This phenomena with simulators was documented back in the mid 80's. If you simply charge a up a capacitor in mid air or inside a plastic housing (ESD Simulator) the cap will immediately begin to bleed off in an attempt to equalize itself with the rest of the world. The higher the voltage, the more noticeable this effect and if you connect a long pointed tip the capacitor (via a resistor or not) this enhances the bleeding and makes it happen quicker. On a humid day, you can actually see the corona around a point tip connected to a simulator charged to > 20kV (if you dim the lights).
Also, as you approach an object that's at a different potential -- let's say a metal chassis -- prior to an actual discharge, electrons begin to stream from the tip of the simulator to the metal chassis, reducing the actual charge voltage at the tip in the process. Again, much more pronounced at voltages >20kV than at lower voltages. This "pre-discharge" discharge is clearly visible in low light as a reddish glow. Many simulators now have circuits which basically monitor the cap voltage through a comparator and supply a trickle charge to the cap to keep the voltage up to where it was set. If you don't do this, the actual test voltage becomes a factor of the time between t=0 (when the cap was charged) and some later time when the tip gets close enough to the victim equipment to cause a discharge. Some years ago, there were some papers done both at the Zurich EMC Symposium and the IEEE EMC Symposium that talked about this, but I'd guess it was mid to late 80's and I no longer have these records. Hope this helps..... Best Regards, Mike Hopkins Thermo KeyTek -----Original Message----- From: richwo...@tycoint.com [mailto:richwo...@tycoint.com] Sent: Tuesday, December 04, 2001 10:32 AM To: emc-p...@majordomo.ieee.org Subject: ESD Simulator Evaluation An engineer was evaluating ESD simulators and observed an interesting phenomenon. > The air discharge tip was placed at a fixed distance of about 1 inch from > a ground reference plane with the simulator set for 30 kV and a healthy > spark was launched by all of the simulators. > > HOWEVER, when he started from some distance and rapidly approached the > grp, one of the simulators appeared to have lost 2/3 of the charge. The > other 3 did not. He base the conclusion on the fact that the spark > launched from about 1 inch for 3 simulators, but launched from about 1/3 > inch from one unit when using the "rapid approach" air discharge method. > Any suggestions on what is happening here? > > Evaluation of simulators is specified using a target with the tip > contacting the target. We > did not see a parameter in the standard that allows one to evaluate the > ability of a simulator tip to hold a charge. Did we miss something? > Richard Woods Sensormatic Electronics Tyco International ------------------------------------------- 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.
