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Wide-band RF filtering for mains - part III

Posted 18th February 2011 at 05:19 AM by abraxalito
Updated 2nd March 2011 at 12:09 PM by abraxalito

OK, slight change of plan. Due to a ****-up on the ordering front with the iron wire, I ended up with some wire which is a bit too thin (0.5mm inner diameter) as they specified the outer diameter on the ordering page, rather than the conductor diameter. On reflection of course, that makes perfect sense since this isn't wire for electrical purposes at all, what use is the 'conductor' diameter? Wire with an external diameter of 1.8mm's on its way, which I hope will have ~1.2mm diameter conductor.

So, in the meantime I've been experimenting with wire I have been able to get hold of, which is shiny steel. It turns out that only 30 paces from my home is a hardware store which sells it - so I've tried winding chokes with it, just to see what happens. I've also acquired an LCR meter to see what kinds of values I'm getting at different measuring frequencies.

The first pair of chokes I wound with some really thick stuff - 2.8mm. Having aimed for 20 turns I calculated the desired length of the wire as 1.7m. It gave me blisters winding it around the metal handle of my floor mop (around 20mm diameter), so I don't really recommend using this. The low frequency (50Hz) inductance measures 13.3uH which falls off to around 3.2uH at 100kHz. I wound a second pair just because I had some wire left.

The third pair I've wound with 2.2mm wire, much easier to wind. Owing to the smaller diameter, more turns will fit into the same volume so I cut lengths of 1.9m. 27 turns later these measure at 14.8uH @ 50Hz and 4.3uH @ 100kHz. I've spaced the turns out nicely to reduce the self-capacitance and they're about 10cm long.

My attached pic shows how I've built the first two chokes into a mains line to a distribution block. To keep them apart, I hacked up some IC packaging rail (having first checked it wasn't conductive). I've decided to build this up in a modular fashion, so the CM chokes for the next stage will go in a separate box, along with the snubbers. Having the first two chokes separate keeps the stray capacitance as low as physically possible.
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