15 ms·
Why do electronic components have such odd values? (2021)
- SOTGO 2y agoCan 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 2y agoI 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 2y ago> 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 2y agoFun 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 2y agoI'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 2y agoIf 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 68kΩ +/- 20%, and it actually ended up at 66kΩ +/- 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 2y agoResistors 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 2y agoMost 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.
- 2y ago
- thornewolf 2y agotolerance should actually go down since the errors help cancel each other out. reference: https://people.umass.edu/phys286/Propagating_uncertainty.pdf 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 2y agoNote 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.
- MobiusHorizons 2y agoIt’s not nearly that simple:) Component values change with environmental factors like temperature and humidity. Resistors that have a 1% rating don’t change as much over a range of temperatures as 5% or 10% components do. This is typically accomplished by making the 1% resistors using different materials and construction techniques than the lower tolerance parts. Just taking a single measurement is not enough.
- immibis 2y agoIf 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.
- Sohcahtoa82 2y agoNope, 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 2y agoCan you assume that +/-5% isn't linearly distributed? If so, the tolerance in practice may likely end up even smaller.
- sophacles 2y agoThere'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 2y agoYou are correct. Two of the comments on the article itself also mention this error.
- LeifCarrotson 2y agoBrilliant, informative writing, and yet people will jump to nit-pick the arithmetic. I'd better spell-check this comment before clicking reply...
- rexer 2y agoI think that was a typo and they meant 22 + 47, which equals ~70 Ohms
- renewiltord 2y agoBut 80 is within 20% of 70 so we're fine ;)
- phkahler 2y ago>> But 80 is within 20% of 70 so we're fine ;) So are the 68 Ohm and 75 Ohm.
- dbcurtis 2y agoI 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 2y agoEven ordering from China the 1% are perfectly fine. E96 resistors are ubiquitous and cheap.
- deleted 2y ago[deleted]
- rylittle 2y agoInsightful 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 2y agodo geometric sequences care about the base?
- perlgeek 2y agoThe ones mentioned in the article return to powers of 10. In contrast, musical notes don't, their frequencies return to powers of 2.
- dmurray 2y agoYes, 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 2y agoIt's a more accessible way of explaining it that doesn't require understanding geometric sequences first.
- timerol 2y agoIt'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."
- deleted 2y ago[deleted]
- Workaccount2 2y agoWikipedia 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#Table 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 2y agoI'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 2y agoIf 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 2y agoFor voltage dividers it's best to use matched networks. Often not much more expensive and orders of magnitude more precise.
- eternityforest 2y agoI definitely agree that 1% or better resistors are easier than calibration, but that doesn't mean you need values outside of E3 most of the time. I might want want an accurate 1/10 divider or something, but a 1/12 divider would probably be fine too, as long as it's consistent. If it doesn't vary between devices, it's just a line of code to change.
- 2y ago
- foxbeneficial 2y ago[dead]
- throw0101d 2y agoThis 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 47kΩ + 20% = 56400, while 68kΩ – 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 2y agoI 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 2y ago
- amelius 2y agoWhy don't resistors show their power rating on the package, always? Or at least more often.
- robxorb 2y agoProbably because only you and I have a problem with it ;)
- dboreham 2y agoCan be inferred from the size usually.
- petsfed 2y agoBecause 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 2y agoI'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/PA0402CRF5P2U5L/21620323 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 2y agoIt probably has the cutest little heat sink.
- tshaddox 2y agoThese sometimes end up being useful in UI/graphics work too. And the math/code is dead simple! https://gist.github.com/tshddx/8341d1bdbe2f83ed4e2c26bc48faf6b9 https://gist.github.com/tshddx/8341d1bdbe2f83ed4e2c26bc48faf...
- eternityforest 2y agoI like the 5-smooth numbers and related sequences, because they include a lot of numbers that are very common in engineering
- pikminguy 2y agoThe 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 2y agoI 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 2y agoIt'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.
- yonatan8070 2y agoAt my local hackerspace we got a donation of a _huge_ 3D printer (~1m³ IIRC), after a while we realized that the number "3" printed on it is actually the serial number
- CliffStoll 2y agoI'd always wondered why 47 ohm resistors were so common! Yellow and Purple striped critters inside of HeathKits.
- dboreham 2y agoHaving been around electronic components since before I could read: these aren't odd values. They're normal expected values.
- ssl-3 2y agoRelated: https://www.veith.net/e12calc.htm https://www.veith.net/e12calc.htm It quickly calculates pairs of resistors from E12 (and other) resistor series to meet a target.
- PhasmaFelis 2y agoSlight 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?
- yobert 2y agoThink about tethering a zeppelin with a curved body shape, where you need to attach in multiple places. Combine that with needing to tether at different elevations and it will get out of hand pretty quick.
- PhasmaFelis 2y agoYou're thinking that shorter lines can be thicker without tearing under their own weight, so the optimal mix is a few heavy lines supported by a larger number of light ones? And/or heavy lines for safety, many light lines for stability? I suppose that makes sense, though it still seems weird that they needed that many types.
- susa_berry 2y ago[dead]
- deleted 2y ago[deleted]
- hw-guy 2y agoI worked at a company with a technician who clearly did not understand this. When I asked him to order an assortment of resistors with a range of values, he came back to me and said that Digikey did not have most of them. Turns out he had submitted a request for quote, listing desired values in a linear progression: 1 ohm, 2 ohm, 3 ohm, 4 ohm, etc.