Mains filters

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published: Jun-17-2015, last edit: Mar-16-2017

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Mains filters

Mains filters in electronic equipment is usually there to prevent unwanted signals to get onto the mains when the equipment produces contaminations. This is mostly caused by switching components in SMPS, electronic transformers used in CFL, LED and Halogen lighting. Dimmers are notorious for this as well as motor controls (washing machines, kitchen appliances). These filters are there to ensure these devices comply to certain rules about emission. Usually the amount of components used is minimised so they can just pass all the tests with the least possible investment in parts.

Mains filters are also used in electronic equipment to prevent influence from garbage that could be present on mains. This will depend on what the device is for and how it may and may not ‘fail’ under certain circumstances. In this case the filter is there to improve immunity of that device. In other words, garbage on the mains is lowered in amplitude (NOT removed !) so it reaches levels where no influence is detected or remains below minimal required limits.


Above a schematic diagram of a very common filter that has a good suppression of both common mode and differential mode ‘noise’.  For those interested in filters a brief description of how such a filter works.

It consists of 3 MOV’s (Metal Oxide Varistors) that can ‘dissipate’ short peaks by transforming them into HEAT. RV1 from Line and Neutral to absorb differential mode peaks (meaning peaks between Live and Neutral). Also 2 MOV’s (RV2 and RV3) from both power lines (Neutral as well as Live) to ground. These are to absorb peaks that are present between both power lines and ground (common mode) or when present from only one power line to ground. There is a risk in using RV2 and RV3 and that is that IF they fail and become conductive or if they are triggered the ground will ‘lift’ when the ground connection is open (untended by plugging it in a faulty earth mains socket or in a non-earthed socket) so can ONLY be applied when you are certain the ground connection is made AND functioning properly, certainly when the earth lead is used to connect with equipment that is intended for earthed operation.
For this reason the fuse is of much importance for a filter using MOV’s in particular, even when the circuit behind the filter is already fused..
When these parts have done their job they sometimes create a short or can even catch fire !
Make sure the fuse is ‘Time-Lag’ or “Slow’ and the rating is based on the load after the filter.

C1 and C4 (at least class X2) are used for filtering differential mode ‘noise’ way above the mains frequency, L1 helps too. L1, however, is intended for filtering common mode noise. This is noise between both power lines and ground. C2 and C3 are extra components that divert the noise, that is already greatly attenuated by L1, even further by sending these signals to ground. For those into DIY …. C2 and C3 MUST be class Y type capacitors, they must be able to handle very high voltages, at LEAST as high as the peak voltages of RV2 and RV3. Very important for your safety ! R1 doesn’t do anything for the filter action but is only there to discharge the filters capacitors when mains has been turned off.

In both cases manufacturers will not go to extreme lengths to get the best possible performance as filter components are expensive and manufacturers like costs to be minimal and profits maximal. Instead they use the minimal and cheapest possible components in their filters to pass the tests they should pass to be allowed to be sold. Not all companies (especially cheap Chinese brands) are equally conscientious  about it and could not care less about the rules and simply print markings as if they were tested on it and/or skimp on parts on the actual production devices. They can get away with it because of the way certain rules can be interpreted. Manufacturers will therefore use a minimal amount of parts so they can pass the tests.

This means that all household equipment should not influence each other. BUT when (multiple) devices are connected together or not connected together in the right way circumstances may have changed and limits that would otherwise not be reached may be ‘breached’. For this reason in audio, especially for analog audio and small signals are present, some influence may be there under circumstance A but may not be present under circumstance B. Also it is possible that ‘setup A’ may play wonderful with person A but that same ‘setup A’ may be plagued with all sorts of problems for person B. In SOME, but certainly not all, cases this could be blamed on the mains circumstances. For those occasions a mains filter could prevent influence or lower it to levels where it isn’t audible any more.

Incorrect circumstances that can EASILY happen is when equipment that has a 3 prong mains plug (earthed/grounded) is connected to a 2 pin socket. In this case a big portion of the built-in filter does not work (correctly) any more and you could even feel a tingling sensation when touching metal parts of that device or even get a small jolt. So… be warned… equipment with 3 prong plugs MUST be connected to 3 prong sockets. Equipment with 2 prong plugs (the ones used on double insulated equipment) may be used in BOTH types of mains sockets.

In THIS SCHEMATIC DIAGRAM there are some suggestions for a DIY mains filter for different circumstances such as 3 pin and 2 pin mains plugs.

When working with mains voltages always ensure your safety, use appropriate fuses and make sure you cannot touch parts that are ‘live’. Common sense and a dose of experience comes in handy.

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