Designing my first AC Mains Filtering Circuit - what are the jellybean components

Hi Colin,

I take it you just want a separate filter specifically for the powerline comms, rather than a filter for the AC/DC supply (in which case it's easiest to get a ready-made filter, if it's not already on the AC/DC power supply module).

In that case (separate filter for the powerline comms portion), the most jellybean is probably just a ferrite core, and you can self-wind it. Looking at the spec of the RAK product (they make great stuff btw!), it appears the 30-86 MHz spectrum is used, so the desired common-mode choke should be effective for that range. A type 43 ferrite material is excellent for that. Looking at the Farnell website, there is this 2643801002 ferrite core . It is a bit on the large side (30 mm max diameter) but if space isn't a constraint (and it will be easier to get wires through such a large core) then it should be a good option. According to the coil64 software (which is reliable):

You'd only need three turns on that core, to get 5 uH of inductance, which would get you about 1 kohm of impedance at 30 MHz.

So, you could just take your two wires, and loop them through, at least 3 times. That should be a sufficient filter! However, I've never worked with powerline comms, so if the datasheet specifies anything more, then that should be followed. In the absence of any other specific requirements, I'd use the above ferrite core choke if it were me.

Thanks Shabaz, your comment really helps.

As far as application is concerned, I would say that my filtering question is primarily for the Power Line communication module only but could well be required for both as I would envisage the following setup. However, as you've already noted if using a decent AC-DC converter it will probably include some sort of filtering internally so hence I never saw that as part of the core design criteria but I would see it as worth keeping it in mind (as part general EMI filtering design).

I have noticed in many designs (the diagram below was pinched from STM UM1818 User manual for their Street Lighting power line EVK) that they also includes a signal transformer rather than a differential choke (I assume that they do much similar). I was not sure if this was for good measure or whether it's seen as necessary in this context... or is a signal transformer used for a different purpose to say a differential choke. I suppose if not common then neither would be considered as a jellybean component.

In sense, I've almost answered this question by looking at an all-in-one component designed specifically for power-line. Here, it looks like they only use common mode chokes (L) - diagram on page 3. https://www.farnell.com/datasheets/3975509.pdf

The part labeled ZT1 is serving a different purpose, it is providing safety isolation, which is not what the common-mode choke does (that just removes EMI noise). Any common-mode choke would be placed to the right of that schematic, i.e. where the L and N wires are labelled. 

The all-in-one filter that you mention would normally be used with the AC/DC power supply (if it doesn't have an in-built filter). The AC/DC power supply module could cause interference too, so if it doesn't have a built-in filter, you may not want the powerline comms to be connected to the L and N connections of the module, you may want the all-in-one filter to be connected to the AC/DC module, i.e. this is what I thought was envisaged:

The powerline comms circuit would still need its ZT1 transformer, since that is the only thing providing electrical isolation.

Oops! Thanks Shabaz for pointing that out... I should've referred back to that Kemett YouTube video first before commenting as I can now see all my obvious mistakes (I interchanged DM and CM and was thinking a CM is same as ZT1 transformer). Learning by mistake, in this case.

Oops! Thanks Shabaz for pointing that out... I should've referred back to that Kemett YouTube video first before commenting as I can now see all my obvious mistakes (I interchanged DM and CM and was thinking a CM is same as ZT1 transformer). Learning by mistake, in this case.