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From: dmturne@PacBell.COM (Dave Turner)
Newsgroups: sci.electronics
Subject: Re: How were component values selected?
Message-ID: <6142@ptsfa.PacBell.COM>
Date: 20 May 91 22:55:43 GMT
References: <1991May19.020604.13608@deeptht.santa-cruz.ca.us>
Reply-To: dmturne@PacBell.COM (Dave Turner)
Organization: Pacific * Bell, San Ramon, CA
Lines: 48

In article <1991May19.020604.13608@deeptht.santa-cruz.ca.us> spcecdt@deeptht.santa-cruz.ca.us (John DuBois) writes:
>     I've been trying to figure out how the lists of valid component values
>for parts of various tolerance were generated.  It seems to me that the
>
>So, does anyone know how the standard list came to be?
>

According to the *Reference Data for Engineers* (7th Edition Sams), Chapter 5
Components or Parts:

	  "To maintain an orderly progression of sizes, preferred numbers
	are frequently used for the nominal values. A further advantage is that
	all parts are salable as one or another of the preferred values.
	Each preferrred value differs from its predecessor by a constant
	multiplier, and the final result is conveniently rounded to two
	significant figures.
	  "ANSI Standard Z17.1-1973 covers a series of preferred numbers
	based on (10)**(1/5) and (10)**(1/10) as listed in Table 2. This
	series has been widely used for fixed wirewound power-type
	resistors and for time-delay fuses.
	  "Because of the established practice of using +/-20-, +/-10-,
	and +/- 5-percent tolerances, a series of values based on (10)**(1/6),
	(10)**(1/12), and (10)**(1/24) has been adopted by the EIA, and is
	now an ANSI Standard (C83.2-1971) (EIA RS-385). It is widely used
	for such small resistors and fixed ceramic, mica, and molded paper
	capacitors. These values are listed in Table 2. (For series with
	smaller steps, consult the ANSI or EIA Standard.)"

Table 2 shows the following step sizes:

Series	Step Multiplier			Percent step size

"5"	(10)**(1/5)	= 1.58			60
"10"	(10)**(1/10)	= 1.26			25
E6	(10)**(1/6)	= 1.46		       ~40
E12	(10)**(1/12)	= 1.21			20
E24	(10)**(1/24)	= 1.10			10

Two other series are also standard:

"20"	(10)**(1/20)	= 1.222			12
"40"	(10)**(1/40)	= 1.059			 6


E24	(10)**(1/24)	= 1.10

-- 
Dave Turner	415/823-2001	{att,bellcore,sun,ames,decwrl}!pacbell!dmturne