electronic question my wife asked me that I can't readily answer - Page 1
Forum section: electronics / beginners

author: KTP
author_rank: Frequent Contributor
timestamp: July 20, 2012, 10:41:01 pm
content: Ok, here is what she asked:"How did 47K become such a common resistor value? Why is it much more common than say, 50K?"She was looking at some reels I had, and noticed 1K, 10K, 20K, 47K and 100K...sort of stumped me for the moment.

author: bilko
author_rank: Frequent Contributor
timestamp: July 20, 2012, 10:46:35 pm
content: My guess is that it is because the 'preferred values' can make up lots of values using series parallel configurations. If the resistors were even numbers, how would you get for example 49k ?

author: David_AVD
author_rank: Super Contributor
timestamp: July 20, 2012, 10:48:38 pm
content: I just did a quick Google search for "E12 range why" and this was one result:http://www.learnabout-electronics.org/resistors_05.phpIt's funny though that potentiometers (panel and trim types) are usually 20K, 50K, etc these days where they used to be 22K and 47K, etc. Go figure!

author: baljemmett
author_rank: Unknown
timestamp: July 20, 2012, 10:51:09 pm
content: Quote from: KTP on July 20, 2012, 10:41:01 pmAs I understand it, the values are chosen in a geometric sequence so that the largest percentage error between any arbitrary value and the nearest preferred number is minimised -- if they were chosen to be equally-spaced linearly, the lower end of the scale would have a higher percentage error for intermediate values. (Although bear in mind it's almost midnight and I've been on the beer so I might be thinking that one backwards...)Also, the values are ones that can't easily be made up from series/parallel combinations of others in the same series -- e.g. two 100s in parallel gives you the 50, but finding a suitable substitute for 47k using E12 resistors is trickier!

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 20, 2012, 11:11:56 pm
content: Why had you reels in the kitchen?:p

author: digsys
author_rank: Unknown
timestamp: July 20, 2012, 11:15:31 pm
content: As already noted, E12 .. E192 series ranges are chosen by the max tolerance between values, starting at 1.000 (...9.999)For the common garden variety at 5%, that is 1K0 1K2 1K5 ... In your list you have a RING-IN !! 20K and 47K aren't in thesame %E series. In E12 it is 22K (or 18K).There are charts of all the Exx series around egwww.logwell.com/tech/components/resistor_values.html

author: gxti
author_rank: Frequent Contributor
timestamp: July 20, 2012, 11:35:36 pm
content: The puzzling part to me has always been why the E12 values aren't simply 10^(n/12) rounded to the nearest tenth. For example, according to that formula it should be 83, not 82. I ran into that difference face-first when I wrote a script to "round to E12", so that rather than an elegant and generic formula to round geometrically, it has to pick from a predefined list. Oh well.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 21, 2012, 01:30:02 am
content: Quote from: David_AVD on July 20, 2012, 10:48:38 pmBack when all the pots used in Australia were made locally,standard values for pots were 1k,2.5k, 5k,10k,25k,50k & so on...When most of the pots were sourced from Japan,the 22k,47k,etc types became the norm.Now they are mostly from China,we get the Chinese standard values,like 20k,& 50k.

author: amspire
author_rank: Super Contributor
timestamp: July 21, 2012, 02:03:53 am
content: Once you have chosen 6.8, it works out that 8.2 is a marginally better fit then 8.3.If you choose 8.2, then there is a 0.2 overlap between 8.2+10% and 10.0 - 10%.If you choose 8.3, there is a 0.1 overlap between 6.8 + 10% and 8.3 - 10%So they could have chosen either, but 8.2 wins by a tight margin.The choice of 6.8 is a no brainer - the ideal value was 6.81, whereas 8.2/8.3 was the 50/50 toss-up. The ideal value for the 8.2 E12 value is 8.254, but given the choice of 6.8, the ideal value shifts to 8.237. Rounding to one place makes that 8.2Richard.

author: KTP
author_rank: Frequent Contributor
timestamp: July 21, 2012, 02:17:18 am
content: Thanks! She liked the answers explaining the tolerances and %error but her favorite was the one about why did I have resistors in the kitchen.I wonder if this was more important back in the day when we could not get 1% tolerance resistors at just about any value next day from Digikey.

author: amspire
author_rank: Super Contributor
timestamp: July 21, 2012, 02:24:36 am
content: Quote from: KTP on July 21, 2012, 02:17:18 amThe E12 numbering dates back to the days when 20% and 10% carbon composition resistors were the common resistors. To get 1%, you probably had to get a very expensive wire wound resistor.

author: gxti
author_rank: Frequent Contributor
timestamp: July 21, 2012, 04:45:53 pm
content: Even if your parts are 1% and not 10%, having a selection of E12-valued resistors means that you'll always have one that's within 10% of the ideal value for your circuit. It (or a finer geometric series) is the ideal allocation for which values to manufacture, rather than just counting up. The tolerance simply dictates the largest number of values you'd bother making, because any finer and they overlap completely.