EMI Mains Filter

Circuit : Rodney Byne, UK
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Modern society reliance on electrically driven appliances, creates Electro-Magnetic Interference (EMI) which manifests as "electrosmog", a gathering phenomenon to which susceptible people are becoming unusually allergic as recently reported in the press.

This is prevalent from power lines, transmitters, electrical and electronic devices, plus data hash from computers in industry, shops, the home and is unavoidably all around us.

A little local problem
So in not wanting to worsen ambient EMI levels, I was having trouble some years ago, tracing a serious source of interference to radio receivers in my house. It came to light that a so called "wall wart" switched mode power supply for a then new Freeview tv digital decoder box was the culprit.

My immediate problem was that high level hash spikes were being directly injected into the house ring-mains wiring. This ring acting as an aerial, was radiating interference to my other home equipment and out to the nearest neighbours.

However replacing the psu would have been pointless, as the factory batch design was unsuppressed, so an attack strategy had to be my only contribution against the menace of increasing background EMI.

In a corner of our lounge is a stand unit, housing a CRT television, a video recorder and two Freeview decoders. One channel is to watch and a different channel is for recording.

They are all supplied from a four-way mains extension block and the only solution I could think of, was to modify this by adding a low-pass RF ferrite ring filter of bifilar construction close up to the block as practical, in series with the cable to the mains wall socket, as the photo below illustrates.

emi mains filter circuit

To be recommended as a good notion, this task proved to be a complete success and no noticeable interference was afterwards detected on sensitive home radio receivers. The filter remains a permanent fixture in the house, as the offending psu source though now tamed is still in constant use.

This suppression method can just as effectively be used to treat the mains distribution to a computer workstation with its peripheral equipment, as collectively this is notorious for the generation of hash.

Suggested build guide
Notwithstanding that a low pass mains filter and clip-on ferrite beads to cables can be added or inserted anywhere as required to suppress mains-borne noise, here is a short shopping list to get the bits needed for this particular home project as an example:

Any shape mains extension block with attached mains cable of required length, bought cheaply from almost anywhere.

One, roughly 40mm o/d grey ferrite ring of Maplin code QT26D.

A double two-way 3A or 5A terminal (term) block cut from a strip, as to easily fit the guage of stranded mains wire used. Obtainable from hardware & electrical stores, or from Maplin of codes L96AR or L97AR.

One, Class Y2 (X1) metallised polyester-film 250V AC suppression capacitor, value 100000pf or 0.1mfd, of Maplin code SC88V. This small grey block component is non-axial wire-ended, for the convenience of mounting to a term block as explained below.

Reel of black pvc electrical tape.

Method of assembly emi mains filter circuit
WARNING:   Please be careful when building any circuit that uses live voltages. See also the sites General Disclaimer.

If this diagram layout is familiar to the reader, then it would be easy to construct without further information, but full details of how to make this EMI filter follow for the uninitiated. Caveat precautions of working safely with mains voltage are self-explanatory.

Isolate the mains block and remove it completely away to work on.

Remove screws and open up the block. Make careful note where mains wires Brown (L), Blue (N) and Green/Yellow (E) are connected to inside the block. Avoid disturbing the copper busbar placements in their mouldings. Remove the three mains wires.

Of this now free end of mains cable, carefully cut the outer sheath along lengthwise, then cut it off by at least 400mm. It may need more sheath length cutting off if required. Peel away to one side BUT DO NOT CUT the Green/Yellow wire.

Now for the trial and error parts.

Thread the ring down to 50mm from where the newly cut outer sheath ends. Tightly wind the brown and blue wires in pairs through the ring, until the ring is almost full. Don't overwind turns back on themselves. So visually the turn pairs look as: brown-blue, brown-blue, brown-blue etc.

Leave a tail of at least 130mm of wire pair at the upper end to the block. Check for them being long enough to reach inside the block so they would insert easily back to their correct respective connections. If the upper tail is too short to reach as above, remove the turns, strip back more outer sheath as required, then start rewinding again until the tail is long enough.

As a guide when all three connections are eventually remade, the completed ring should be about 40mm from the block's cable-entry hole.

When the brown/blue tail length is finally correct but before reconnecting, offer up the green/yellow wire which may need trimming to fit.

Cut about 50mm of removed outer sheath split along its length and wrap it over the three wires.

Thread the covered together section of wires underneath the loosened cable clamp that is near the cable-entry hole. Don't tighten the clamp down yet.

Reconnect all three wires ensuring they are each comfortably inside their respective moulding grooves without appearing to be stress-taut. Tighten down the cable clamp over the wrapped wires and refit the block cover.

In its still unfinished state, power up the block just to check that the neon light if fitted still lights and check mains is available at each outlet. If all is ok, unplug power to continue building the filter.

PART 2: Where brown and blue wires exit the ring at the bottom end near the sheath, cut them off about 40mm after the ring.

Trim off the ends insulation and offer them to one side of the four-way term block to make sure they enter, ensuring the wire guage is not too tight a fit inside the screw sleeves. If too tight, use 5A term blocks. Don't fully enter the wires just yet.

Carefully space each of the suppressor capacitor legs to offer up to comfortably fit the term block hole spacing.

Turn the term block over to view the flat underside. Insert the capacitor legs but leave 20mm showing outside. Carefully bend the capacitor body to fold over the flat underside. This avoids obscuring the screws on the other side. Trim excess off the legs so the ends sit under the sleeve screws, then remove the capacitor again without disturbing its folded over shape.

Separately, neatly press all the exposed strands each of the brown and blue wires and solder-tin the two ends. Solder-tin the ends of the cap legs. Offer the tinned cap legs to the tinned brown and blue wires and solder each to each. Ensure just enough solder flows evenly through each of the two joints.

While ensuring the capacitor is again folded over the flat underside, insert each soldered new pair of wires into one side of the term block, and tighten down each of the two sleeve screws.

Trim off the ends insulation from the remaining loose brown and blue cable sheath-end wires, then insert those into the opposite side of the term block. Tighten down the sleeve screws. Observe that the green/yellow wire runs loosely outside of the filter assembly and enters the four-way block as in its original state.

Keep fingers away and power up the four-way block again. Ensure the neon lights and mains is still available at each outlet. Unplug the block again.

Finally as per the photo, carefully and tightly wind black pvc electrical tape round the entire filter assembly, to conceal all exposed items including the green/yellow earth wire.

Fit the completed four-way block with filter in its required position and plug in the four electrical items to be powered.

Assuming that a noise problem was originally noted, this should no longer be detected, as the diagram shows the added low-pass filter is now facing the house ring-mains which will be safeguarded against outgoing RF interference.

Finished Project
Click Images to Zoom

The above photos illustrate another compact single 13A socket version of the filter intended as a quickly insertable test unit. These show both an interior view of the constructed filter and as could be used in a practical situation where direct EMI mains injection is suspected of being troublesome and to be confirmed by adding the filter in line between the device under test and the house mains. Purchased from electrical stores, this socket type is usually sold for non-indoor use and contained in a robust metal housing which also conveniently acts as an RF screen.

Shopping List
The 0.1uF capacitor available from Maplin code SC88V has been discontinued and replaced by Wima type N69CN.
The replacement cap has short legs intended for pcb mounting, so need extending with initially 60mm thin tinned copper wires soldered to each leg to effect the assembly as described in the article.
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