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What is an EMI Filter?

By Andrea Stein
An EMI filter on a circuit board decreases circuit disruptions.
printed circuit board image by Goran Bogicevic from Fotolia.com

Line filters are electronic filters that are used to enhance or remove certain frequency components. Line filters are used to to attenuate, or lower, EMI (electromagnetic interference) between electronic equipment and an external power line.


Electromagnetic interference, also referred to as RMI (radio frequency interference), is a disruption in an electrical circuit caused by electromagnetic radiation emitted from an external source in the vicinity. Electromagnetic radiation contains electric and magnetic components that produce self-propagating, or self-replicating, waves of energy.


Line filters contain inductors, which are electrical components that store energy in a magnetic field generated by an electric current passing through it. Inductors are comprised of conducting wire shaped into a coil. The loops of the coil help facilitate the production of a strong magnetic field. Inductors block high-frequency signals while managing the flow of low-frequency signals through the line filter.


AC (alternating current) line filters are commonly used to filter EMI between AC power lines and equipment. Alternating current refers to moving electrical charges that can reverse direction. PCB (printed circuit board) line filters are mounted on the circuit board and are used to block EMI in certain electronic devices. Facility AC line filters are typically mounted in a cabinet or room at the location where the AC power is received.