Precise Resistor Values

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Gentlemen,

During the course of recent internal probings, I've come across the usage  
of an awful lot of resistors of unusually precise value: 1251 ohms @ 1%  
and many of them are doing a job which clearly does not require such  
precision (many used as current limiting for LEDs for example). I'm  
guessing the manufacturer must have had millions of these things in stock  
from some previous production run of a different instrument and just  
wants to get rid of 'em? But then that begs the question: what (on earth)  
kind of role would they have originally been specified for?? Come on you  
high-end designers, throw me a mere hobbyist a bone here.



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Re: Precise Resistor Values
On a sunny day (Sun, 1 Sep 2019 09:09:27 -0000 (UTC)) it happened Cursitor

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Not a 'high end' designer here, but I guess (wild) somebody did
something like:

Volts 5
LED drop 1.65
Current required 10 mA

So resistor is (5 - 1.65) / 10e-3 = 335.000000 Ohm  (Linux wcalc)
then put 335 .001% in the circuit diagram
Spice freaks.

No sense of reality,
Where the world goes, F35 stealth,  LOL, any passive RF radar can see it.

We, the neural nets, REALLY need to build and test circuits and get a feeling
for real values, else it all is lost.

AI will defeat us and robots will rule.. oops those will make the same mistakes.
one AI  will invent math and an other one spice, and 1224.5 Ohm .001 % will be required.

Take a hot air gun, heat up your circuit, so much for monte carlo and put it in the fridge,.,,
Drop if from some height, <snipped worse tests>, high volts, low volts.



Re: Precise Resistor Values
On Sun, 01 Sep 2019 09:27:06 +0000, Jan Panteltje wrote:

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Nice theory, Jan, but it's not correct in this instance at any rate. The  
actual physical component has markings backing up the precise value and  
it's stated as such on the parts inventory too.


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LOL! I like your thinking! :-D



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Re: Precise Resistor Values
On Sunday, 1 September 2019 10:32:38 UTC+1, Cursitor Doom  wrote:
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Another possibility is that somebody once designed a product that used
1250 ohms.  The resistor itself was probably marked 1251.  Somebody
else copied the design into another product and, looking at the
resistor markings decided that it was a 1251 ohm resistor.
Millions were ordered and this became a company standard part.
All future designs that needed about 1250 ohms then used this
part because there would be too much administrative hassle in  
specifying a 1250 ohm resistor instead.

I have encountered a similar situation where I needed to design
a 600 ohm resistor into a product.  The nearest value in the
company database with the right power rating was 601 ohms. Nobody
could explain why this value had been chosen.  I think the tolerance
was +/-5%.  The resistor marking was 601, so I am sure it was
a mistake.

Large companies will put each resistor value needed for a product
out to tender and the manufacturers will happily quote for any
value if the quantity is large enough.  Once this has been done,
all future products that need a similar value will be forced to
use what is already there unless there is a compelling reason
to change.  It can take weeks to get a new part into the "system"
and there is significant cost involved.  The "large company" I  
refer to is one that everyone here will have heard of.

John


Re: Precise Resistor Values
On Sun, 01 Sep 2019 04:16:20 -0700, jrwalliker wrote:

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HP perchance?




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Re: Precise Resistor Values
On Sunday, 1 September 2019 12:29:51 UTC+1, Cursitor Doom  wrote:
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Think of blue bat wings.

Re: Precise Resistor Values
On Sun, 01 Sep 2019 04:36:03 -0700, jrwalliker wrote:

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Motorola? Why not just say the name?



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Re: Precise Resistor Values
On Sunday, 1 September 2019 13:20:36 UTC+1, Cursitor Doom  wrote:

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You had no trouble working it out, but it is less obvious to
search engines.

Re: Precise Resistor Values

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  Not if it destroys the entire radar site 150 miles before it even  
reaches the radar site's view horizon base.

  Oh and that 'passive radar' 'picture' appears as if to be a small  
bird.  Usually ignored by the passive radar's operator or computer  
analyzers.  Way past "too late" time and also guess what...  ignored  
by radar guided air-to-air missiles.

  The detection schema change requisite to track them would have  
missiles flying off after non existent noise signatures.  Way too  
loose to call guided any more.

Re: Precise Resistor Values

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  Then you can toss it off the back dock and if it still works you can  
then get your CSA cert.

Re: Precise Resistor Values
On Sun, 1 Sep 2019 09:09:27 -0000 (UTC), Cursitor Doom

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In these days of laser trimming, 1% is free. We buy all 1% resistors,
except when we need something better. I think the best we now buy is
0.05% 10PPM, which costs 20 cents in modest quantities.

1% surfmount resistors cost under a penny by the reel.

1251 ohms is an oddball value. "1251" could mean 1250 ohms, also
oddball.


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John Larkin   Highland Technology, Inc   trk

jlarkin att highlandtechnology dott com
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Re: Precise Resistor Values
On Sun, 01 Sep 2019 07:37:29 -0700, John Larkin wrote:

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Yeah, but this is mid eighties we're talking here.

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There are *plenty* of oddball values I've since discovered, having gone  
to the trouble of seeking them out on the schematics:

316
19.6k
61.9
56.2
23.7
17.8k
3.83k

To name but a few of very many such examples!

In fact these weird values seem to outnumber the regular ones by some  
measure. Weird!



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Re: Precise Resistor Values
On Sun, 1 Sep 2019 15:43:52 -0000 (UTC), Cursitor Doom

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The Susumu thinfilms seem to come in oddball values.



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John Larkin   Highland Technology, Inc   trk

jlarkin att highlandtechnology dott com
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Re: Precise Resistor Values
On Sunday, September 1, 2019 at 11:43:55 AM UTC-4, Cursitor Doom wrote:
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Those are all standard E96 1% values.

https://en.wikipedia.org/wiki/E_series_of_preferred_numbers

Re: Precise Resistor Values
On 2019/09/01 9:31 a.m., Michael Terrell wrote:
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Michael, thanks for the link! I had always wondered where those values  
came from - that will help with our resistor and capacitor stock.

Wikipedia is most useful when it is accurate!

John :-#)#

Re: Precise Resistor Values
Michael Terrell wrote...
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 Right, very common, if they were E192, not quite as common.


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 Thanks,
    - Win

Re: Precise Resistor Values
wrote:

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Win, I haven't looked particularly for this in your book but do you
have the equations for calculating 1%  5%  and other standard resistor
values in there ?

I remember the old basic program to find the nearest value standard
resistor value from way back when.



Re: Precise Resistor Values
boB wrote...
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 No, sorry, we don't, but it's readily available.


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    - Win

Re: Precise Resistor Values
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Google for E192 "E series of preferred numbers"
--  
Uwe Bonnes                 snipped-for-privacy@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
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Re: Precise Resistor Values
On Sunday, September 1, 2019 at 5:18:05 PM UTC-7, boB wrote:
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It is nothing more than rounded log-spaced (geometrically spaced) numbers o
ver a decade. For m-code, E96 is:

po = 96
n=0;
p = (round(10.^(2:(1/po):(3-1/po))))';  
p = (10.^n).*p(:,ones(size(n,2),1))  

yielding:
p =
  100
  102
  105
  107
  110
  113
  :
  :
  887
  909
  931
  953
  976

for E192 (po19%2) it yields:

p =
  100
  101
  102
  104
  105
  :
  :
  931
  942
  953
  965
  976
  988

The 5% class (E24) and 10% class (E12) are tricky because there are wrongly
 rounded values, and you have to fix them with special treatment. Here is t
he compare of "correct rounding" versus what we actually have:

10 10
11 11
12 12
13 13
15 15
16 16
18 18
20 20
22 22
24 24
26 27
29 30
32 33
35 36
38 39
42 43
46 47
51 51
56 56
62 62
68 68
75 75
83 82
91 91

It is interesting that the "bad rounding" of E12/24 works to our favor in c
ertain resistor ratio problems. We can actually get some ratio combinations
 with the distorted E24 family that we can't with the E96 family. I mean th
ere is less error in the ratio for some desired ratios. This is because the
 "always correct" rounding (to discrete values) tends to cluster the availa
ble ratios at "ratio-spectral lines" and leaves gaps elsewhere.

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