Well your calculation shows that they are approximately equal, with a slight advantage to the tube. Both intuitively and calculations seem correct. When I wrote the comparison I was thinking of massive conductors not tubes, as tube conductors normally find no place in electronics. Especially when they are 0.75 in diameter.
Massive conductors are too often abused for emc grounding, and I needed a simple rule of thumb to show the difference. Regards, Ing. Gert Gremmen [email protected] www.cetest.nl Kiotoweg 363 3047 BG Rotterdam T 31(0)104152426 F 31(0)104154953 Before printing, think about the environment. Van: [email protected] [mailto:[email protected]] Namens [email protected] Verzonden: Friday, November 06, 2009 2:28 AM Aan: [email protected] Onderwerp: RE: [PSES] Discharge capacitors Flat rectangular is NOT better than a round tube if each have the same circumference. The round tube has lower inductance than a flat ribbon conductor of comparable circumference. Makes sense, because Nature abhors 'pointy' stuff. This statement is based upon a comparison analyzing the two structures using finite element analysis, femm 4.2. Given: solid ribbon copper conductor, 1 inch wide and 120 mils thick copper tube, 0.75 inch outside diameter and 120 mils wall thickness The two circumferences are approximately the same. Rectangle 2 * (0.12+1) = 2.24 inches Tube pi * 0.7 = 2.20 inches Which is a better conductor? Assume infinitely long, straight conductors. Assuming most of lightning energy is significant between 1MHz to 100MHz, calculate each conductor's characteristics at 10MHz and at 100MHz using 2D finite element analysis. Method: place each conductor in a 24 inch diameter metal 'tube' to provide return current and represent infinity. Note: I also used single conductors in free space with current return at infinity. Values changed, but the conclusions did not change. Use a 12 inch length to reference values per ft. Mesh was set to be fine near the surfaces of the conductors, so that even with hgih frequency currents inside the conductors were accurately represented. Skin depth was more than 3 nodes. >From results, the current as a function of depth into the conductors matched expected values. Plots of current/eddy currents verified mesh was of suficient density for these calculations. TABLE RESULTS: Ribbon - Solid mesh 69,406 10MHz 210 nH/ft 0.00347 W 13.2j ohm/ft 83 milliohm/ft 100MHz 210 nH/ft 0.0115W 132j ohm/ft 0.15 ohm/ft Tube - Hollow mesh 90,233 100MHz 203 nH/ft 0.00239W 12.8 ohm/ft 69 mOhm/ft 100MHz 203 nH/ft 0.00732 128 ohm/ft 0.12 ohm/ft It was interesting to note that with a wall thickness of more than 10 mils, at these frequencies the metal was doing nothing but physically supporting the outside layer. CONCLUSION: Round is always better. Robert > As a rule of thumb, a conductor > > has an (self)inductance proportionally inverse with > > it's circumference (if fact the shortest way the magnetic field lines > will take). > > Big flat conductors always perform better then round ones, as they > > have the highest circumference per kilo. > > Litz and silver(gold) coated conductors do contribute to the > > real part of the impedance (=resistance) properties only. > > Litz by increasing the conductive surfaces so reducing > > the resistance increase caused by the skin effect. > > > > > > > > Regards, > > Ing. Gert Gremmen > > > > [email protected] <mailto:[email protected]> > > www.cetest.nl > > > Kiotoweg 363 > > 3047 BG Rotterdam > > T 31(0)104152426 > F 31(0)104154953 > > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
