Take E12 series - 12 values based on 10^1/12 = 1.212 - you calculate the next value by multiplying the last by this number 1.212 and rounding thus:
1.0 1.0 x 1.212 = 1.2 1.212 x 1.212 = 1.47 = 1.5 1.47 x 1.212 = 1.78 = 1.8 Regards, Chris From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Barker, Neil Sent: 13 March 2007 15:34 To: 'Tarver, Peter'; PSTC 1 Subject: RE: standard component values Peter, My understanding is that each series, E12, E24, etc increments approximately according to the corresponding tolerance such that adjacent values approximately meet at the upper tolerance of one value and the lower tolerance of the next greater value. This is rounded off to the nearest integer value, hence 10, 12, 15, 18, 22, etc for a tolerance of ±10%; i.e. E12 series for ±10%, E24 series for ±5%, and so on. Best regards Neil R. Barker CEng MIET FSEE MIEEE Manager Quality Engineering e2v technologies (uk) ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: (+44) 1245 453616 Fax: (+44) 1245 453571 Mob: (+44) 7801 723735 P Please consider the environment before printing this email. From: Tarver, Peter [mailto:peter.tar...@sanmina-sci.com] Sent: 13 March 2007 15:24 To: PSTC 1 Subject: OT: standard component values An off-topic question that hopefully someone can shed a little light upon. I have wondered for as long as I've been involved in things electrical, why standard component values are what they are. Text books are of no use and I've done more than a few internet searches on this in the last ten years (or so; maybe I've used the wrong search terms, but I haven't found an answer). I'm interested to hear what others might know or think about this arcane topic. It's probably lost to history now, but it may well be based on some limitations of physical characteristics of early devices or one manufacturer dominating the early production of components and they had some idea that they could sell more components if they were in strange increments. Or maybe there's a mathematical reason that hasn't occurred to me, like some arithmetic progression, or even simple phobias or prejudices. Maybe someone gave it the deepest thought and surmised that particular values would yield the minimum of mixing of components in series/parallel combinations. Maybe they're still laughing from the grave. Regards, Peter L. Tarver ptar...@ieee.org CONFIDENTIALITY This e-mail message and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail message, you are hereby notified that any dissemination, distribution or copying of this e-mail message, and any attachments thereto, is strictly prohibited. If you have received this e-mail message in error, please immediately notify the sender and permanently delete the original and any copies of this email and any prints thereof. 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