There are virtually no electronic products today that can comply with the conducted emission regulatory requirements without the use of some form of power supply filter being inserted where the power cord exits the product. Sometimes, properly designed transformers can provide inherent filtering, and so can obviate the need for an "intentional" filter. Mains EMI (electromagnetic interference) filters carry potentially high currents at dangerously high voltages, so care is essential in their choice. The working voltage and current rating of components can be decided once the specification is known. The basic specification should also include mechanical details such as the enclosure size, and the limit of weight. The electrical specification should include the voltage and current rating. In addition the EMC performance and the allowable leakage current should be specified. The electrical specification must also comply with national safety standards. In the Electrical Equipment Laboratory of the Technical University from Cluj-Napoca, we have made studies on several types of EMI filters and have performed procedures for determining their main differential mode characteristics, presented in the paper for a study of case.
