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Wednesday, 4 March 2015

RFI from old PC fan - using copper mesh to fix.

Whilst waiting for the solder iron to heat up I thought I'd have a quick look at a a couple of cooling fans and the RFI they were creating.

Two 80mm (c 3") PC fans bolted to a heatsink, 30 amp 13.8V supply I need for the K2 I am building. They have a simple set of DC leads to powerpoles.

When in use the RFI they create is a tad obvious, does not effect operation of the supply at all. Without them the supply gets warm :-) I cannot change the fans (although I might look at doing that another time maybe).

Why I do not know but was thinking about PhD Chemistry days (93-96) I used to use a reflux still to purify some solvent (I am sure you homebrew moonshine guys know the stuff I mean) I was using at the time, even AR grade 99% wasn't good enough and was effecting the laser dye output ... the what/why doesn't matter but the how is I remember using some pure copper woven cloth, looks a bit like one of those holey jumpers, in the distillation setup. We fed in the solvent, distilled it and hey presto laser dye worked.

So quick visit to my ex-labs and chat with a technician and I now hold a new 2 foot length of it. It is considered a consumable so they have it on a big drum and just hacked off a piece. They use it for one experiment and ditch it! and use a new length.

It is a woven 'tube' so it will expand a bit, but this is made from 99.99% pure copper wire. So the idea is to wrap the fans with it.... reattach to the heatsink, add a couple common mode chokes to the DC feed line. The holes will allow the fans to keep doing their job and hopefully the copper screen will help a bit.

Took all of 5 mins to put into place and tried both without and with -61 ferrite common mode choke on the DC line (12 turns in opposite direction of the feed line about a torroid)  and seems to have reduced the noise to the point where it is usable again.

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