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 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"
