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

    AAC Fanatic!

    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

    Well-Known 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.