from Hacker News

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

by mtm on 6/4/24, 4:20 PM with 157 comments

  • by throw0101d on 6/4/24, 6:47 PM

    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 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.

  • by Workaccount2 on 6/4/24, 6:15 PM

    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.

  • by pikminguy on 6/4/24, 6:59 PM

    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.

  • by SOTGO on 6/4/24, 5:28 PM

    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?

  • by tshaddox on 6/4/24, 6:56 PM

    These sometimes end up being useful in UI/graphics work too. And the math/code is dead simple! https://gist.github.com/tshddx/8341d1bdbe2f83ed4e2c26bc48faf...

  • by rylittle on 6/4/24, 5:28 PM

    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?

  • by ssl-3 on 6/4/24, 7:35 PM

    Related: https://www.veith.net/e12calc.htm

    It quickly calculates pairs of resistors from E12 (and other) resistor series to meet a target.

  • by PhasmaFelis on 6/4/24, 8:05 PM

    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?

  • by CliffStoll on 6/4/24, 7:20 PM

    I'd always wondered why 47 ohm resistors were so common!

    Yellow and Purple striped critters inside of HeathKits.

  • by hw-guy on 6/5/24, 2:45 PM

    I 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.

  • by dboreham on 6/4/24, 7:22 PM

    Having been around electronic components since before I could read: these aren't odd values. They're normal expected values.

  • by amelius on 6/4/24, 6:48 PM

    Why don't resistors show their power rating on the package, always? Or at least more often.