Hi Group, I am working on a small Class II product (A) i.e. no earth. The product is powered by 12Vdc via a 1M cable with RJ type connections from another similar Class II product (B) which itself is powered via an external in-line AC/AC power block (basically an AC/ac transformer) and rectified to 12Vdc internally. It has a CMC on the supply lines. The system (A+B) EMC tests were fine. However, since product A may be used in future as a stand alone product i.e. powered from it's own 12Vdc power supply, I had to add a small common mode choke to the input supply lines, the same lines as the RJ connection method, to enable me to meet the EMC requirements. Since I had added a major EMC component I decided to re-test the EMC of the system. To my horror the system (A+B) readily failed the mains EFT tests on product B. Removing the CMC on product A resolved the EFT problem. I now have to create 2 product A builds depending on the option of usage, one with CMC (Stand alone usgae) and one without a CMC (system usage).
I am sure I have had a similar problem in the past but at that time I did not realise that the addition of the CMC (to resolve conducted radiated emissions) was the source of the EFT failures. I think the EFT problem must have something to do with high/low impedance mis-match due to the CMC but I am not sure. I would appreciate a more experienced member of the group explaining as to why this scenario can occur? and more importantly as a warning to others! As always I look forward to your valuable comments Kind Regards Alex McNeil Principal Engineer Tel: +44 (0)131 479 8375 Fax: +44 (0)131 479 8321 email: alex.mcn...@ingenicofortronic.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
