Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

digilent.com

Post reply on HN
Print view View original

21–30 of 168 posts

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

#21

Post by projektfu »

riedel wrote: post #7
rylittle wrote: post #5

Earlier quoted context omitted.

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%?

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.

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.

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

#22

Post by Workaccount2 »

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.

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

#23

Post by renewiltord »

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?

But 80 is within 20% of 70 so we're fine ;)

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

#24

Post by dylan604 »

dmurray wrote: post #13
riedel wrote: post #7

Earlier quoted context omitted.

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.

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 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, per…

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

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

#25

Post by wongarsu »

dmurray wrote: post #13
riedel wrote: post #7

Earlier quoted context omitted.

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.

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 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, per…

The issue is probably volume. Very few applications need a resistor that's exactly 66kΩ, but a lot of applications need resistors that are in the ballpark of 68kΩ (but nobody would really notice if some 56kΩ 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.

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

#27

Post by dbcurtis »

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”

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

#28

Post by RetroTechie »

retrac wrote: post #19
dmurray wrote: post #13

Earlier quoted context omitted.

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 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, per…

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 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.

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

#29

Post by projektfu »

retrac wrote: post #19
dmurray wrote: post #13

Earlier quoted context omitted.

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 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, per…

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".

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.

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

#30

Post by afiori »

immibis wrote: post #14
thornewolf wrote: post #8

Earlier quoted context omitted.

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 re…

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 manufactur…

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

Post reply on HN