I may have not phrased my response as clearly as it could have been. What I meant was that with a very high volume product, it is important to minimize the costs associated with each product unit (recurring costs) and thus it would be more economical when faced with an EMC noncompliance to find the source and snub it, as compared to adding a band-aid type fix. So in this case they would go back and re-layout the PCB in order to reduce the source of the emissions that was being snubbed by the ferrite cores. But in a very low volume product line, there isn¹t enough volume over which to spread a nonrecurring cost like re-laying out a board, and it is less expensive to just add those ferrites and ship.
Ken Javor Phone: (256) 650-5261 From: John Woodgate <jmw1...@btinternet.com> Date: Wed, 24 Aug 2016 16:45:27 +0100 To: 'Ken Javor' <ken.ja...@emccompliance.com>, <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: RE: [PSES] EMC & its role in reliability AKA 'customization', which is immediately understood. But I don't see why adding ferrites to combat EMI would be non-recurrent. Anyway, it's all speculation. With best wishes DESIGN IT IN! OOO Own Opinions Only www.jmwa.demon.co.uk <http://www.jmwa.demon.co.uk/> J M Woodgate and Associates Rayleigh England Sylvae in aeternum manent. From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Wednesday, August 24, 2016 4:08 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] EMC & its role in reliability Non-recurring engineering. Can¹t afford very much when the product line is a very small number of high dollar units and when no two shipped products are exactly the same, because each must be tailored to the needs of a unique customer¹s needs. Ken Javor Phone: (256) 650-5261 From: John Woodgate <jmw1...@btinternet.com> Date: Wed, 24 Aug 2016 15:52:52 +0100 To: 'Ken Javor' <ken.ja...@emccompliance.com>, <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: RE: [PSES] EMC & its role in reliability What is NRE? Any relation of NREVIII? (-;) With best wishes DESIGN IT IN! OOO Own Opinions Only www.jmwa.demon.co.uk <http://www.jmwa.demon.co.uk> <http://www.jmwa.demon.co.uk/> J M Woodgate and Associates Rayleigh England Sylvae in aeternum manent. From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Wednesday, August 24, 2016 3:25 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] EMC & its role in reliability I believe it comes down to the tiny volume of this sort of product, and the consequent need to minimize NRE. I mean compared to the automotive market. I couldn¹t remember the source for the digression, but it makes sense a Brit would remind me. Being reminded of the source, I recall hearing the quotation on a series of lectures on Churchill by Larry Arne, the president of Hillsdale College, who is a Churchill biographer. Ken Javor Phone: (256) 650-5261 From: John Woodgate <jmw1...@btinternet.com> Date: Wed, 24 Aug 2016 08:01:19 +0100 To: 'Ken Javor' <ken.ja...@emccompliance.com>, <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: RE: [PSES] EMC & its role in reliability The point isn't so much 'added parts' but 'added parts without thinking'. In a vehicle, *everything* must be tied down so that the inevitable vibration doesn't turn it into a hammer. It's pretty astonishing that it was forgotten in the case of these toroidal cores, and QA *should* have caught it. Perhaps they were added as a field fix by an insufficiently instructed dealer. Re: 'I digress', no, you are quoting Churchill, which is always in order. With best wishes DESIGN IT IN! OOO Own Opinions Only www.jmwa.demon.co.uk <http://www.jmwa.demon.co.uk> <http://www.jmwa.demon.co.uk> <http://www.jmwa.demon.co.uk/> J M Woodgate and Associates Rayleigh England Sylvae in aeternum manent. From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Wednesday, August 24, 2016 3:14 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] EMC & its role in reliability Totally anecdotal, but this just happened to me. My wife uses a powered wheelchair. A very high end, high quality chair. We were on vacation driving north and had just crossed the 45th parallel in the state of Michigan. The 45th parallel anywhere is halfway between the equator and the North Pole. In Michigan it¹s also halfway to nowhere, and it was there my wife¹s chair broke down. The chair weighs close to 250 pounds empty, and is not much fun to move around less the electric motors, especially say up and down the handicap van¹s ramp. The chair has enough smarts to give you a code when something fails. Maybe not the correct code, but a code. In this case, it told us the joystick module had gone bad, so we called a repair place and got one ordered to be sent to the place we were headed. I mentioned we were in the middle of nowhere, but we had pulled over in a cell phone-serviced spot just off I-75 because I was supposed to support a telecon with a customer and I had been looking for just such an oasis. While looking at the chair, I saw a cable had been abraded and the sheathing was damaged and so were internal wires. I repaired those, and although that cable had nothing to do with the joystick module, once I got it repaired the fault code changed to bad power module cable. We haven¹t priced the joystick module, because essentially price was no object with the chair stalled at the side of the road north of the 45th parallel, but I know it will be well north of $500 US. Seeing a new code unrelated to the joystick module did not lighten my mood. Several days later after cooling our heels at what is called the ³tip of the mitt², the northern most point of Michigan¹s lower peninsula, the new joystick module arrived and I installed it and got the same cable fault code as before and no life from the chair. So I removed all the chair plastic covers and traced the cable from the joystick module back to its other end, and I ended up at two plastic connectors with various wires emanating, and ferrite cores around them. Now we are finally getting near the end of the shaggy dog story. Or maybe not the end, but the beginning of the end. Or maybe just the end of the beginning. But I digress. One of those connectors or a wire/pin connection within had loosened and when I pushed on it, the chair woke up and worked. So clearly an intermittent. The chair is several years old, so this was not some sort of infant mortality problem, nor something to be caught by QA before shipping the chair. The ferrite cores I mentioned earlier are not the typical snap-on type, but they are actual toroids around which one could wind a choke or transformer. But they weren't so wound; they were just encircling the cables. Anyway, pushing on one of the two cores caused the intermittent to change states, so clearly over time that heavy core bouncing around the cable had caused something to open up. So the lesson here is that adding EMI fixes can actually decrease reliability. Stating that in some circles where EMC is held to be a reliability issue is nigh on to heresy and can literally cause a violent argument. I could tell that story, but I won¹t. But it is in fact the case that adding parts decreases reliability it¹s just probability theory. Snapping a core around a cable at the last minute may be attractive and relatively risk-free on a piece of portable electronics, but when it is a vehicle and/or people¹s lives depend on its functionality, then the modification must be subjected to review. Related to this problem, I once worked a spacecraft that had a long RS-232 cable interface, for which they had chosen some very fast logic family, and the cable was electrically long relative to the transition time. There was considerable ringing on leading and trailing edges, and there was an expressed concern that such ringing could look like an extra bit. The fix here was to slow down the rise time (the data rate was OTOO 100 kbits/sec), but no one wanted to change anything, so I suggested one of these press-in connector filters (ee-seal by Quell), and that was demonstrated to work. But the ancient program manager, confronted with this news, asked the question: ³Do capacitors fail open or closed?² The answer was ³closed.² The program manager, the recipient of no telling how many whippings by Murphy, responded, ³Not on my spacecraft,² and that was that. Intelligence guided by experience. Ken Javor Phone: (256) 650-5261 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to <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) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
