Lexicon Netfilter

A mains filter is a component which is looped into the mains cable of a device in order to either prevent interference from entering the device or to prevent interference from leaving the device. This concerns line-borne interference; a mains filter does not help against interference radiated as radio waves.

In many devices, mains filters are already built in. This happens typically with devices with switching power supply, because the legislator requires that the disturbances produced by such power supplies may not come out in too large strength from the equipment. So it is more a matter of protecting the environment than the device itself from interference. However, a mains filter usually has a certain effect in both directions.

These mains filters are usually designed to sufficiently attenuate interference frequencies above 100kHz. So we are talking about radio frequencies. Harmonic distortions of the mains frequency are hardly attenuated, because their Frequencies are much lower. One assumes that these do not harm.

With mains filters for external connection, which are available as accessories, it is meaningful to make the line between filter and equipment short, so that here no disturbances are caught again.

Most devices are already filtered well enough by themselves, so that additional mains filters do not bring any advantage. It is therefore worthwhile to check whether there is a corresponding problem at all before purchasing. The available filters also have different characteristics, so that one should select the filters if necessary suitably to the problem.

A simple and inexpensive filter measure consists of the use of ferrite cores, which are laid either as folding ferrite around the power line, or as a ferrite ring, through which the line is led once or several times. The ferrite should be close to the device.

See also:

• Hifi forum thread: mains interference
• Wikipedia articles ferrite, toroid and electromagnetic compatibility.