Re: Baffled by EFT test results (EN61000-4-4)
I investigated the phenomenon of capacitor damage by transients and found caps to be very resistant to damage from short duration, especially high source impedance (50 Ohm) spikes. There is a complete write up entitled, Investigation Into the Effects of Microsecond Power Line Transients on Line-Connected Capacitors NASA/CR-2000-209906 at: http://see.msfc.nasa.gov/ As I write this, I cannot access the site and get you the precise page it is on, but it is usually accessible to the public (a pdf download). I also have the original in MS Word that I can send if necessary. -- From: Chris Chileshe chris.chile...@ultronics.com To: 'EMC-PSTC List' emc-p...@majordomo.ieee.org Subject: Baffled by EFT test results (EN61000-4-4) Date: Fri, Oct 25, 2002, 6:55 AM Hi Group, So I went ahead and built a test PCB with my inductors and ferrites in PI filter formation with capacitors - just like I threatened to do a few months ago ( e-mail subject header Designing for low power radiated and conducted immunity describing a small 3-wire low power DC pressure sensor). The analog circuit simulating a pressure sensor is simply a 5V regulator (LP2985) supplying a AD623 IN-amp monitoring the differential voltage across a wheatstone bridge (4k7) connected to the same 5V rail. The output then simply connects to an Avometer (through a PI filter of course). A variant of the test PCB doesn't even have the regulator. It is that simple. I took the PCB through radiated immunity testing to EN 61000-4-3 and met 100V/m, and through conducted immunity testing to EN 61000-4-6 and met the 10V level. All tests were done with unscreened cable. I also proved the protected circuit was susceptible when the filter was removed. Having decided I had sized my inductors, ferrites and caps appropriately, and done the PCB layout correctly, I then proceeded to add small SMT varistors for transients to EN 61000-4-4. The test PCB was again good for 2kV. I then shuffled the relative positions of the varistors in the circuit (initially pre- PI filter, then post PI filter ) and it didn't seem to make any difference. The idea was to get some permutations of varistor location and then repeat the RF immunity tests in case the varistors introduced susceptibility (an observation I have made in the past). I then took the varistors off the board altogether and still the PCB meets the 2kV stress levels ( I am using the levels of EN 61000-6-2 ). I am not too keen to raise the stakes to 4kV just now because there are more tests I would like to perform on the PCB before I let it go up in smoke. This is probably the first time I have seen anything get through transients testing without any explicit measures to mitigate against failure. Has anyone experienced this? Is there an explanation for this? I have always used varistors almost as a matter of course, so you can understand my surprise. Could it be I have over-designed my RF filter to the point it is good enough for the EFTs as well? The caps are only rated for 50V though ( the clamping voltages of the varistors). Could it be the caps are in fact rated for such transients although this may not be stated explicitly? Should I prolong the exposure of the varistor-free circuit at 2kV to see if I am dealing with delayed failure potentially? I have been setting my burst duration to 1 minute. Any pointers? Regards - Chris Chileshe - Ultronics Ltd This e-mail has been scanned for all viruses by Star Internet. The service is powered by MessageLabs. For more information on a proactive anti-virus service working around the clock, around the globe, visit: http://www.star.net.uk --- 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 --- 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
Re: Baffled by EFT test results (EN61000-4-4)
I read in !emc-pstc that Chris Chileshe chris.chile...@ultronics.com wrote (in 01c27c25.d813b6c0.chris.chile...@ultronics.com) about 'Baffled by EFT test results (EN61000-4-4)' on Fri, 25 Oct 2002: This is probably the first time I have seen anything get through transients testing without any explicit measures to mitigate against failure. Has anyone experienced this? Is there an explanation for this? Yes, sometimes you get lucky! I have always used varistors almost as a matter of course, so you can understand my surprise. Could it be I have over-designed my RF filter to the point it is good enough for the EFTs as well? The caps are only rated for 50V though ( the clamping voltages of the varistors). If you can afford it, you haven't over-designed it. (The converse is not, unfortunately, always true.) Could it be the caps are in fact rated for such transients although this may not be stated explicitly? Should I prolong the exposure of the varistor-free circuit at 2kV to see if I am dealing with delayed failure potentially? I have been setting my burst duration to 1 minute. Without knowing what type the capacitors are, and the exact circuit configuration, one cannot say. How much of the 2 kV actually appears across the caps? (Measure, it might not be what you expect). Leave the varistors in if you can afford to and they don't cause a problem if/when they explode. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- 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
Baffled by EFT test results (EN61000-4-4)
Hi Group, So I went ahead and built a test PCB with my inductors and ferrites in PI filter formation with capacitors - just like I threatened to do a few months ago ( e-mail subject header Designing for low power radiated and conducted immunity describing a small 3-wire low power DC pressure sensor). The analog circuit simulating a pressure sensor is simply a 5V regulator (LP2985) supplying a AD623 IN-amp monitoring the differential voltage across a wheatstone bridge (4k7) connected to the same 5V rail. The output then simply connects to an Avometer (through a PI filter of course). A variant of the test PCB doesn't even have the regulator. It is that simple. I took the PCB through radiated immunity testing to EN 61000-4-3 and met 100V/m, and through conducted immunity testing to EN 61000-4-6 and met the 10V level. All tests were done with unscreened cable. I also proved the protected circuit was susceptible when the filter was removed. Having decided I had sized my inductors, ferrites and caps appropriately, and done the PCB layout correctly, I then proceeded to add small SMT varistors for transients to EN 61000-4-4. The test PCB was again good for 2kV. I then shuffled the relative positions of the varistors in the circuit (initially pre- PI filter, then post PI filter ) and it didn't seem to make any difference. The idea was to get some permutations of varistor location and then repeat the RF immunity tests in case the varistors introduced susceptibility (an observation I have made in the past). I then took the varistors off the board altogether and still the PCB meets the 2kV stress levels ( I am using the levels of EN 61000-6-2 ). I am not too keen to raise the stakes to 4kV just now because there are more tests I would like to perform on the PCB before I let it go up in smoke. This is probably the first time I have seen anything get through transients testing without any explicit measures to mitigate against failure. Has anyone experienced this? Is there an explanation for this? I have always used varistors almost as a matter of course, so you can understand my surprise. Could it be I have over-designed my RF filter to the point it is good enough for the EFTs as well? The caps are only rated for 50V though ( the clamping voltages of the varistors). Could it be the caps are in fact rated for such transients although this may not be stated explicitly? Should I prolong the exposure of the varistor-free circuit at 2kV to see if I am dealing with delayed failure potentially? I have been setting my burst duration to 1 minute. Any pointers? Regards - Chris Chileshe - Ultronics Ltd This e-mail has been scanned for all viruses by Star Internet. The service is powered by MessageLabs. For more information on a proactive anti-virus service working around the clock, around the globe, visit: http://www.star.net.uk --- 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
