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EMI Line Filters: What role do they play in Electrical Devices?

EMI Line filter

As customer spending is increasing around the world, the use of electrical devices in day-to-day lives is growing rapidly. Whether it is telecommunications, entertainment, networking, medical, power conditioning or information technology, electronics is everywhere. But did you know these devices generate electromagnetic fields which create external interference within the radio frequencies? Line Filters, also known as EMI Filter or RFI Filter, are developed to get rid of this. They help to mitigate the Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) present in the power and signal lines. Then, they transmit pure signals, so that the electronic equipment functions properly.

In this blog, we’re going to explore why Line Filters are used and what role they perform for your applications.

Role/Functions of the Line Filter in Switching Power Supplies

  • The EMI line filter suppresses electromagnetic interference signals transmitted through conduction.
  • They extract any unwanted current passing through cables or wires, while allowing desirable currents to pass through freely.
  • They prevent the electronic control circuit from having peak voltage on the mains input that can be produced by electromechanical switches.
  • Filters act in opposite directions to attenuate interference variables so that an acceptable level of interference can be achieved.
  • They can also suppress noise from grid power, which is why they are also known as EMI power line filters.


EMI Line Filters are used in the following applications:

  • Household appliances
  • Military/Aerospace Systems
  • Medical Equipment
  • Computers
  • Energy Management Systems
  • Automotive Battery Charger
  • Industrial Equipment

All the above machines produce electromagnetic interference. Hence, now it is mandatory to use line filters, especially in household appliances, to limit the noise.

In industrial units, there is a little relaxation on the amount of interference that the machines can produce, but in domestic units, stringent norms are in place.

Importance of Line Filters

EMI Line Filters are crucial to ensure that the power systems have good Electromagnetic Compatibility (EMC). EMC is an indicator used to measure the quality of electronic components. Its importance is growing largely in the whole world. While designing power systems, the presence of electromagnetic compatibility can improve the product’s ability to resist interference, extend its lifetime, and ensure the safety of use.

Structure of the EMI Line Filters

EMI Line Filters are passive filters designed using capacitors, resistors, and inductors. They also contain UT ferrite cores as an essential component. They are used to separate the electromagnetic signals. The filters transmit AC to the power supply and attenuate the EMI interference coming from the AC. They suppress the EMI noise generated by power supplies effectively. This helps to prevent interference from entering the AC power grid that affects the performance of electronic devices.

How to select the EMI Line Filters?

While selecting the EMI line filters, three things have to be considered- voltage and current, size and structure, and insertion loss. As the filter is usually potted, the environment in which it is used is not a major concern. However, the temperature of the potting materials and filter capacitors creates an influence on the environmental characteristics of the line filter.

It is even better if the power supply to be filtered has undergone preliminary EMC testing to get a baseline of the conducted emissions. The results of the test will tell the manufacturer about the frequencies at which the product failed and how much. You can compare this data with the insertion loss of the EMI filter to measure if it offers enough attenuation at frequencies where it failed to pass the test.

As there is an inconsistency in attenuation across the frequency spectrum, you must ensure that all failed or marginal frequencies will be attenuated. If there is a single attenuation value received in the place of an insertion loss graph, you must ensure that the single value is higher than the largest margin of failure.


We can conclude that EMI Line Filters are an appropriate choice for you to suppress interference in electronic products & other power systems. While selecting them, you must determine their volume and shape to ensure they resonate with your applications.

Alisha Coils & Transformers (A Division of Cosmo Ferrites Ltd) is a leading manufacturer and supplier in the soft ferrite industry. They offer EMI Line Filters in a range of configurations and designs to help customers meet the FCE and CE requirements on EMI. We also ensure that our product meets all governmental emissions and safety standards. Contact us to know more or explore our range of products.