I am looking for any information that would help me reduce the power rail noise and EMI of DC brush motors. I am building a robot that is powered by 2 x 250 W 24V DC motors.

Thank you in advance


The noise could be generated by either the brushes (commutation) or the driver if its PWM. Easiest fix for brush noise (which would be random or 'static') is a capacitor right across the PMDC motor leads, something like a 1 µF, 100 V ceramic capacitor might be a good starting point.

If the PWM supply is causing problems (something with narrow spectral content), you would need a choke between the PWM power rail and your main supply.

  • \$\begingroup\$ If it is a PWM, you may also need to shield the wiring between the motor and controller to prevent EMI. \$\endgroup\$ – Jesse Oct 14 '10 at 3:38

The capacitor that Nick T mentions is the most important -- directly across the 2 motor leads, mounted directly on the motor.

Occasionally that is not enough.

Some people get further improvements by adding one capacitor per motor lead between that motor lead and the metal case of the motor.

A few people go even further, adding ferrite bead "choke" and another capacitor to form a "pi filter" -- see "Nophead's inteference suppressor" and the pages it links to for the ugly "before" o'scope traces and the pretty "after" o'scope photos.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.