49.9ohms, 51 ohms

Discussion in 'General Electronics Chat' started by dannybeckett, Jan 30, 2013.

  1. dannybeckett

    Thread Starter Senior Member

    Dec 9, 2009
  2. t06afre

    AAC Fanatic!

    May 11, 2009
  3. MrChips


    Oct 2, 2009
    As t06afre says, resistors come in standard values and 50Ω is not a standard value. However, manufacturers do make non-standard values such as 50Ω.
  4. dannybeckett

    Thread Starter Senior Member

    Dec 9, 2009
    Simple as that then! Thanks
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    If you want more information, read on. Otherwise, skip it.

    Resistors are usually sold in an "E Series" That is, E6, E12, E24 or E96.
    The E series simply tells you how many proportionally spaced resistors are between each power of 10 (e.g. 10 to 100, 100 to 1000, ...).

    In the case of E6, start with 10 ohms and then, to evenly space them by % difference, multiply 10 by the "magic number" 1.4677993 to get the next in the series, multiply that new value by the same number, 1.4677993 to get each additional value in the series. The number gets rounded to the nearest 2 significant digits. Note that the rounding may be slightly off - I don't know the historical reason for this (e.g. 33 is the common value but, doing the math, 32 appears to be the correct number).

    For the E12 series, the magic number is slightly smaller 1.2115277. By the time you get to the 12th number, you are at the next decade (e.g. starting with 10 you get to 100).

    You will notice that each higher E-series will have to have tighter and tigher tolerances (allowable error) to keeps is position correct without overlapping with neighboring values. Notice in an E12, you have 10% error allowed above and below the standard value before any resistor is at risk of being above or below is neighboring value (if one value is high in limit and the next higher resistor is low in its tolerance range). By the time you get to E96, tolerances must be very narrow 1% ("magic number is 1.024275221). Because tolerances are so tight (1%), these resistors must have more color bands to properly describe their value. You also get to strange decimal values to stay proportinally (%) different from the neighboring values.
    dannybeckett likes this.
  6. dannybeckett

    Thread Starter Senior Member

    Dec 9, 2009
    Thank you for the details. I didn't realise it was based on a proportional system, but it does make sense and accounts for the strange value resistors. I was thinking that a 49.9ohm resistor is preferable in a 50ohm input impedance device so that it's slightly less resistance than the source - I don't suppose this would have affected the system in any significant way, though I just wanted to make sure. Much appreciated Gopher!
  7. #12


    Nov 30, 2010
    The magic number is, for instance, the 6th root of 10 for an E6 series, the 12th root of 10 for the E12 series, etc. I assume someone didn't make an E7 series of resistors because 6 was enough to get all the possible values of resistance, give or take the percentage of accuracy guarantee, covered in each decade.

    You could make an E7 series or an E11 series now that you know how the magic number is calculated.