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"
