# Silly question about 47uf caps

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

1. ### JFSGuest

Why 0.47uf or 47uf? Is there a significance to that value or is it just a
randomly chosen value?

2. ### Michael BlackGuest

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. ### vicGuest

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. ### John PopelishGuest

E12 value 38 should be 36 and 83 should be 82

5. ### Clint SharpGuest

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. ### Geir KlemetsenGuest

But the value 38 becomes an E12 value of 39. 36 is too close to 33.

7. ### Fred StevensGuest

No, it should be 39.

8. ### vicGuest

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. ### GPGGuest

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