Hi Enci & Other Folks Comments:
1) The actual value of the capacitor may not the most significant factor - it is the actual voltage effect on the "user" that is most important, and that can be significant even if the filter meets the 0.1uF limit requirements. Even if the capacitor is smaller than that, the "shock" can be enough to cause the reflex reaction to which I previously refered. 2) If the unit has a switch, then the worst effect is often with the switch in the OFF position if it then isolates the capacitor from any internal discharge paths - So much for "switch off before disconnection" or similar notices! (In fact, it was due to that particular issue that I first encountered the problem - around 20 years ago!) 3) Linear transformer units are generally better than SMPU's in providing an internal discharge path as the impedences are much lower and the number of components between them and the filters are also lower. Regards John Allen -----Original Message----- From: Enci [mailto:e...@cinepower.com] Sent: 29 October 2001 12:35 To: emc-p...@majordomo.ieee.org Subject: RE: EMI filter hazards At 09:31 29/10/01 -0000, John Crabb wrote: > >IEC60950 requires that capacitor exceeding 0.1uF should have a means of >discharge resulting >in a time constant not exceeding 1 second for pluggable equipment Type A. ".. a means of discharge" As I understand it there is no need to blindly install a resistor. Depending on the product, it may discharge the filter caps. This is how I test when the need arises: Hook up a relay normally closed, providing power to your appliance. Connect one input of your scope (battery powered scope is best) as the trigger on the relay coil. Connect the other input in voltage capture mode across the appliance terminals. When you open the relay, the second input is triggered by the first input and captures the residual voltage across the appliance filter. (in reality you see a few mains cycles as well due to the operating time of the relay) You will also need a small dc supply for the relay. Repeat the test about 50 times and if you have a good portable scope you should be able to print off the discharge waveform and stick it in the design file. Cheap. :) Enci ------------------------------------------- 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: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net 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: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- 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: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net 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: No longer online until our new server is brought online and the old messages are imported into the new server.
