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Silly question about 47uf caps

Discussion in 'Electronic Basics' started by JFS, May 4, 2004.

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  1. JFS

    JFS Guest

    Why 0.47uf or 47uf? Is there a significance to that value or is it just a
    randomly chosen value?
     
  2. There was, though I can't actually remember the reasoning.

    I gather there was a time when there weren't such standard values. Or
    maybe it was that they weren't placed at quite the best values.

    So at one point, the "standard values" we know today came along. It
    was something like with a given tolerance, and we're talking when 20%
    tolerance was common, you didn't need values at every point on the scale,
    but on the other hand, the tolerance set the stage for the next value up.
    Add 20% to 47K and you get 56K and add 20% to that and you get just under
    68K.

    When higher tolerance resistors came along, of course they filled in some
    of the blanks.

    That's not an exact answer, but it goes something along those lines. It
    was most definitely not a random choice.

    Michael
     
  3. vic

    vic Guest

    The standard resistor/capacitor/inductance values are build the same way
    as music notes frequencies. For example the E12 series has 12 different
    values between each decade, the factor between successive values is
    calculated like this :

    k = 10^(1/12) = 1.2115
    V1 = 10 and Vn = k*V(n-1)

    Which gives the following values (rounded)

    calculated values: 10, 12, 15, 18, 22, 26, 32, 38, 46, 56, 68, 83, 100
    E12 values : 10, 12, 15, 18, 22, 27, 33, 38, 47, 56, 68, 83, 100

    For some reason there are slight differences but it's very close.
    You can notice that there is about 20% difference between each values,
    which is coherent with the tolerance of usual components.

    vic
     
  4. E12 value 38 should be 36 and 83 should be 82
     
  5. Clint Sharp

    Clint Sharp Guest

    They had to think of a number to be the 'usual' fault and 47 got it.
    It's a basic rule of electronics repair, replace all components with a 4
    and a 7 in the value, if, on the rare occasion it still doesn't work,
    get the test gear out. (Years of SMPS repair can do this to you!)
     
  6. But the value 38 becomes an E12 value of 39. 36 is too close to 33.
     
  7. Fred Stevens

    Fred Stevens Guest

    No, it should be 39.
     
  8. vic

    vic Guest

    E12 values : 10, 12, 15, 18, 22, 27, 33, 38, 47, 56, 68, 83, 100
    Sorry I should have checked ... the correct series is
    10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100
     
  9. GPG

    GPG Guest

    k = 10^(1/12) = 1.2115 OK
    Multipler = k^n For n = 0 to 11
     
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