Why don't resistors show their power rating on the package, always? Or at least more often.
Can be inferred from the size usually.
Why do electronic components have such odd values? (2021)
51–60 of 168 posts
Re: Why do electronic components have such odd values? (2021)
#52Earlier 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…
> 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.
Re: Why do electronic components have such odd values? (2021)
#53Why don't resistors show their power rating on the package, always? Or at least more often.
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.
Re: Why do electronic components have such odd values? (2021)
#54Earlier 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…
> 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.
Re: Why do electronic components have such odd values? (2021)
#55Wikipedia 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.
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)
Re: Why do electronic components have such odd values? (2021)
#56Related: https://www.veith.net/e12calc.htm
It quickly calculates pairs of resistors from E12 (and other) resistor series to meet a target.
Re: Why do electronic components have such odd values? (2021)
#57Wikipedia 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.
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.
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.
Re: Why do electronic components have such odd values? (2021)
#58Earlier quoted context omitted.
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.
Can you assume that +/-5% isn't linearly distributed? If so, the tolerance in practice may likely end up even smaller.
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.
Re: Why do electronic components have such odd values? (2021)
#59Wikipedia 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.
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.
Re: Why do electronic components have such odd values? (2021)
#60Earlier quoted context omitted.
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.
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…
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)