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

["McCoy, Bart O." <McCoy.Bart@xxxxxxxx>]

I'm fairly sure K = 2 when units are in cm.

Take, for example, a typical 1 mil diameter wirebond
1 mm in length.   It's well known that they are usually 
near (I'd say slightly less than) 1 nH per mm.  

Diameter = 1 mil = 2.54E-3 cm
Length   = 1 mm  = 0.1 cm
K        = 2

L  =  K * length * [ ln(4*length/Diam) - 0.75 ]
L  =  2 * 0.1 * [ ln(4*0.1/2.54E-3) - 0.75 ]
L  =  0.2 * [ ln(157.5) - 0.75]
L  =  0.2 * [ 4.31 ]
L  =  0.86 nH

That's very much in the ballpark-- I'd say extremely
close based upon electromagnetic sims I've done which
came out a bit less than 1 nH for a 1 mm wirebond.

If you use 0.002, that would make it 3 orders of magnitude
less, which I know for sure isn't correct.   Even
1 order of magnitude smaller would be WAY off base.

  - Bart




-----Original Message-----
From: Ken Cantrell [mailto:Ken.Cantrell@xxxxxxxxxxxxxxxx] 
Sent: Thursday, September 04, 2003 5:04 PM
To: McCoy, Bart O.; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: FW: Inductance of wire section


K should be = .002, shouldn't it?

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of McCoy, Bart O.
Sent: Thursday, September 04, 2003 3:11 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] FW: Inductance of wire section



Well, you'll probably get a million replies with the same
equation in 50 different forms, but here's one that I use
for a straight-wire of circular cross-section inductance 
calculation that will give you first order effects:


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


22 Gauge wire....   I'll let you do the calculation given:

20 Gauge AWG  = 0.0813 cm diameter
26 Gauge AWG  = 0.0406 cm diameter


Hope that helps.

   - Bart




-----Original Message-----
From: sdb@xxxxxxxxxx [mailto:sdb@xxxxxxxxxx] 
Sent: Thursday, September 04, 2003 2:53 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Inductance of wire section


Hi gang --

Please pardon me if this is a FAQ, but does anybody have a quick
rule-of-thumb for the inductance to expect from a 1 cm length of 22
gauge wire-wrap wire in free space?   Yes, I've searched the web &
looked at a few pages which are far too detailed to be really useful
-- all I want is an order-of-magnitude number, and the websites are
all about calculating the number by integrating Maxwell's equations.  

So far, I've been using a figure of around 360 nH/m, which is the
inductance of a common 50ohm transmission line.  If the real number is
larger or smaller by a factor of 2 to 5 I don't care a lot, but I would
like to make sure that the "rule-of-thumb" number is *not* orders of
magnitude larger or smaller than this.

What rule-of-thumb do you use?

Stuart
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