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Why do electronic components have such odd values? (2021)

digilent.com

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Re: Why do electronic components have such odd values? (2021)

#71

Post by xxs »

dbcurtis wrote: post #27
SOTGO wrote: post #2

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?

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 ”

Even ordering from China the 1% are perfectly fine. E96 resistors are ubiquitous and cheap.

Re: Why do electronic components have such odd values? (2021)

#72

Post by [deleted] »

xw390112 wrote: post #64
throw0101d wrote: post #34

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……

[dead]

[deleted]

Re: Why do electronic components have such odd values? (2021)

#73

Post by neuralRiot »

xw390112 wrote: post #64
throw0101d wrote: post #34

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……

[dead]

And before digital circuits. All I see now is 10k, 1k and 100 Ohm resistors.

Re: Why do electronic components have such odd values? (2021)

#74

Post by pikminguy »

utensil4778 wrote: post #69
pikminguy wrote: post #42

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.

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

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.

Re: Why do electronic components have such odd values? (2021)

#75

Post by nomel »

jjmarr wrote: post #45
nomel wrote: post #31

Earlier quoted context omitted.

> 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…

> 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…

> 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.

Re: Why do electronic components have such odd values? (2021)

#76

Post by nomel »

Dylan16807 wrote: post #54
nomel wrote: post #31

Earlier quoted context omitted.

> 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…

If you're going to say "Complete nonsense." you shouldn't get the calculation wrong in your next sentence.

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.

Re: Why do electronic components have such odd values? (2021)

#77

Post by Gibbon1 »

RetroTechie wrote: post #28
retrac wrote: post #19

Earlier quoted context omitted.

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".

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 t…

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.

Re: Why do electronic components have such odd values? (2021)

#78

Post by rchowe »

xw390112 wrote: post #66
Denvercoder9 wrote: post #63

Earlier quoted context omitted.

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 you…

In 2024, if your resistor vendor has even 5% tolerance, you need to find another vendor.

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 mΩ and you're getting a 5% part at best.

Re: Why do electronic components have such odd values? (2021)

#79

Post by xxs »

neuralRiot wrote: post #73
xw390112 wrote: post #64

Earlier quoted context omitted.

[dead]

And before digital circuits. All I see now is 10k, 1k and 100 Ohm resistors.

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.

Re: Why do electronic components have such odd values? (2021)

#80

Post by willis936 »

nick238 wrote: post #59
eternityforest wrote: post #43

Earlier quoted context omitted.

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.

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.

For voltage dividers it's best to use matched networks. Often not much more expensive and orders of magnitude more precise.
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