Sunday, November 30, 2008
I have tested it and am pleased to report that it works at least as well as my prototype, possibly better.
Here is the noise spectrum without it fitted.
And here is the result of fitting it: -
More details here: hydraraptor.blogspot.com/2008/11/suppression.
Labels: DC motor suppression
The ferrite was Zach's choice, he reckons there are plenty in stock at Mouser.
My version had a diode but that prevents reversing the motor, which the RepRap motor driver can do.
The filter will reduce the speed of the back emf spike so having the clamping diode at the other end of the cable probably doesn't make much difference from an RFI point of view. It may well do without the filter.
For a reversable motor and electronics, a TVS (transient voltage suppression) diode -- basically back-to-back zeners in a single package, might be a good addition. SA12CA is a popular family, available in leaded package, and appropriate for a +/- 12 volt system. This would crowbar any spikes above 13.3 volts, either polarity.
There are also lower-voltage versions, handy for protecting 5 volt or lower inputs from harm. Cheap insurance....
-- Larry Pfeffer
No really unless you could mount them on the rotor, TVS diodes would just clamp the signal to 13V pk-pk. That is still a lot of RF energy.