In relation to this discussion, what is the significance of the vertical lines either side of the |L| or |N|? Google doesn't recognise it as a searchable term, possibly thinks it is a logical OR symbol? Thanks James
From: Elliott Martinson [mailto:elliott.martin...@etcconnect.com] Sent: 30 March 2016 22:05 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN Sorry if this is being over exhausted. I don't think I'm straying far from the real-world here. Let's say for sake of argument, CM and DM currents from a single source are necessarily 100% in phase, ignoring whether it's true/false. In a real device of moderate complexity, I don't see any reason why there can't be more than one independent source of noise at a particular frequency. Whether differential-mode or common-mode noise dominates one source, the dominant modes of the other sources don't follow. They also aren't necessarily in phase with each other (they aren't necessarily out of phase either). There's an entire range of possibilities, all affecting the proportion of DM and CM noise required to account for measuring |L| = |N| at a particular frequency, meaning it could be all DM and no CM, it could be all CM and no DM, it could be equal parts both, 60/40, or anything else between. The whole point of this hypothetical measurement is that we don't know enough about the noise source(s) to begin with, so the assumption of a single source could be a bit of a leap, unless we do in fact know our source for sure (such as a switching circuit). Personally, I disagree with stating a rule-whose truth depends on a number of assumptions-as fact without acknowledging and justifying those assumptions. I've read an EMC textbook by each of the authors to whom you referred (great books, btw), and they do argue that if |L| = |N| it follows that one must dominate. Neither acknowledges the assumption of either 0 or 180 degree phase difference between the two. The derived equations may be true, but the interpretation of their implications ignores the fact that a current can have a complex amplitude (i.e. phase shift). The rule may just be stated backwards, mixing what follows from what. It's not that if |L| and |N| are similar, either DM or CM must dominate. Rather, if either DM or CM dominates, |L| and |N| will be similar! ...Besides, If |L| and |N| can be measured individually, that same current probe can also be used to measure |L+N| and |L-N| directly. No assumptions necessary ;) ALSO, How common-mode currents arise and predicting their behavior accurately isn't always very easy to wrap my head around, so correct me if I'm completely wrong here: Common-impedance coupling through parasitic resistance from a DM current leads to a common-mode voltage exactly in phase with that DM current. That CM voltage can then leak through stray capacitance as a CM current that's out of phase with the DM current/CM voltage. The above effect, if correct, is very unlikely to produce CM current of any significance, so the DM current will almost certainly dominate. It's not an example of how |L| can equal |N| without CM or DM dominating but rather an example of CM current out of phase with DM current. Elliott Martinson Product Assurance Specialist I Electronic Theatre Controls 3031 N PLEASANT VIEW RD MIDDLETON WI 53562-4809 Work: 608.824.5696 / Cell: 608.209.9897 elliott.martin...@etcconnect.com From: John Woodgate [mailto:jmw1...@btinternet.com] Sent: Wednesday, March 30, 2016 9:09 AM To: Elliott Martinson <elliott.martin...@etcconnect.com<mailto:elliott.martin...@etcconnect.com>> Subject: RE: [PSES] Commom mode current vs. differential mode current and LISN I wouldn't want to conjecture whether something *might* happen. Real-world issues are enough, I think. You could look at the textbooks by Henry Ott and Clayton R Paul for more general studies. With best wishes OOO - Own Opinions Only www.jmwa.demon.co.uk<http://www.jmwa.demon.co.uk> J M Woodgate and Associates Rayleigh England From: Elliott Martinson [mailto:elliott.martin...@etcconnect.com] Sent: Wednesday, March 30, 2016 3:01 PM To: John Woodgate <jmw1...@btinternet.com<mailto:jmw1...@btinternet.com>> Subject: RE: [PSES] Commom mode current vs. differential mode current and LISN Possibly. I am certainly no expert-in fact I only just found out EMC exists shortly before getting my most recent job last July. Could common-impedance coupling from a purely capacitive or inductive impedance in the return cause common-mode noise that's exactly out of phase with the differential-mode noise? That sounds really unlikely at low frequency where resistance is significant, but at high frequency, maybe on a thin return conductor? Elliott Martinson Product Assurance Specialist I Electronic Theatre Controls 3031 N PLEASANT VIEW RD MIDDLETON WI 53562-4809 Work: 608.824.5696 / Cell: 608.209.9897 elliott.martin...@etcconnect.com From: John Woodgate [mailto:jmw1...@btinternet.com] Sent: Wednesday, March 30, 2016 1:47 AM To: Elliott Martinson <elliott.martin...@etcconnect.com<mailto:elliott.martin...@etcconnect.com>> Subject: RE: [PSES] Commom mode current vs. differential mode current and LISN But your counterexample is not 'real world', I think. Nothing produces DM and CM that satisfies those criteria. Unless, of course, you can find something. With best wishes OOO - Own Opinions Only www.jmwa.demon.co.uk<http://www.jmwa.demon.co.uk> J M Woodgate and Associates Rayleigh England From: Elliott Martinson [mailto:elliott.martin...@etcconnect.com] Sent: Tuesday, March 29, 2016 11:36 PM To: EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN I was about to post about discovering my "erroneous" line of thinking. Something similar to what follows: Line has combination of DM and CM, lets say L = DM + CM Neutral has combination of DM and CM, lets say N = DM - CM Then, when converting to magnitude, my mind said L = ||DM| + |CM|| and N = ||DM| + |-CM|| = ||DM| + |DM|| Therefore L = N In reality, it's L =|DM + CM| and N = |DM - CM| If DM is similar to CM or (-CM) then it initially appears that the magnitudes of L and N cannot be the same barring a zero signal. --------------------EXCEPT------------------------- After yet even further thought, I have discovered a very simple counterexample. Let's say DM = jCM, and |DM| = |CM| L = |DM + jCM| N = |DM - jCM| Since DM and CM are orthogonal, |L| = |DM + jCM| = ||DM| + |jCM|| = ||DM| + |-jCM|| = N Elliott Martinson - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org<mailto:emc-p...@ieee.org>> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)<http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org<mailto:sdoug...@ieee.org>> Mike Cantwell <mcantw...@ieee.org<mailto:mcantw...@ieee.org>> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org<mailto:j.bac...@ieee.org>> David Heald <dhe...@gmail.com<mailto:dhe...@gmail.com>> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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