In message <be3336be85968d49be01e66d6e365b1e01b59...@sjc1amfpew01.am.sanm.corp>, dated Tue, 13 Mar 2007, "Tarver, Peter" <peter.tar...@sanmina-sci.com> writes:
>Or maybe there's a mathematical reason that hasn't occurred to me, like >some arithmetic progression, or even simple phobias or prejudices. Ah, you youngsters! (Strokes long, white beard.) It's based on geometric progression and it's about selling all the parts you make, with the fewest nominal values, even if the process results in a wide spread of values. It's simplest to explain if we go back to the agricultural tolerances on component values of 50 years and more ago. Then, the closest sensible tolerance available on carbon composition resistor values was +/-10%. +/- 5% parts were available, but values tended to drift with age more than that. OK, so a 1 k resistor could be as high as 1.1 k. If that is also the lowest limit for the next nominal value, that nominal value is 1.22 k, rounded to 1.2 k. And the next is 1.2 x 1.1/0.9 = 1.47 k, rounded to 1.5 k. The next is 1.8 k, and then 2.2 k, but if you carry on too far in that way, you find anomalies; a ratio of 1.2 gives values lower than the actual '10%' series, while 1.22 gives higher values. The reasons are often disputed, and I'm not going there, thank you. Another way of looking at it is that successive values are multiplied by the 12th root of 10, 1.211.., but that doesn't fit exactly, either. But you can now see the basic principle. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
