are the ferrite rings supposed to be on the wires generating the noise, or the ones you want to protect from it?
When I built a large machine tool using brushless motors (grad school) we used ferries on the motor leads to reduce radiated noise. They were quite effective. I imagine, though, that ferrites on the signal wires may also help by suppressing the induced noise. One would think that they should be separate ferrites, right?
Not open for further replies.