Finding a Unknown Inductor Equation

Discussion in 'Homework Help' started by trav, Dec 13, 2011.

  1. trav

    Thread Starter New Member

    Dec 8, 2011
    Does anybody know the formula to calculate for a unknown inductor?
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    Can you elaborate on the problem :)
  3. trav

    Thread Starter New Member

    Dec 8, 2011
    Well I've got the formula F = 1 / 2pi sqrt L x C
    and need rearrange to find L but dont really know if my answer is correct.
  4. trav

    Thread Starter New Member

    Dec 8, 2011
    I got the formula of L = 1 / 4(pi^2)(f^2)xC anyone can clarify this?
  5. thatoneguy


    Feb 19, 2009
    Is this for a simple, air core inductor with a single winding?

    Or to determine the Fo of an LC circuit?

    I see both your formulas have f and C in your equations. I must admit I do not understand the question you are asking.

    There is a formula for a wound air coil of Length L and diameter d with a winding density factor.

    There is a formula for an inductor wound on a ferrite core, but the permiability of the core would be needed.

    The resonant frequency, F_o=\frac{1}{2 \pi sqrt{L C}}

    Please be more verbose with your question, explaining what you do know, and what you'd like to figure out.

    First, keep in mind: \omega=2\pi F and F=\frac{\omega}{2\pi}

    --ETA: Your Post #3 states you want to solve F_o=\frac{1}{2 \pi sqrt{L C}

    The equation is derived from:


    to get L out, we need to square both sides:


    now we can "swap" the left with the denominator


    woops, moved one too many:

    L=\frac{1}{\omega^2 C}

    Obviously, the formula is the same to find C if you know f and L

    C=\frac{1}{\omega^2 L}
    Last edited: Dec 14, 2011
  6. debjit625

    Well-Known Member

    Apr 17, 2010
    I also didn't understood your question...
    Anyway to find out inductance for a coil you can use Wheeler's Formula

    <br />
Inductance (\mu H) = {{0.8(NA)^2} \over {6A + 9B + 10C}}<br />

    N = number of turns
    A = average coil radius
    B = coil length
    C = coil thickness

    All quantities are in inches and the result is in microhenries (\mu H).

    Good Luck
  7. Adjuster

    Late Member

    Dec 26, 2010
    The OP's problem appears to be stated in terms of the resonant frequency in a tuned circuit, not the coil's construction.