@ WBahn

I didn't; you did (post #5)

Clearly I made a typo in Post #13. You said 216 and all of my references say 215. So I'm curious what reference you used that said 216. Sorry for the confusion.

 

You can also use the formula

R = 10 ^(i/E)
where E is the number of steps in the series and belongs to {12, 24, 48, 96,...}
and i belongs to {0,1,...,E}

Depending on how you view the roundoff process, either 215 or 216 is a reasonable choice.
You get 215 if you truncate, or round down, or round to the closest integer.
You get 216 if you round up.

While either 215 or 216 might be reasonable choices, only one of them was actually chosen and the other was not.

You can get nominal values this way and I think it is a great exercise for people to understand the concept of a geometric progression and where the values nominally come from. Even better, don't tell them the E number but instead ask them how many values should be in the sequence for, say, 10% resistors. But the values in the standard sequence are not always the nominal values, even allowing for roundoff. More to the point, the entries in the sequence are not subject to change due to how one person rounds, they are official elements of an officially adopted sequence (namely IEC 60063:1963).

 

You can get nominal values this way and I think it is a great exercise for people to understand the concept of a geometric progression and where the values nominally come from. Even better, don't tell them the E number but instead ask them how many values should be in the sequence for, say, 10% resistors. But the values in the standard sequence are not always the nominal values, even allowing for roundoff. More to the point, the entries in the sequence are not subject to change due to how one person rounds, they are official elements of an officially adopted sequence (namely IEC 60063:1963).

True enough, but somebody or a group of somebodies had to make a decision at some point in time in order to promulgate a standard. Is there a consistency to what they did? I dunno, but I'm not sure I'm that concerned about it.

 

True enough, but somebody or a group of somebodies had to make a decision at some point in time in order to promulgate a standard. Is there a consistency to what they did? I dunno, but I'm not sure I'm that concerned about it.

Some years ago me and a few friends actually spend a bit of time trying to track down when and why the original values were chosen and were unable to do so. Like so many things, the origins and reasoning are very possibly lost forever. I can come up with lots of guesses as to why the adopted values were chosen, but they are just that. Some of those guesses are based on rationales for why the chosen value might have advantages over the nominal value. But my best guess is that the committee (and, yes, it was a committee as these things usually are) included people with vested interests in having particular values included, probably because of what their companies already produced, and so the final values represented the interplay of political, practical, and theoretical forces.

 

You can also use the formula

R = 10 ^(i/E)
where E is the number of steps in the series and belongs to {12, 24, 48, 96,...}
and i belongs to {0,1,...,E}

You would think so but why is 33 the common value when 10^(6/12) = 3.16? Why not 31 or 32?

 

How close are you going to try to hit those nominal values? Remember that with 1% resistors, you are already working with a much wider window than the nominal values imply. For instance, let's say that you are focusing on the 6838 Ω resistor. If you start off with a 6810 Ω resistor then you might think that you only need to add another 28 Ω resistor in series. But remember that the 6810 Ω resistor has a tolerance of ±68 Ω, well over twice the nominal value of the resistor that you would think you need to add such that there is a fair chance that the actual resistance is already too large and that adding the 28 Ω resistor is only making it worse. Also, at 25ppm/°C, it only takes a 6°C temperature change to result in a 1 Ω change in the resistance, so trying to hit 6838 Ω exactly just doesn't make a lot of sense.

I also found that it's more of a stock issue. Using the .1% values and relatively high parallel resistances from the 1% chart I can easily hit the values in 3 resistors, but finding stock, not so much. Nice thing is they're cheap so buying 10-25 of each doesn't break bank compared to buying exact trimmed values. Mouser has a series called MELF just have to find stock combos.

 

Some years ago me and a few friends actually spend a bit of time trying to track down when and why the original values were chosen and were unable to do so. Like so many things, the origins and reasoning are very possibly lost forever. I can come up with lots of guesses as to why the adopted values were chosen, but they are just that. Some of those guesses are based on rationales for why the chosen value might have advantages over the nominal value. But my best guess is that the committee (and, yes, it was a committee as these things usually are) included people with vested interests in having particular values included, probably because of what their companies already produced, and so the final values represented the interplay of political, practical, and theoretical forces.

As a participant in a standards committee, I can assert that this is an accurate picture of how decision making works. Every time somebody stands up with a baby in their arms and says to the assembled multitudes: "Isn't my baby beautiful?" The resounding reply from the chorus is: "Sitdown and shutup, because you're baby is ugly." Thus was born in the summer of 1982, "The Ugly Baby Bus". It never took the world by storm.

 

I also found that it's more of a stock issue. Using the .1% values and relatively high parallel resistances from the 1% chart I can easily hit the values in 3 resistors, but finding stock, not so much. Nice thing is they're cheap so buying 10-25 of each doesn't break bank compared to buying exact trimmed values. Mouser has a series called MELF just have to find stock combos.

Can you give an example of three specific resistances that you are planning to use for a particular target value?

 

As a participant in a standards committee, I can assert that this is an accurate picture of how decision making works. Every time somebody stands up with a baby in their arms and says to the assembled multitudes: "Isn't my baby beautiful?" The resounding reply from the chorus is: "Sitdown and shutup, because you're baby is ugly." Thus was born in the summer of 1982, "The Ugly Baby Bus". It never took the world by storm.

I have a good friend that was on the IEEE 754 Floating Point standard and he remarked that making both sausages and laws was clean and comprehensible compared to the standards-making process. I talked to him about why certain choices were made and he was able to recall some of the reasoning on some of them but others escaped him totally and he couldn't even guess why the final standard included certain features (not so much why a particular feature MIGHT have been adopted, but rather why a particular feature WAS adopted). This just underscores the likely fact that the reasoning behind many decisions that have profound influences on the way things are done for generations to come often become completely lost to history and eventually come down to "because this is the way it was handed down from those that came before." And who says science and engineering have nothing in common with religion?

 

...
And who says science and engineering have nothing in common with religion?

That's a puffball question. All the fundamentalists who insist that the Texas School Board of Education adopt science books that have nothing to do with science.

 

@ WBahn

You said 216 and all of my references say 215. So I'm curious what reference you used that said 216.

I think it was this one:
http://www.radio-electronics.com/info/data/resistor/e-series-e3-e6-e12-e24-e48-e96.php

Edit: Perhaps you were looking at the E48 series? That has 215.

 

Hello,

Here are three tables:
1) from logwell about the EIA resistor E series:
http://logwell.com/tech/components/resistor_values.html
Where we see the 215 value at the E48 E96 ans E192 series.

2) from Radio-electronics site, also claiming the EIA series:
http://www.radio-electronics.com/info/data/resistor/e-series-e3-e6-e12-e24-e48-e96.php
But there the 215 is at the E48 series and 216 at the E96 series.

3) from "the resistorguide" :
http://www.resistorguide.com/resistor-values/
But there the 215 is at the E48 series and 216 at the E96 series, but 215 again in the E192 series.

Bertus

 

I have a good friend that was on the IEEE 754 Floating Point standard and he remarked that making both sausages and laws was clean and comprehensible compared to the standards-making process. I talked to him about why certain choices were made and he was able to recall some of the reasoning on some of them but others escaped him totally and he couldn't even guess why the final standard included certain features (not so much why a particular feature MIGHT have been adopted, but rather why a particular feature WAS adopted). This just underscores the likely fact that the reasoning behind many decisions that have profound influences on the way things are done for generations to come often become completely lost to history and eventually come down to "because this is the way it was handed down from those that came before." And who says science and engineering have nothing in common with religion?

I was on a standards committee and. as a member of a large company, lawyers quickly inform their technical staff participating on standards committees that they must not pass a standard that will interfere with a smaller organization's ability to continue operations. That means, almost anything a small organization does is included in a new standard. Not all markets have mom&pop shops serving markets but, when they exist, it becomes difficult to pass standards. Most standards-setting organizations are already aware of this hurdle and steer the conversation appropriately.

 

Never make anybody adapt to a changing world? That should make the whole planet grind to a halt pretty soon.

 

@ WBahn
I think it was this one:
http://www.radio-electronics.com/info/data/resistor/e-series-e3-e6-e12-e24-e48-e96.php

Edit: Perhaps you were looking at the E48 series? That has 215.

In all of the references I looked in, the E48, E96, and E192 all had 215.

It is my understanding that the lower count E-series, namely E3, E6, E12, and E24, are all proper subsets of the next higher series and that the same applies to the higher count E-series, namely E48, E96, and E192. However, E24 is not a proper subset of E48. The actual standards document is $8 and I don't want to know the answer that badly. But I did find several more sites that seem to be tightly referenced to the standard and they all agree. Here's a selection of the ones I looked at:

http://bourns.com/pdfs/standard_decade_values_02.pdf

http://logwell.com/tech/components/resistor_values.html

So my guess is that the site you used just had a typo. It would have been interesting to look at
their E192 table, but they stopped at E96.

I did, however, find one that agrees with the one you used:

http://www.resistorguide.com/resistor-values/

As with so many things on the internet, it's hard to tell when two things that agree are actually nothing more than one copying the other.

 

We call those parochial interests.

 

Never make anybody adapt to a changing world? That should make the whole planet grind to a halt pretty soon.

There is a fine line between modifying existing standards (existing players must be allowed to continue operations) and making a new (next generation) standards - (sufficient time and intellectual property licensing opportunity must be given to all members of the market).

 

Never make anybody adapt to a changing world? That should make the whole planet grind to a halt pretty soon.

It's a real problem with no easy solution (or perhaps no solution at all). It is entirely too possible for a standards body to craft a standard that is intended to have the effect of putting many/most of the small players out of business (or at least out of that market). So, particularly in a nice litigious society such as ours, they tend to bend over backwards to prevent even the possibility of being seen as doing this even by accident. Combined with the desire not to cause too many near-term disruptions (which is a legitimate concern and goal), the long-term results are often far from optimal.

 

Can you give an example of three specific resistances that you are planning to use for a particular target value?

Not sure I'm expressing this correctly

2k162= (3m300/1+130k0/1+2k200/1)/1
216r2=(221/1+10k20/1+400k0/1)/1
6k838=(6k980/1+634k0/1+715k0/1)/1

The online calc, http://www.sengpielaudio.com/calculator-paralresist.htm was a bit flaky but I ran these several times

 

So for a 6838 Ω resistor you are planning to use 6980 Ω, 634.o kΩ, and 715 kΩ all in parallel. I agree that this will give you nominally 6837.96 Ω. But the question is whether there is any point. I think you said you were going to use 0.1% resistors. That means that the tolerance on the 6838 Ω resistor is 6.8 Ω all by itself (which is going to dominate the tolerance range). If all of the resistors are at the lower end of their limits then the resistance could be as low as 6831 Ω while if they are all at the upper end of their limits, then the resistance could be as high as 6844 Ω, giving a tolerance of right at 6.5 Ω or just under 0.1% (not too surprising).

What if you just put three 20.5 kΩ resistors in parallel? That would give you a nominal resistance of 6833 Ω with a tolerance of that same 6.5 Ω and at least you could buy all the same values. The value you want is within the tolerance range of this combination, so is there any real benefit to trying to get the ideal value any closer?