# Finding a Unknown Inductor Equation

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

1. ### trav Thread Starter New Member

Dec 8, 2011
7
0
Does anybody know the formula to calculate for a unknown inductor?

2. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Can you elaborate on the problem

3. ### trav Thread Starter New Member

Dec 8, 2011
7
0
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
7
0
I got the formula of L = 1 / 4(pi^2)(f^2)xC anyone can clarify this?

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
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:

$\omega=sqrt{\frac{1}{LC}$

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

$\omega^2=\frac{1}{LC}$

now we can "swap" the left with the denominator

$LC=\frac{1}{\omega^2}$

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
790
186
@trav
I also didn't understood your question...
Anyway to find out inductance for a coil you can use Wheeler's Formula

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

where
N = number of turns
B = coil length
C = coil thickness

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

Good Luck