A power line filter, also called EMI power line filter, is a passive bi-directional network, electrical equipment which effectively filters the particular frequency point in the power line or the frequency out of the particular frequency point.

Power line filters offer multiple levels of protection to reduce the effects of electrical noise or power anomalies on electric/electronic equipment. Devices available include power line EMI/RF filters with surge protection, drive-rated power line EMI/RF filters, general-purpose power line EMI/RF filters, and power entry modules.

It is a kind of filters and can also be called reflection filter according the working principle. It provides high series impedance and low parallel impedance in the filter stop band, severely mismatching noise source and its impedance with load impedance, thus transmitting the undesired frequency components back to the noise source.

When selecting the power line filter, three main indicators should be considered: first is voltage and current, the following is insertion loss, and finally it is size and structure. Since the filter inside is typically potted, environment is not a main concern. However, the temperature characteristics of all potting materials and filter capacitors have some influence on the environmental characteristics of the power filter.

Power line filters are generally designed as passive filters composed of resistors, capacitors, and inductors, without active components like transistors. According to the characteristics of EMI at the power port, the EMI line filter is a passive low-pass filter, which transmits AC to the power supply without attenuation and greatly attenuates the EMI noise incoming with AC. At the meantime, it effectively suppresses EMI noise generated by power equipment, to prevent them from entering the AC power grid to interfere with other electronic devices.