[HN Gopher] Why do electronic components have such odd values? (...
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Why do electronic components have such odd values? (2021)
Author : mtm
Score : 234 points
Date : 2024-06-04 16:20 UTC (6 hours ago)
(HTM) web link (digilent.com)
(TXT) w3m dump (digilent.com)
| SOTGO wrote:
| Can someone explain the last paragraph? The author gives the
| example of trying to find a 70 Ohm resistor and how the 68 Ohm
| and 75 Ohm are a little off. They conclude by saying you should
| just use 33 and 47 Ohm resistors, but wouldn't that give an
| resistance of 80, not 70?
| rylittle wrote:
| I also thought that was interesting. Also, wouldn't the
| tolerance be doubled when you add them in series? Or does it
| still average out to +/- 5%?
| aleph_minus_one wrote:
| > Also, wouldn't the tolerance be doubled when you add them
| in series? Or does it still average out to +/- 5%?
|
| Neither.
|
| Let R_{1, ideal}, R_{2, ideal} be the "ideal" resistances;
| both with the same tolerance t (in your example t = 0.05).
|
| This means that the real resistances R_{1, real}, R_{2, real}
| satisfy
|
| (1-t) R_{1, ideal} <= R_{1, real} <= (1+t) R_{1, ideal}
|
| (1-t) R_{2, ideal} <= R_{2, real} <= (1+t) R_{2, ideal}
|
| Adding these inequalities yields
|
| (1-t) (R_{1, ideal} + R_{2, ideal}) <= R_{1, real} + R_{2,
| real} <= (1+t) (R_{1, ideal} + R_{2, ideal})
|
| So connecting two resistors with identical tolerance _in
| series_ simply keeps the tolerance identical.
| riedel wrote:
| Fun fact is that afaik component values are often distributed
| in a bi-modal way because actually +-5% often means that they
| sorted out already the +-1% to sell as a different more
| expensive batch. At least it used to be that way. Wonder if
| it is still worth doing this in production. So I guess one
| could also measure to average things out otherwise the errors
| will stay the same relatively.
| bluGill wrote:
| I'm not sure where the line is, but at some point things
| like temperature a matter and so a low % resister cannot be
| high % that passes tests.
| dmurray wrote:
| If you can measure them with that precision, would it make
| sense to sell them with that accuracy too? So if you tried
| to manufacture a resistor at 68kO +/- 20%, and it actually
| ended up at 66kO +/- 1%, couldn't you now sell it as an
| E192 product which according to TFA are more expensive?
|
| Selling with different tolerances only makes sense to me if
| the product can't be reliably measured to have a tighter
| tolerance, perhaps if the low- quality ones are expected to
| vary over their life or if it's too expensive to test each
| one individually and you have to rely on sampling the
| manufacturing process to guess what the tolerances in each
| batch should be.
| retrac wrote:
| Resistors with worse tolerances may be made out of
| cheaper, less refined wire, which will vary resistance
| more by temperature. The tolerance and resistance is good
| over a temperature range. For more reading looking up
| "constantan".
| RetroTechie wrote:
| Most resistors don't use wire, but some film of carbon
| (cheaper, usually the E12 / 5% tolerance parts) or metal
| (E24, or 1% and tighter tolerances) onto a non-conducting
| body. Wires mean winding into a coil, which means
| increased inductance.
|
| I suspect in most cases the tolerances are a direct
| result from the fabrication process. That is: process X,
| within such & such parameters, produces parts with Y
| tolerance. But there could be some trimming involved
| (like a laser burning off material until component has
| correct value). Or the parts are measured & then binned /
| marked accordingly.
|
| Actual wire is used for power resistors, like rated for
| 5W+ dissipation. Inductance rarely matters for _their_
| applications.
| Gibbon1 wrote:
| Accuracy depends on the technology used. Carbon comp
| tends have less accuracy then carbon film. And it's not
| true that higher accuracy is always better.
|
| Some accurate resisters are essentially wound coils and
| have high inductance and will also induce and pick up
| magnetic interference. Stuff like that matters often a
| lot.
| projektfu wrote:
| Thanks, always good to remember that the tolerance of a
| resistor is not just a manufacturing number but also
| defined over the specified temperature range.
| dylan604 wrote:
| Depends on where in the production line they are being
| tested. If they are tested after they've had their color
| bands applied, then you wouldn't be able to sell it as a
| 66kH since the markings would for a 68kH
| wongarsu wrote:
| The issue is probably volume. Very few applications need
| a resistor that's exactly 66kO, but a lot of applications
| need resistors that are in the ballpark of 68kO (but
| nobody would really notice if some 56kO resistors slipped
| in there).
|
| For every finely tuned resonance circuit there are a
| thousand status LEDs where nobody cares if one product
| ships with a brighter or dimmer LED.
| projektfu wrote:
| Unless the components are expensive, that proposition seems
| dubious. It's much more economical to take a process that
| produces everything within 12% centered on the desired
| value and sell it as +-20%. 100% inspection is generally to
| be avoided in mass production, except in cases where the
| process cannot reach that capability, chip manufacturing
| being the classic example. For parts that cost a fraction
| of a penny, nobody is inspecting to find the jewels in the
| rough.
| riedel wrote:
| Actually it seems to be really the case that multimodal
| distribution are rather the result of batches not having
| a mean. So it is rather the effect of systematic error
| [1]. I guess it is really a myth (we did low cost RF
| designs back in 2005 and had some real issues with
| frequencies not aligning die to component spread and I
| really remember that bi modality problem, but I guess
| okhams razor should have told me that it makes no
| economical sense)
|
| [1]
| https://www.eevblog.com/2011/11/14/eevblog-216-gaussian-
| resi...
| thornewolf wrote:
| tolerance should actually go down since the errors help
| cancel each other out.
|
| reference:
| https://people.umass.edu/phys286/Propagating_uncertainty.pdf
|
| disclaimer: it will be a relatively small effect for just two
| resitors
|
| aleph's comment is also correct. the bounds they quote are a
| "wost-case" bound that is useful enough for real world
| applications. typically, you won't be connecting a
| sufficiently large number of resistors in series for this
| technicality to be useful enough for the additional work it
| causes.
| rexer wrote:
| Note that tolerance and uncertainty are different.
| Tolerance is a contract provided by the seller that a given
| resistor is within a specific range. Uncertainty is due to
| your imprecise measuring device (as they all are in
| practice).
|
| You could take a 33k Ohm resister with 5% tolerance, and
| measure it at 33,100 +/- 200 Ohm. At that point, the
| tolerance provides no further value to you.
| immibis wrote:
| If values are normally distributed, random errors
| accumulate with the square root of the number of
| components. Four components in series have 2x the
| uncertainty over all, etc, but if you divide that double
| uncertainty by four times the resistance, it's half the
| percentage uncertainty as before. (I avoid using the word
| "tolerance" because someone will argue whether it really
| works this way)
|
| In reality, some manufacturers may measure some components,
| and the ones within 1% get labeled as 1%, then it may be
| that when you're buying 5% components that all of them are
| at least 1% off, and the math goes out the window since it
| isn't a normal distribution.
| afiori wrote:
| I wonder about the effect of different wiring patterns.
| For example you can can combine N^2 resistors in N
| parallel strips of N resistors in serie.
|
| I expect that in this case the uncertainty would decrease
| afiori wrote:
| Iterating either of
|
| f(x) = 3/(1/x + 1/110 + 1/90)
|
| g(x) = 1/(1/(3 _x) + 1 /(3_110) + 1/(3*90))
|
| Seems to show that 100 is a stable attractor.
|
| So I will postulate without much evidence that if you
| link N^2 resistors with average resistance h in a way
| that would theoretically give you a resistor with
| resistance h you get an error that is O(1/N)
| RetroTechie wrote:
| In the article's example, I'd prefer 2 resistors in
| parallel. That way result is less dramatic if 1 resistor
| were to be knocked off the board / fail.
|
| Eg. 1 resistor slightly above desired value, and a much
| higher value in parallel to fine-tune the combination. Or
| ~210% and ~190% of desired value in parallel.
|
| That said: it's been a long time since I used a 10%
| tolerance resistor. Or where a 1% tolerance part didn't
| suffice. And 1% tolerance SMT resistors cost almost
| nothing these days.
| nomel wrote:
| > tolerance should actually go down since the errors help
| cancel each other out.
|
| Complete nonsense. The tolerance doesn't go down, it's now
| +/- 2x, because component tolerance is _the allowed
| variability, by definition_ , worst case, not some
| distribution you have to rely on luck for.
|
| Why do they use allowed variability? Because determinism is
| the whole point of engineering, and no EE will rely on luck
| for their design to work or not. They'll understand that,
| during a production run, they _will_ see the combinations
| of the worst case value, and they _will_ make sure their
| design can tolerate it, regardless.
|
| Statistically you're correct, but statistics don't come
| into play for individual devices, which need to work, or
| they cost more to debug than produce.
| tempestn wrote:
| The total tolerance is not +/- 2x, because the
| denominator of the calculation also increases. You can
| add as many 5% resistors in series as you want and the
| worst case tolerance will remain 5%. (Though the likely
| result will improve due to errors canceling.)
|
| For example, say you're adding two 10k resistors in
| series to get 20k, and both are in fact 5% over, so
| 10,500 each. The sum is then 21000, which is 5% over 20k.
| jjmarr wrote:
| > Statistically you're correct,
|
| The Central Limit Theorem (which says if we add a bunch
| of random numbers together they'll converge on a bell
| curve) only guarantees that you'll get a normal
| distribution. It doesn't say where the _mean_ of the
| distribution will be.
|
| Correct me if I'm wrong, but if your resistor factory has
| a constant skew making all the resistances higher than
| their nominal value, a bunch of 6.8K + 6.8K resistors
| will not on average approximate a 13.6K resistor. It will
| start converging on something much higher than that.
|
| Tolerances don't guarantee any properties of the
| statistical distribution of parts. As others have said,
| oftentimes it can even be a bimodal distribution because
| of product binning; one production line can be made to
| make different tolerances of resistors. An exactly 6.8K
| resistor gets sold as 1% tolerance while a 7K gets sold
| as 5%.
| Dylan16807 wrote:
| > The Central Limit Theorem (which says if we add a bunch
| of random numbers together they'll converge on a bell
| curve) only guarantees that you'll get a normal
| distribution. It doesn't say where the mean of the
| distribution will be.
|
| That's kind of overstating and understating the issue at
| the same time. If you have a skewed distribution you
| might not be able to use the central limit theorem at
| all.
| _dain_ wrote:
| _> If you have a skewed distribution you might not be
| able to use the central limit theorem at all._
|
| The CLT only requires finite variance. Skew can be
| infinite and you still get convergence to normality ...
| eventually. Finite skew gives you 1/sqrt(N) convergence.
| nomel wrote:
| > Tolerances don't guarantee any properties of the
| statistical distribution of parts.
|
| That's incorrect. They, by definition, guarantee the
| maximum deviation from nominal. That is a property of the
| distribution. Zero "good" parts will be outside of the
| tolerance.
|
| > It will start converging on something much higher than
| that.
|
| Yes' and that's why tolerance is used, and manufacturer
| distributions are ignored. Nobody designs circuits around
| a distribution, which requires luck. You guarantee
| functionality by a _tolerance_ , worst case, not a part
| distribution.
| Dylan16807 wrote:
| If you're going to say "Complete nonsense." you shouldn't
| get the calculation wrong in your next sentence.
| nomel wrote:
| Very true, I was writing as absolute value, not %
| (magnitude is where my day job is). My point still
| stands: it is complete nonsense that tolerance goes down.
| Dylan16807 wrote:
| They said it "should" go down, but that another comment
| saying the worst case is the same is "also correct".
|
| I do not see any "complete nonsense" here. I suppose they
| should have used a different word from "tolerance" for
| the expected value, but that's pretty nitpicky!
| Sohcahtoa82 wrote:
| Nope, still averages to +/- 5%.
|
| To give an example, let's say you've got two resistors of 100
| Ohm +/- 5%. That means each is actually 95-105 Ohm. Two of
| them is 190-210 Ohm. Still only a 5% variance from 200 Ohm.
| sram1337 wrote:
| Can you assume that +/-5% isn't linearly distributed? If
| so, the tolerance in practice may likely end up even
| smaller.
| sophacles wrote:
| There's a fundamental misunderstanding here.
|
| Tolerance is a specification/contractual value - it's the
| "maximum allowable error". It's not the error of a
| specific part, it's the "good enough" value. If you need
| 100 +/- 5%, any value between 95 and 105 is good enough.
|
| Using two components to maybe cancel out the error as you
| describe. On average, most of the widgets you make by
| using 2 resistors instead of one may be closer to
| nominal, but any total value between 95 and 105 would
| still be acceptable, since the tolerance is specified at
| 5%.
|
| To change the tolerance you need to have the engineer(s)
| change the spec.
| Stratoscope wrote:
| You are correct. Two of the comments on the article itself also
| mention this error.
| LeifCarrotson wrote:
| Brilliant, informative writing, and yet people will jump to
| nit-pick the arithmetic.
|
| I'd better spell-check this comment before clicking reply...
| rexer wrote:
| I think that was a typo and they meant 22 + 47, which equals
| ~70 Ohms
| renewiltord wrote:
| But 80 is within 20% of 70 so we're fine ;)
| phkahler wrote:
| >> But 80 is within 20% of 70 so we're fine ;)
|
| So are the 68 Ohm and 75 Ohm.
| dbcurtis wrote:
| I think the author maybe doesn't know how to order 1% resistors
| from Digi-Key??
|
| My intro circuit analysis prof gave these wise words to live
| by: "If you need more than one significant digit, it isn't
| electrical engineering, its _physics_ "
| xxs wrote:
| Even ordering from China the 1% are perfectly fine. E96
| resistors are ubiquitous and cheap.
| rylittle wrote:
| Insightful article. Not something I had considered before, but
| also...isn't this just a fancy way of defining a geometric
| sequence thats convenient for values in base-10?
| csours wrote:
| do geometric sequences care about the base?
| perlgeek wrote:
| The ones mentioned in the article return to powers of 10.
|
| In contrast, musical notes don't, their frequencies return to
| powers of 2.
| dmurray wrote:
| Yes, the values are produced by a geometric series. For E6, the
| series has a ratio of R, where R^6 = 10, and the values are
| further rounded to two significant figures.
| mikewarot wrote:
| It's a more _accessible_ way of explaining it that doesn 't
| require understanding geometric sequences first.
| timerol wrote:
| It's not just a geometric sequence that's convenient for base
| 10, it's the standard set of geometric sequences (that was
| chosen because they're convenient for base 10).
|
| The caption on the graph (and the paragraph before the graph)
| directly addresses this: "This graph shows how any value
| between 1 and 10 is within +-10% of an E12 series value, and
| its difference from the ideal value in a geometric sequence."
| Workaccount2 wrote:
| Wikipedia has a nice table of these values that I actually have
| printed out and hanging above my bench.
|
| https://en.wikipedia.org/wiki/E_series_of_preferred_numbers#...
|
| The fact of the matter is that nowadays, E96 series resistors are
| readily available and dirt cheap. And if you need more precision
| than that, you either don't know much about electronics or you
| know a whole lot about electronics, heh.
| eternityforest wrote:
| I'd say if you need more than E3, you either know a lot of not
| much, unless you're into analog.
|
| I've done stuff that needs high precision resistors, but
| usually the specific value isn't that important, just that it's
| a known repeatable value.
| nick238 wrote:
| If I want a voltage divider, it's a lot easier to just use
| some 1% resistors and forward-calculate the expected output
| (rather than doing a calibration) if you're happy with 1-2%
| error from the resistors and your ADC or the like. Adding
| software and testing hardware to do a full on calibration is
| a lot of work.
|
| But yeah, for digital signals, oft times 1k or 100k make no
| difference.
| willis936 wrote:
| For voltage dividers it's best to use matched networks.
| Often not much more expensive and orders of magnitude more
| precise.
| klodolph wrote:
| Yes--although E96 is cheap, I'm still very fond of E12. You get
| to keep less stock. I'll even use two resistors rather than use
| something outside E12, most of the time. Maybe it's habit?
|
| Hell, I don't even think all of E12 is necessary. I'll stick to
| E6 most of the time.
| joemi wrote:
| How do the tolerances combine when you're using two
| resistors? I'm pretty sure they'd add together if in series
| (so two 5%'s become 10%), but I'm having trouble easily
| intuiting what happens if in parallel. Do they combine in the
| same way that resistances combine when in parallel?
|
| edit: Actually, I'm not so sure anymore that the tolerances
| would add up in series... I should probably just look this
| stuff up, since I'm not awake enough to intuit correctly, I
| think.
| chongli wrote:
| If you have 2 identical resistors that are 5% over nominal
| and you put them in parallel, you'll get a value 5% over
| nominal. Example:
|
| Suppose you had a pair of 105 Ohm resistors that are
| nominally 100 Ohm. In parallel you get:
|
| 1/(1/105 + 1/105) = 105/2 = 52.5 Ohm (5% over expected 50
| Ohm)
|
| If one is over nominal and the other is under, they'll
| cancel out for the most part:
|
| 1/(1/105 + 1/95) = 49.875 Ohm (0.25% under expected 50 Ohm)
| racingmars wrote:
| In series they don't add up... doing a quick example, I
| find that in the worst case (e.g. each resistor out by 5%
| in the same direction):
|
| 22 - 5% = 20.9
|
| 47 - 5% = 44.65
|
| Actual resistance in series: 65.55
|
| Nominal resistance in series: 69
|
| 69 - 5% = 65.55
|
| So the combination of the components still appears to
| maintain the 5% tolerance.
| Brian_K_White wrote:
| Values (for resistors) add in series and sort of divide-
| average in parallel.
|
| In either case though, the tolerance divides.
|
| The combined tolerance becomes more accurate the more
| resistors there are in total, whether parallel or serial.
| The highs and lows, and the chances of high or low, cancel
| each other out and you get a final actual value that is
| closer to the nominal statistical center of the bell curve
| the more individual parts there are. (same goes for other
| components, just resistors are simpler to talk about
| because their behavior is simple.)
|
| In series, a single 10K might really be 9K or 11K, but if
| you chain 10 10Ks in series, you don't get a "maybe 90K
| maybe 110K". That is technically possible but statistics
| means that what what you actually get is if there was N%
| chance that a given 10K is 9K or 11K, the there is 1/10th
| of N% chance (or less, I bet the actual equation is more
| complicated) that the chain of 10 is 90K or 110K. If the
| individual 10Ks were 10%, then you get 100K with something
| like 1% tolerance.
|
| (except also in reality, there is such a thing as batches,
| where all the parts in a given batch are all high or low
| the same way, because the process was drifting a little
| high or low while it was cranking out thousands of them
| that hour. So Ideally your 10 individuals need to come from
| 10 different batches or even 10 different manufacturers if
| that were practical or in a pure math world.)
|
| In parallel, the statistical division is the same though
| the value centers on the 1/N division rather than the sum.
| 10 10% 10Ks in parallel = 1 1% 1k
| londons_explore wrote:
| Being a mostly-digital electronics guy, I think 0.1, 1, 10,
| 100, 1k, 10k, 100k, 1M and 10M is a perfectly fine series for
| pretty much any usecase.
|
| Sense resistor? 0.1 ohm.
|
| Resistor for an LED: 100 ohm
|
| Pull up resistor: 10k
|
| Bias resistor for some mosfet gate: 10M
|
| Voltage divider to measure the battery voltage with an ADC:
| two 100k resistors.
|
| It's super rare I need anything else. I hate fiddling about
| with switching the reels on the pick'n'place anyway.
| picture wrote:
| Have you tried 10 kO for LED and FET pull down?
|
| 100 O sounds like way too much current for modern LEDs. I
| often end up using 100 kO especially for green LEDs. They
| are very visible under indoor lighting even with 1 MO and
| 3.3 V supply.
|
| For pulling down FETs, you want something in the range of
| 10 kO. 10 MO sounds way too high, which makes your circuit
| sensitive to being touched or affected by moisture,
| especially if there are near by components connected to the
| power rail.
|
| My digital electronics grab bag consist of 22 mO for
| sensing, 100 kO for battery voltage divider, 22 kO for one
| of the 3.3 V buck converter feedback dividers, 10 kO for
| everything else like I2C pulling.
| hathawsh wrote:
| Are you sure all those numbers are in the right ballpark?
| With a 3.3V supply and a 1 MO resistor, the most current
| you can get from that circuit is in the neighborhood of
| 3mA, and that's ignoring the LED voltage drop. I would
| think the LED won't be visible until you're around the mA
| range. Or are some LEDs visible in the low mA range?
| londons_explore wrote:
| human eyes are logarithmic and can easily see microamps.
|
| In fact, just hold an LED between your fingers in a dark
| enough room and you'll sometimes see them glow from stray
| magnetic fields inducing enough current in your body to
| light them.
| CamperBob2 wrote:
| _Resistor for an LED: 100 ohm_
|
| Yeah, that's why I can read a book by the blue LEDs on my
| alarm clock...
| MeteorMarc wrote:
| E12 is also great for older users who do not have the keen
| eyesight anymore to read the 1% codes with certainty without
| using tools.
| _benj wrote:
| There's also part of, good designs don't depend on high
| precision components. I think TAoE emphasized that. For high
| precision one can use trim potentiometers or maybe even digital
| potentiometer with an ADC at the other side to measure and get
| as close as possible, but otherwise depending on resistors for
| high precision is kinda rough (I'm think like an RC circuit
| that need a very specific resistance to meet some specific
| timing requirements)
| picture wrote:
| High precision resistors are often necessary for metrology
| applications like very precise and low drift voltage sources.
| Often parts like Vishay's same-substrate thin film resistor
| networks [0] are used, as the temperature of each resistor
| leg are kept the closely relative to each other, resulting in
| the ratio between them being stable against temperature
| changes. Even if you use some adjustable/tunable circuit, you
| usually still require some sort of precision resistor network
| as an original standard.
|
| In general, however, it's much better to measure/sense
| physical phenomenon by first converting it into frequency,
| because it is much easier to measure frequency precisely.
| Using something like a TCXO from Seiko Epson with 1 ppm
| tolerance, and measuring over time, you can easily achieve
| 0.00001% precision and beyond. I know that strain gauges used
| in civil engineering often utilize this concept, where a
| metal string is "plucked" electronically and the frequency is
| then measured.
|
| [0] https://www.vishay.com/docs/61010/ccc.pdf and
| https://foilresistors.com/docs/63120/hzseries.pdf
| contingencies wrote:
| Neat. Next time I see resistors in a splayed or star
| configuration with one leg in shared proximity I will think
| of this comment.
| pclmulqdq wrote:
| One fun thing to do when designing high-precision analog stuff
| (audio) was to choose component values that are about 1.5-2%
| off of a value in the E12 series. You can then go test a whole
| bunch of resistors and you will find a lot within 0.1% of each
| other (even within 0.01%). Everything within 1% of E12 is
| binned as a 1% resistor so those aren't polluting your stock.
|
| Going within 0.1% of an E12 value is a pricey resistor, but
| resistors that are matched nearly perfectly and are 2-3% off
| are cheap and easy to find.
| throw0101d wrote:
| This part is the thing that made me understand the numbering
| series:
|
| > [...] _Continuing this trend, rounding as needed, and we end up
| with the series 10, 15, 22, 33, 47, and 68. Components built to
| the E6 standard have a 20% relative error tolerance, and if we
| look at the values again we'll see a trend. Starting with 10
| again and adding 20% error we end up with 12. Moving to 15 and
| subtracting 20% we get... wait for it... 12. Moving up from 15 we
| get 15 + 20% = 18 and 22 - 20% = 17.6. This trend repeats no
| matter what range of powers of 10 you use, as long as they are
| consecutive. So 47kO + 20% = 56400, while 68kO - 20% = 54400._
|
| > _Look again at the values 47 and 68. The max /min values
| overlap right about 56, don't they? That sounds familiar. The E12
| standard uses all of the same values as E6, but with 6 more
| values mixed in. These 6 additional values are roughly where the
| E6 values overlap, and now in order to cover the entire range our
| %-error is reduced to 10%. Starting again at 10, we have 10, 12,
| 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82. The math holds true
| here as well, with the error values just slightly overlapping._
|
| It's the 'tolerance overlap' concept that makes the numbers work,
| but I don't think I've ever seen it explained so clearly before.
| Denvercoder9 wrote:
| I feel like the author conflates tolerance in component value
| choice and fabrication tolerance. The E-series were chosen so
| that if you have perfect resistors (no fabrication tolerance)
| of only their values available, you can replace any resistor
| value you need with one from the series, and you'll never be
| more off than a fixed error (e.g. 20% for the E6 series).
|
| This only works with perfect resistors, though. If your actual
| resistors have a fabrication tolerance, you might be more off.
| For example, if you need a 41 Ohm resistor, you can use a
| perfect 47 Ohm resistor from the E6-series, and you'll be
| within 20% error. However, if that 47 Ohm resistor has a 10%
| fabrication tolerance, in reality it might be 51 Ohm, and
| that's more than 20% off from the 41 Ohm you needed.
|
| To take the example from the author's last paragraph, if you
| need a 70 Ohm resistor, the idea is not that you could be lucky
| and find an exact 70 Ohm in your E24 resistor set, but that you
| change the design to use a 68 Ohm instead, and don't introduce
| more than 5% off by doing so (regardless of the resistor value
| you needed).
| xw390112 wrote:
| In 2024, if your resistor vendor has even 5% tolerance, you
| need to find another vendor.
| rchowe wrote:
| Thin-film resistor design engineer here! It's dependent on
| value and geometry -- if you order a 0.5 ohm resistor the
| meters on our trimming lasers only go down to 20 mO and
| you're getting a 5% part at best.
| alright2565 wrote:
| What about shunt resistors? I can pretty easily get a 1%
| 5mO resistor, but they don't look to me like they are
| constructed in the same way as a generic resistor.
| gmueckl wrote:
| AFAIK, it used to be that parts binning was used to sort
| parts by tolerance. So the 5% bin wouldn't include <1%
| parts because those were already selected into the 1% bin
| in the factory and so on. Is it still like this?
| rchowe wrote:
| It depends on the component and the company/process.
| Laser trim time for thin film is a significant cost-
| driver, so if possible you want to aim for a specific
| value and reject or bin-sort the rest out. My company
| only makes 1% tolerance resistors by laser trimming.
| bsder wrote:
| Mostly, no. Nobody except for expensive precision
| resistor companies are actually measuring resistors more
| than statistically.
|
| The resistors are manufactured so that they are
| "guaranteed by manufacturing" such that the outliers are
| 1%, 5%, 10%, etc. And they do statistical checks on
| batches, but not really looking for the 10% outlier
| (which is stupendously rare and very difficult to catch)
| but looking for slight drifts off nominal (which are much
| easier to spot) which would result in more outliers than
| expected.
|
| As such, if you measure resistors, you tend to find that
| you get _really_ close to nominal--much closer than you
| would expect for 10%, say. Resistors are so cheap that
| binning simply doesn 't make economic sense.
| LeifCarrotson wrote:
| Is this how LEDs are binned as well, or are they powering
| each node on the wafer before packaging? They're orders
| of magnitude more expensive than resistors, so I figure
| they might...
|
| There are all kinds of crazy parameter variations in
| optoelectronics. I understand that resistors are really
| close to nominal because the manufacturer's ability to
| tune the process controls are so much better than the
| standard 5% and 10% bins, but it seems that LED
| manufacturing is way more difficult and they can't always
| tune the process to get exactly what they want.
| xg15 wrote:
| I didn't understand the Renard Numbers tangent until
| realizing it's the same principle of exploiting "usage
| tolerance": He replaced the 400 different cable lengths with
| 17 "standard" cables that can be stretched to any of the
| desired actual lengths. The choice of numbers ensures that
| the "stretch", i.e. error never exceeds a certain factor.
| xw390112 wrote:
| These things all made sense before laser trim resistors. At
| even a modest volume you can get any value you want at better
| than 1% for basically no money.
|
| Usually the minimum order is something like 10K parts (a.k.a.
| one reel) and you might pay something like $75 for it. $0.0075
| per resistor.
|
| https://www.ppisystems.com/ppi-systems-designs-and-manufactu...
| neuralRiot wrote:
| And before digital circuits. All I see now is 10k, 1k and 100
| Ohm resistors.
| xxs wrote:
| Most resistor in power supplies would be different values.
| For digital stuff and low current applications (along with
| designated FET drivers) you dont need too much.
| mb_72 wrote:
| 'Laser-trimmed resistors' will be the next selling point on a
| boutique guitar FX company's 57th clone of a Tubescreamer.
| analog31 wrote:
| Factored into the tolerance is the temperature coefficient,
| because the tolerance is specified over the operating range.
| There are also 3 basic temperature ranges: Industrial,
| automotive, and military. I've used this to my advantage by
| spec'ing automotive capacitors when I needed tighter
| tolerances for my normally room temperature applications.
|
| They also laser-trim IC's.
| amelius wrote:
| Why don't resistors show their power rating on the package,
| always? Or at least more often.
| robxorb wrote:
| Probably because only you and I have a problem with it ;)
| dboreham wrote:
| Can be inferred from the size usually.
| petsfed wrote:
| Because there's basically no design downside to having a higher
| power rating than needed, aside from BOM cost. If you're
| ordering a bunch to have on hand, you should just order the
| highest power rating you're likely to need in that size.
|
| For me, that means that my 0402s are all 1/16W, 0805 are 1/8W,
| 1206 1/4W, etc. And all of my through-hole resistors are 1/4,
| because the wire stock plays well with breadboards better.
|
| There are probably 1/4W 0402s out there, but that's definitely
| a specialty piece. I'm seeing 16 cents a resistor/each for a 1
| MOhm 1/4W 0402, which is about 4 times what I'd expect to pay
| for a 1/16W of the same resistance and package.
| LeifCarrotson wrote:
| I'd be surprised to find a 1/4W 0402, you'd just about melt
| the solder off. Yageo claims this one is good to 3W, do you
| think it glows cherry red? What trace width and pad geometry
| do you need to push 3W into a 0.0025 ohm resistor?
|
| https://www.digikey.com/en/products/detail/yageo/PA0402CRF5P.
| ..
|
| But to your point, Digikey has >70,000 0402s in 1/16W. There
| are 900 rated for 0.05W, and they're all exotic high-
| frequency/low temp coefficient/high-precision specialty
| parts.
| petsfed wrote:
| It probably has the cutest little heat sink.
| utensil4778 wrote:
| The package _is_ the power rating ;)
| tshaddox wrote:
| These sometimes end up being useful in UI/graphics work too. And
| the math/code is dead simple!
| https://gist.github.com/tshddx/8341d1bdbe2f83ed4e2c26bc48faf...
| eternityforest wrote:
| I like the 5-smooth numbers and related sequences, because they
| include a lot of numbers that are very common in engineering
| pikminguy wrote:
| The thing that's blowing my mind here is that this standard was
| adopted as ISO 3. It reminds me of the Simpsons joke that Mr.
| Burns' social security number is 000-00-0002.
| utensil4778 wrote:
| I think a lot of people are surprised to learn just how old the
| field of electronics is. It's an easy mistake to make with the
| relative novelty of digital electronics, but the science has
| been around for a good long time
| pikminguy wrote:
| It's less about the field of electronics being old and more
| about being surprised that ISO apparently just started
| counting with number 1 and that the preferred numbers would
| be so early relative to other things you might want to
| standardize.
| CliffStoll wrote:
| I'd always wondered why 47 ohm resistors were so common!
|
| Yellow and Purple striped critters inside of HeathKits.
| dboreham wrote:
| Having been around electronic components since before I could
| read: these aren't _odd_ values. They 're normal expected values.
| ssl-3 wrote:
| Related: https://www.veith.net/e12calc.htm
|
| It quickly calculates pairs of resistors from E12 (and other)
| resistor series to meet a target.
| PhasmaFelis wrote:
| Slight sidetrack:
|
| > We have to go back a few years to 1877 France. The French
| military used balloons for various purposes and of various sizes,
| and they had to be anchored using cables. Over time, they ended
| up with 425 different sizes of mooring cables that had to be
| individually ordered and inventoried. Talk about a nightmare. > >
| Enter Charles Renard. He was tasked with improving the balloons,
| but discovered this rat's nest of cables in the inventory closet
| instead. He spent some time thinking about it and came up with a
| series of 17 cable sizes that would allow for every type of
| balloon to be properly moored.
|
| I'm astonished that 425 distinct mooring-cable sizes were ever
| allowed to happen, and I'm also slightly astonished that even the
| cleaned-up version used 17. Anyone have more info about that?
| What were they doing with all those different-sized ropes? How
| many different balloon models could there have been?
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