Gabi, The topic of an air-discharge spark gap is a little bit more complicated.
1) The breakdown voltage in air for a homogeneous field is given by the Paschen-equations, providing that the breakdown is a gas discharge process, not an explosive surface process (happens at high pressure, distances less than about 5um and if the gap is highly overvoltaged) Even for a homogenous field, the breakdown fieldstrength is a strong function of voltage. 2) A spark gap will need some time to turn on. There are two processes: Statistical time lag: this is the time it takes before the first electron appears that can start the avalange process. Time lags may be ps to seconds, depending on the field strength and many other factors. 3) Formative time: The time the spark needs from its start until its impedance is low. The time may be ns to us. 4) Clamping voltage. Typically spark gaps clamp at about 25 V for currents of less than 100 A in time frames of 10ns to a few hundred ns. I do not know the physical reason for the 25 V. If anyone knows, please let me know. 5) In my experience a PCB using the footprint of an 0805 part (not loaded) will break down at about 2000-3000V. 6) It is not easy to get breakdown voltages consistantly below 500 V with spark gap structures in air. The needed distances are so small that surface properties, contamination etc. start to dominate. Regards David Pommerenke Associate Professor Electromagnetic Compatibility Laboratory ECE Department University of Missouri-Rolla 1870 Miner Circle Rolla, MO 65409-0040 pommere...@ece.umr.edu Phone: (573) 341-4531 Home: (573) 341 5835 FAX: (573) 341-4532 -----Original Message----- From: Gabi Hoffknecht [mailto:gab...@simex.ca] Sent: Wednesday, February 20, 2002 10:33 AM To: emc-p...@majordomo.ieee.org Subject: Using PCB traces as transient voltage suppressor Hi all, I have seen PCB designs with two triangular shaped copper pads pointed towards each other at very close proximity, meant as an air gap discharge path for transients. Does anyone have information about such designs, whether they work and how well ? At a breakdown voltage for air of 1 Megavolt per meter, they should theoretically work: 10mil distance would have a breakdown voltage of only 254V. Such a PCB design basically comes for free, so I was thinking of adding it on top of my already existing series impedance - TVS network. Thanks in advance for your comments. Best regards, Gabi Hoffknecht ------------------------------------------- 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...@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: 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...@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: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
