Author Topic: Electro-Magnetic Compliance - traps for young players  (Read 1571 times)

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Offline wosser

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Electro-Magnetic Compliance - traps for young players
« on: May 30, 2012, 07:21:33 am »
I'd like to hear Dave do a video about the difficulties of designing a product to be EM Compliant.

I've recently been battling with EMC issues and no doubt falling into all the obvious pitfalls that everyone falls into.  Now we've got the problem solved and it's relief all round, but this would have been easier and cheaper if I had given EMC more effort at the beginning of the project.

I'd love to see a good EMC "briefing" (I think a proper tutorial would be too difficult to fit into a video blogging format like EEVBlog).  Just warning people of the potential issues and stressing the importance and good economical sense of addressing these EMC issues before the design process begins, would be a very useful episode for many designers.

Problems I found during my EMC battle...
  • Plastic enclosures with "EMC coatings" DO NOT WORK!
  • Metal enclosures with even the slightest gap between panels don't work either
  • A badly designed box may actually cause WORSE EMC performance than the naked electronics (the lighthouse effect)
  • Ground all your connectors securely to the metal enclosure - otherwise they are a path for RFI run amok inside your design.
  • Ferrites are your friends
  • Conductive gaskets are vital
  • Spectrum analysers are expensive, but in the long run it's worth it because it saves you running off to the expensive EMC lab all the time.

I'm sure Dave has experience with this kind of design challenge.
« Last Edit: May 30, 2012, 07:32:32 am by wosser »
 

Offline Neilm

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Re: Electro-Magnetic Compliance - traps for young players
« Reply #1 on: May 30, 2012, 08:27:19 pm »
It is interesting the number of times this comes up. Each time it does I point the poster to a series of articles by Keith Armstrong that are published in the compliance club. Find them at http://www.compliance-club.com/] [url]http://www.compliance-club.com/[/url]. I have found the explanations simple to understand and not overly heavy on maths.

The single biggest price of advice is think about the EMC at the start of the project. Before any work starts, including housing, prototype circuits and the like. It is at this point you can explain to your boss that spending a little more money on a PCB will save money in the longer term. (KM did an article on this about 2/3 years ago)

On your points;

  • Plastic enclosures with "EMC coatings" DO NOT WORK!
Plastic boxes with  "EMC coatings" can work, but the coating has to be sprayed on correctly and correctly connected to a reference.  I have used this in the past when I had a board that was a bit noisy but the coating was very expensive and an absolute pig to ensure it was connected to 0V.

Quote
  • Metal enclosures with even the slightest gap between panels don't work either
See the definition of a slot antenna. (also a metal enclosure with holes covered in sprayed plastic doesn't work either)
Quote
  • A badly designed box may actually cause WORSE EMC performance than the naked electronics (the lighthouse effect)
  • Ground all your connectors securely to the metal enclosure - otherwise they are a path for RFI run amok inside your design.
Oh yes
Quote
  • Ferrites are your friends
Yes, but preferably only as a last resort. If you can install them on a PCB rather than clipping them on leads then so much the better.
Quote
  • Conductive gaskets are vital
It will close the gaps, especially if you are having problems at very high frequencies.
Quote
  • Spectrum analysers are expensive, but in the long run it's worth it because it saves you running off to the expensive EMC lab all the time.
These are generally only any good in the near field, some standards will actually say that these can't be used to make the measurement. They are invaluable for finding what areas are radiating however these might not be the cause of the far field problems.

Yours

Neil
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