FreeLists / si-list / [SI-LIST] Re: Inductance of wire section

[Kim Flint <kim@xxxxxxxxxxxx>]

At 01:05 PM 9/5/2003, bpanos wrote:
>Frederick Terman gives some brief background and assumptions about the formula
>everyone has been mentioning,
>in his book:  Electronic and Radio Engineering (written back in the 40's it is
>still applicable today)


Thanks, but that's not a really useful suggestion seeing as how that book 
has been out of print for quite a while. I guess we could all run off to 
the rare book emporium, but wouldn't it be a little more efficient to 
simply state the assumptions of the formula?

In any case, I wasn't necessarily looking for a reference. I guess my 
read-between-the-lines point was too subtle and polite so I'll be direct:

"when providing an approximate formula or rule-of-thumb that is based on 
some assumed conditions, please provide the assumptions along with the 
formula."

The original problem here was about a segment of wire wrap wire in space 
with no statement about the geometry of the rest of the circuit connected 
to the wire. The wire could be 1mm from it's return path or 1 meter, we 
don't know. That distance could affect the inductance quite a lot. If a 
formula is provided that doesn't include this distance variable, it seems 
to me it needs some further explanation of the assumed return path in the 
formula for it to be useful to anybody. So again I'll ask, in the case of 
this particular formula, can somebody provide those assumptions that go 
with it?

kim



>Kim Flint wrote:
>
> > Hi-
> > I'm curious about assumptions made in some of these inductance formulas. I
> > understand inductance as requiring a coil of wire, wherein magnetic flux
> > would intersect the area of the coil. "Inductance of a wire segment"
> > doesn't really mean anything without understanding where the rest of the
> > current path forming our coil is located. So I'm guessing these formulas
> > are approximations based on some set of assumed conditions, like height
> > above ground or something like that? Does anybody know what those
> > assumptions are in this case? I'd like to understand where I can expect
> > this formula to be reasonably valid.
> >
> > thanks!
> > kim
> >
> > At 08:57 AM 9/5/2003, Stuart Brorson wrote:
> > >Hi Guys --
> > >
> > >Thanks for all your answers.  I used the following formula from Bart:
> > >
> > >L  =  K * length * [ ln(4*length/Diam) - 0.75 ]
> > >
> > >L      = Inductance in nanohenries
> > >D      = Diameter of wire in cm
> > >length = Length of wire in cm
> > >K      = 2
> > >
> > >stuck it into Gnumeric (Linux-based Excel replacement), and got the
> > >following answers:
> > >
> > >                 Unit    qnty    qnty    qnty
> > >K                       2       2       2
> > >Length          cm      1       1       1
> > >Diam            cm      0.08    0.06    0.04
> > >
> > >Inductance      nH      6.324   6.899   7.710
> > >
> > >depending upon the wire diameter.  Using the below figure of 20nH/in,
> > >and knowing that 1in = 2.54cm, I get L = 7.87 nH.
> > >
> > >Therefore, I think we have nailed it: L = 7nH/cm +/- 10% or so.
> > >
> > >Thanks again for all the answers!
> > >
> > >Stuart
> > >
> > > >
> > > > Hi Stuart: The ROT I use is 20nH/in.
> > > > BTW, I verifed this using a 2D extractor
> > > > and increasing the boundary distance far
> > > > away from the wire to minimize mutual indutance.
> > > >
> > > >
> > > > Hope this helps
> > > >
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_______________________________________________________
Kim Flint                       http://www.kimflint.com
kim@xxxxxxxxxxxx

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