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Standard Resistor Values: Why not a true geometric series?

Discussion in 'Electronic Design' started by Rick Nungester, Oct 15, 2007.

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  1. 10% standard resistor values are close to a geometric series with
    multiplier 10^(1/12), but not exactly. 1 decade of standard values,
    followed by the rounded 10^(1/12) series values follows. (To generate
    the 2nd column set a hand calculator to round to the nearest integer,
    and multiply repeatedly by 10^(1/12).)

    Why were the 5 "off by one" values chosen as standard?

    10 10
    12 12
    15 15
    18 18
    22 22
    27 26 <<<
    33 32 <<<
    39 38 <<<
    47 46 <<<
    56 56
    68 68
    82 83 <<<
    100 100
     
  2. Phil Allison

    Phil Allison Guest

    "Rick Bungester"


    ** What values are in the 20% tolerance ( E6 ) group - eh ???

    Wanker.




    ........ Phil
     
  3. John Larkin

    John Larkin Guest


    It's a shame the ratios are nearly geometric. It means that the
    ability to design voltage divider ratios is severely restricted.

    John
     
  4. ** What values are in the 20% tolerance ( E6 ) group - eh ???

    The 20% (E6) group has the same issue, deviating from a 10^(1/6)
    multiplier. Standard versus geometric series follows.

    10 10
    15 15
    22 22
    33 32 <<<
    47 46 <<<
    68 68

    http://en.wikipedia.org/wiki/Preferred_number includes history of when/
    who picked the values, and a discussion of the "odd values" issue
    under its "Discussion" page. But it doesn't answer the "Why?"
     
  5. Maybe use 1% resistors?
     
  6. John Larkin

    John Larkin Guest

    We use 0.1% resistors, and sometimes 0.05%. But they are still nailed
    to the 1% values, which are geometrically spaced. So if you have some
    assortment of resistors in stock, various combinations tend to keep
    hitting the same ratios, and some ratios can't be done.

    John
     
  7. Ummmm, gee... how close do you need to be?
     
  8. Exactly, and the "standard 1%" family is always "correctly" rounded,
    unlike the "standard 5%" family.

    It is perhaps ironic, but the skewing of the "standard 5%" family
    values often serendipitously provides a better ratio match than
    correctly rounded "standard 1%" family values will. I've encountered
    that many times. So is it really serendipitous? I've wondered if
    "they" skewed them for exactly that reason.

    There are 8 skewed values (82 is unique too,as it is a skew down
    rather than up):

    10 18 33 56
    11 20 36 62
    12 22 39 68
    13 24 43 75
    15 27 47 82
    16 30 51 91

    10 18 32 56
    11 20 35 62
    12 22 38 68
    13 24 42 75
    15 26 46 83
    16 29 51 91

    100 147 215 316 464 681
    102 150 221 324 475 698
    105 154 226 332 487 715
    107 158 232 340 499 732
    110 162 237 348 511 750
    113 165 243 357 523 768
    115 169 249 365 536 787
    118 174 255 374 549 806
    121 178 261 383 562 825
    124 182 267 392 576 845
    127 187 274 402 590 866
    130 191 280 412 604 887
    133 196 287 422 619 909
    137 200 294 432 634 931
    140 205 301 442 649 953
    143 210 309 453 665 976
     
  9. Guest

    As to 'cannot be done', that does not always have to be a problem.
    For example I have this AD box (lots of analog inputs connected to a
    computer),
    it needed a 5.00V reference.
    Now I had a LM317 with 2 resistors that gave anywhere near 5.xx.
    Measured it, made a #define in the program, and: any accuracy you
    want.
    Same with PICs, just calibrate in the software, set value in its
    EEprom.
     
  10. Nominally perfect would be nice; failing that at least 10-20x better
    than the resistor tolerances. But I don't mind much putting a much
    smaller value 1% resistor in series (or a much larger value in
    parallel) with a 0.1% resistor if it's that kind of deal.

    Best regards,
    Spehro Pefhany
     
  11. Tom Bruhns

    Tom Bruhns Guest

    Ah, but for small quantities, it's much cheaper to buy standard
    values. You _can_ get any value you want, if you pay enough; but if
    your volume isn't great enough, it's much more economical to do series
    and/or parallel combinations. With an E24 set of high accuracy
    values, two in parallel or two in series will always get you within
    0.23% of a desired value, and if you're stuck trying to get to a low
    value where series won't work, or a high value where parallel won't
    work, you still can get within 0.45%. And that's without being very
    creative about picking values (just one closest to the desired and one
    to trim it). For ratiometric stuff, it generally gets even better:
    with one "stock" value and one trimmed value, you get to 0.23%, but
    add a trim to the stock value and you get much finer ratio adjustment.

    I have some very low TC matched resistor sets in my junque box, from
    HP voltmeter scrap, that have "weird" values. Given the needed
    precision and the relatively high volume in the voltmeter market, the
    custom values made sense for HP. For the work I do, trimming as
    needed with series and/or parallel is fine: I do need 0.1% parts at
    times, but never in high volume.

    Also, it helps a lot to design systems in such a way that really close
    arbitrary ratios aren't necessary. For practically everything we do,
    stability is far more important than absolute accuracy; the accuracy
    is obtained by calibration. Calibration used to involve adjusting
    pots and possibly variable capacitances and inductances; that's
    practically all been replaced with processor-based calibration. I
    realize that on rare occasion, there's no good way around some
    precision ratio, but at least in what I've seen for quite a few years
    now, those occasions are pretty rare.

    Cheers,
    Tom
     
  12. Tom Bruhns

    Tom Bruhns Guest

    This may be a good question for a different group. Someone that
    checks into rec.radio.amateur.boatanchors may have a reasonable
    answer, or there may be some other group that delves into history
    more.

    Of course, when the values were standardized (some time around WWII, I
    believe), people didn't use pocket calculators or Excel or Matlab to
    easily calculate a set of values, with rounding applied. One may
    forgive them for rounding incorrectly. But some of the values are off
    worse than that: 33 ohms "should be" 31.6.

    Before that time, the "standard" values were even crazier. Count your
    blessings??

    Cheers,
    Tom
     
  13. ªºª rrock

    ªºª rrock Guest

    It's far cheaper to sell resistors that either do not meet mil-spec or
    have been over-produced relative to the military customer's consumption
    rate, to the commercial market rather than make them separately.
    See MIL-PRF-39005 for an example of the resistance values.
     
  14. I wrote a program to calculate the optimum combination of 3 resistors
    to get a given ratio. Yes, I know there are lots of these about - but
    mine only uses the values I carry in stock!

    I started out thinking how to write it efficiently, then realised I
    could simply try all possible combinations and pick the closest...
     
  15. Tim Williams

    Tim Williams Guest

    Micromicrofarads ("mmf") and that's the way I likes it! :^)

    Tim
     
  16. Phil Allison

    Phil Allison Guest

    "Rick Bungester"



    ** Not the point at all - you SNIPPING cretin.




    ........ Phil
     

  17. Exactly. Sheesh. Talk about overkill!
     
  18. Repeatability in a production setting is always an issue, and should be
    of somewhat serious import to a designer. 1% (E96) using two in series
    or parallel will nearly ALWAYS result in a combination that requires ZERO
    matching or culling in a batch. Using a five percent mix can result in
    "chaining errors" that are worse than the original tolerance, even WITH
    matching and culling procedures in place.

    Yet, it all depends on the application. Precision, instrumentation op
    amp circuit where you want the entire production lot to have the same
    gain ratio, or a simple current limit value for an LED driver circuit.

    Application is everything, particularly if large volumes are involved.

    One penny savings over a ten Million piece run is:

    $100,000.00

    There is typically far more than one penny difference between 5% and 1%
    values.
     

  19. Did you know that Joe Walsh is a big Ham Radio relic collector?
     

  20. It isn't the values. Mil spec resistors are produced in an entirely
    different manner, and the process controls are much tighter. They have
    to be or else they will only succeed in getting a 3% yield.

    3 out of every 100 parts made is not very profitable. Better to make a
    process that guarantees a more promising production yield, and yes, that
    IS why they also have a higher attached price.

    Where do you people get this shit from?
     
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