Path: utzoo!attcan!uunet!psivax!torkil
From: torkil@psivax.UUCP (Torkil Hammer)
Newsgroups: sci.electronics
Subject: Re: Preferred Values of Electronic Components
Keywords: preferred values resistors capacitors inductors
Message-ID: <2408@psivax.UUCP>
Date: 20 Jan 89 22:19:21 GMT
References: <1989Jan18.120059.16644@mntgfx.mentor.com>
Reply-To: torkil@psivax.UUCP (Torkil Hammer)
Organization: Pacesetter Systems Inc., Sylmar, CA
Lines: 43

In article <1989Jan18.120059.16644@mntgfx.mentor.com> bobk@mntgfx.mentor.com (Bob Kelley) writes:
#I've been wondering how the standard values for resistors,
#capacitors, and inductors are derived.  Does anyone have
#tables of 1%, 5%, 10% resistor, capacitor, and inductor
#values?  Better yet, is there an analytical approximation
#that can be used to determine these values?


Components come in E series.  Standard series are E3, E6, E12, E24, E48 and
E96.  The E number is the number of steps per decade.  The analytical
formula for the values of the i'th component in the N'th series (3,6,12..)
is 10 to the power of (i/N).  The value is rounded to nearest 2 or 3
digits, but some of them have been chosen slightly off for some reason.

Example: the E12 series.  2 digit values should be
10,12,15,18,22,26,32,38,46,56,68,83  based on above formula but
10,12,15,18,22,27,33,39,47,56,68,82  are the customary values.
I would appreciate to know why they are off.  Did somebody really optimize
that hard for the future, back in the '20s?

The rationale for using 3,6,12.. series is, that the step size for the
E12 series is 20%, hence it ideal for silver resistors of +/- 10%
[now you know my approximate age...], the E24 series is perfect for
gold resistors of +/- 5%, while the E6 is good for (what used to
be ordinary) tolerance of +/- 20%, and E3 for capacitors of 50 schmifty %.

Even today it shows.  Hobby kit boxes of diverse resistors come
typically in the E24 series of 5% tolerance, but the component count
peaks markedly at E3, E6, E12 values in decreasing order - because oldtime
designers are so used to thinking 10 15 22 33 47 68, that they still
swear to those numbers in uncritical applications, so a hobbyist
following a cookbook diagram will most likely use more of these values.
Semi-oldies like yours truly will also intersperse 12 18 etc and
cause minor peaks in the demand.  We are still reluctant to use the
full range because it used to be that 10% resistors were more
expensive and harder to get than 20%, and 5% were special orders
requiring requisition paperwork and delivery time, so whatever today 
happens to be in the rack is likelier to be E6 or 12 than E24 for
self perpetuating reasons.

You are welcome.

torkil