Why were the 5 "off by one" values chosen as standard?
10 10 12 12 15 15 18 18 22 22 27 26"Rick Bungester"
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
Maybe use 1% resistors?
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
Ummmm, gee... how close do you need to be?
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
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.
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
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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
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...
-- John Devereux
Micromicrofarads ("mmf") and that's the way I likes it! :^)
Tim
-- Deep Fryer: A very philosophical monk. Website @
"Rick Bungester"
** Not the point at all - you SNIPPING cretin........ Phil
Exactly. Sheesh. Talk about overkill!
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.
Did you know that Joe Walsh is a big Ham Radio relic collector?
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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