Preferred Values of Electronic Components
Bob Kelley
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?
Torkil Hammer
#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
Richard S D'Ippolito
>In article <1989Jan18.1...@mntgfx.mentor.com> bo...@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.
Yeah, I noticed that a long time ago. Beats me why the marking is wrong!
But, you forgot the important reason for picking such a series: it allows a
full range coverage of all possible values with minimal overlap. (That
means the manufacturer gets to sell all of his production!) For fun, take
some semilog-paper and plot the centerpoints and the rangebands.
By the way, I remember when some maunfactures of resistors took the center
5% bands out of the 10%-ers so that it was impossible to go to the bin and
select a 10%-er within 5% of its marked value.
Allen-Bradley never did that -- their process was so controlled that all of
the units of a lot were within a percent or so of each other. It used to
drive me nuts trying to find a range of values to trim a circuit, until the
price differential shrunk so much that we began buying all 5%-ers. (BTW,
can you sill buy 10% resistors?!)
>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?
To see what he means, the highest (10%) value for a 22-ohm unit is 24.2 and
the lowest 27 is 24.3, leaving a hole. You can't properly mark and sell a
24.25-ohm unit! If I remember correctly, some of the 1%-ers are like that.
Rich
--
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You can lead a horse to water, R...@sei.cmu.edu
but you can't make him fish.
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John Eaton
< I remember reading *years* ago that the "standard" 10% resistor
< values were really closer to +/- 12%, (there was some root-type
< function in there, I think) and the reason for it was so that
< absolutely *any* value they made would fit in one or another
< slot. I think that might also be true for the 5 or 20%
< ranges.
----------
This may be an urban myth but aren't 5% obtained by sorting? If thats the
case then a 10% that is a fallout from a 5% sort is certain to be at least
5% off and maybe up to 10 (or 12). You get what you pay for.
John Eaton
!hpvcfs1!johne
William K. McFadden
>E96. The E number is the number of steps per decade. The analytical
Don't forget the E192 series used for 0.5% resistors. I guess these values
are also used for 0.25% and 0.1% resistors. I don't think there are E384 and
E960 series, are there?
--
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