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Author Topic: How calculate L of edge-wound coil?  (Read 3340 times)
W0BTU
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« on: January 01, 2013, 03:54:28 PM »

Does anyone know the equation for calculating the inductance of an edge-wound inductor? Here's what I have, and I can't find the formula anywhere. Any help would be appreciated.

What I have is made from silver-plated edge-wound flat conductor .073" thick x .377" wide/deep
Coil is 4.831" OD, 4.077" ID
.250" pitch (4 turns per inch)
23 Turns - 5.75" long
(actually is 22.75" turns, 5.687" long)

It's a rotary inductor - Gates model# 30-VB-2344

TIA
« Last Edit: January 01, 2013, 03:56:43 PM by W0BTU » Logged

KE3WD
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« Reply #1 on: January 01, 2013, 04:09:39 PM »

Why not ring it and find out empirically? 


73
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G3RZP
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« Reply #2 on: January 02, 2013, 01:04:11 AM »

Why should it be that much different to a similar coil wound with round wire? You're not looking for 1% accuracy. ARRL Handbooks for years have had Wheeler's formula in them.


L = 0.2aE2, nE2/(3a+9b+10c)

a is diameter in inches, b is winding length in inches, c is radial depth, n i number of turns and L is inductance in microhenries.

the E2 means squared.
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VK2TIL
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Posts: 331




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« Reply #3 on: January 02, 2013, 12:21:08 PM »

I think that the "30" in the part number indicates 30 uH.

As KE3WD suggested, resonating it with a known value of C would be a good check.

I agree, too, with G3RZP; I entered your dimensions, roughly-averaged in the case of the diameter, into an on-line calculator and got 34 uH.

Close enough for jazz!  Smiley
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N3OX
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« Reply #4 on: January 02, 2013, 02:14:33 PM »

I would try using the self-impedance effective diameter derived here:

http://fermi.la.asu.edu/w9cf/articles/conform/index.html

for a rectangular strip of a certain width to thickness ratio and plug that into your favorite round wire calculator.



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KE3WD
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« Reply #5 on: January 02, 2013, 04:24:30 PM »

Use of the math is fine when designing and actual parts are not in hand. 

When you have the actual coil in hand, a cap, a signal generator and a scope can tell you *exactly* what's going on. 

73
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VK2TIL
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« Reply #6 on: January 02, 2013, 06:04:05 PM »

Just did a bit more Googling and learned a bit about Gates/Harris inductor part numbers.

For a 30-VB-2344;

"30" = Inductance in uH.

"V" = Variable.

"B" = A winding of 3/8" flat strip/bar with a current rating of 20A.

"23" = Nominal length in inches.

"4" = TPI.

"4" = Nominal diameter (possibly ID).
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W0BTU
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« Reply #7 on: January 03, 2013, 10:19:06 AM »

Thanks for all the replies! I appreciate it.

It does appear that the extra capacitance between the windings has little to do with the inductance, only the coil's self-resonant frequency.

But somewhere, long long ago in a book far far away, I know I saw the formula specifically for edge-wound coils. :-)

Thank you again for all the useful info.
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G3RZP
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« Reply #8 on: January 03, 2013, 10:24:20 AM »

There's two sorts of edge wound coils, with different equations applicable. One is a coil as you describe, the other is a pancake, rather like a compressed volute spring. Don't often see them these days.
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W0BTU
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« Reply #9 on: January 03, 2013, 10:36:50 AM »

Those old pancake coils were used a lot back in the days of spark transmitters.

Photos of my coil are at http://www.w0btu.com/files/misc/Gates_rotary_inductor/ .
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WB6BYU
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« Reply #10 on: January 03, 2013, 11:00:47 AM »

I don't remember the planar or "pancake" coils being called "edge-wound", but
I'll have to go back to my books to see.  I know they had a different formula,
and were used extensively as antenna coils in AM BC receivers (usually wound
flat against the back panel) before ferrite rods became common.
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KD0REQ
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« Reply #11 on: January 03, 2013, 11:01:21 AM »

RZP, used to call those "solenoid" wound coils back when they had varnished cotton insulation.
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W5DXP
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« Reply #12 on: January 03, 2013, 11:38:25 AM »

It does appear that the extra capacitance between the windings has little to do with the inductance, only the coil's self-resonant frequency.

One feature of those coils is that they can simultaneously be used as a loading coil and top hat capacitance. That design won the mobile antenna shootout in El Paso, many years ago.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
G3RZP
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Posts: 4718




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« Reply #13 on: January 03, 2013, 01:19:32 PM »

Imagine a volute spring. Take a piece of strip and wind it over itself, like paper towel on a roll. You now have a strip wound coil: separate the turns to reduce self capacity. Not a solenoid, and not really like the old BC band antennas in that the turns are separated.
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VK2TIL
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Posts: 331




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« Reply #14 on: January 03, 2013, 02:48:44 PM »

There has been renewed interest in the old technique of conical coils which have reduced inter-winding capacitance and high self-resonant frequency; coil manufacturers now offer a range of conical coils.

There are some interesting designs being used at multi-GHz frequencies;

http://i49.tinypic.com/15x76ms.jpg

http://i48.tinypic.com/30uvb05.jpg

http://i46.tinypic.com/yfj3d.jpg

I particularly admire the person who thought this one up;

http://i47.tinypic.com/zmgojm.jpg



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