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 

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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 

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