AC Power Line Noise Filter Frequently Asked Questions

What is EMI/RFI?

EMI/RFI stands for “Electro-Magnetic Interference” and “Radio Frequency Interference”. EMI/RFI is a high frequency / low energy noise typically of a continuous nature that doesn’t directly destroy electronic systems, but rather disrupts the proper function by overwhelming the normal signal information within the electrical circuitry. A good example of the type of noise is the household vacuum cleaner. If the vacuum is running and you are attempting to watch TV, you may notice lines of interference on the TV screen or even sound distortion. This interference on the TV is being conducted from the motor of the vacuum through the power lines and into the TV. Although you may be able to still see and hear the TV, it is not functioning correctly. EMI/RFI in interfering with the proper function of the TV. This interference may be minor to the TV, but imagine that same interference in medical equipment or an industrial computer controlling a manufacturing process. That interference is not just a nuisance and could potentially cause a failure, which might injure or kill a patient or damage a major manufacturing process costing thousands of dollars.

What does a filter do?

EMI/RFI filters are designed to stop the EMI/RFI electrical noise from entering an electronic system and causing disruptions.

Why use filters?

The purpose of an EMI/RFI filter is two fold. First it stops noise from entering and disrupting the operation of your electrical equipment. Secondly, it stops your electrical equipment from putting EMI/RFI noise onto the power lines.

The first is to protect your equipment from malfunction or failure. The second is to protect other electrical equipment. The FCC, IEC and other regulatory agencies have rules and regulations concerning the amount of electrical noise your equipment will be allowed to place onto the AC Power line. These limits must be met before your equipment will be allowed to be connected to the AC Power line. The European limitations are much more stringent then the North American limits.

Why are safety agency approvals important?

Many times, the EMI/RFI filter is the first line of defense against electrical noise entering your equipment. As such, it must be able to withstand the rigors of the AC Power line. Safety Agency approvals assure that the components used in an EMI/RFI Filter meet the standards required, to survive the AC Power line, while protecting your equipment.

How do I select a filter for my application?

The selection of an EMI/RFI filter begins by determining the voltage and current requirements of the application. Since the filter is a series as well as a parallel element in the power circuit, both the voltage and current ratings of the filter are important. Many filters are rated 250VAC which means the will work in any voltage applications up to and including 250VAC. The current ratings of a filter series can range from as small as 1 ampere to as large as 30 ampere for single phase applications and 250 ampere and greater in 3-phase application. 3-phase filters can also have voltage ratings much higher than 250VAC.

After the voltage/current decisions it becomes a physical decision as to the style of filter required for the applications. IEC inlet filters allow a line cord to be plugged directly into them. Other styles allow interconnect by screw terminals, faston connector or solder connection. The physical size of the filter can also be determined at this point.

The final decision, although it probably should be higher up on the list, is the insertion loss characteristics. This is what the filter is all about. How much electrical noise will the filter prevent from being passed from input to output. To determine the insertion loss characteristics of a filter, electrical noise, of a known amplitude and at a given frequency, is generated at the input to the filter. The amount of noise present at the output is measured and the difference between the two is the insertion losses, ie: how much noise did the filter prevent from passing from input to output. Repeated testing at selected frequencies is done to determine the level of insertion losses over a frequency range. The typical frequency range of this testing is from 150kHz to 30mHz. However, test data at frequencies as low as 10kHz and as high as 300mHz is not uncommon.

The Safety Agency concerns about electrical noise levels generated by equipment and inserted onto the power line system is what established the 150kHZ to 30mHz range. However, since the filtering capability is bi-directional, you, as the design engineer, maybe concerned with electrical interference at noise levels lower than 150kHz or higher than 30mHz as they affect the operation of your equipment. Thus the reason for insertion losses data at frequencies as low as 10kHz and as high as 300mHz.

Keep in mind the Insertion Loss Characteristics, which are shown in filter data books are generated under laboratory conditions and under defined parameters. This data is for reference only and actual results may vary in your application.

Need a larger filter?

If you need a single phase filter larger than 30 Ampere, you may find it difficult to find large amperage single phase filters, but keep in mind that a 3-Phase filter can be used in a single phase application and these filters typically have amperage ratings as high as 250 ampere. Using a 3-phase filter in a single phase application just means that one leg of the filter is left unconnected.