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27th May 2007, 12:51 AM  #1 
Wizard of Kelts
diyAudio Moderator

Calculating the length of wire in a flat, slender coil
Suppose I wanted to build a flat, slender coil using 40 gauge wire, (.0078 in).
The air core will be 0.25 in diameter. The ouside diameter of the coil is 2.0 in diameter. The depth of the coil will be 0.0078 inchjust the width of a single strand of wire. How would I go about calculating the length of the 40 gauge wire necessary for this coil?
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
27th May 2007, 04:36 AM  #2 
Wizard of Kelts
diyAudio Moderator

Just thought I would put an illustration to what I mean. The coil is only a single strand deep, but 2 inches across.
Since the winding in this example is 40 gauge, that is 0.0078 in diameter. The winding is 0.75 inches on each side of the air gap. If I am figuring correctlythis is the first time I have attempted to calculate such thingsthat means the coil has 75 / 0.0078 turns. That is 9, 615 turns of 40 gauge wire. But since each turn requires a different length of wire, I don't know the total length required for the winding. Any help would be appreciated.
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
27th May 2007, 05:35 AM  #3 
diyAudio Member
Join Date: Jul 2003
Location: San Diego, CA

It's 96 turns and you'll need about 379 inches (plus lead length). You can approximate it by multiplying the number of turns by the average diameter (1.25 inches in your case), and then by pi.
Cheers, Casey 
27th May 2007, 05:49 AM  #4 
Wizard of Kelts
diyAudio Moderator

Much obliged.
Thank you.
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
27th May 2007, 06:55 AM  #5 
Warp Engineer
On Holiday

KW,
Just noticed that your description in your first post does not match your description in your second post. In your First post you list ID as 0.25" and in your second post you list ID as 0.5" ... I'll assume that your second post is infact correct since it matches your illustration but I though I would mention it "just incase".
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 Dan 
27th May 2007, 01:51 PM  #6 
Wizard of Kelts
diyAudio Moderator

Audio Freak:
See what happens when you do things late at night, lol? We'll use the 0.5 in for the calculations, even though the 0.25 in would be more desirable. I'm just trying to arrive at a method, I honestly don't know what gauge wire I'll be trying, anyway. Besides, I just looked up a different website and got a different measurement for the the diameter of 40 gauge wire, so that throws things off even more.
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"A friend will help you move. A really good friend will help you move a body." Anonymous 
27th May 2007, 02:14 PM  #7 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Hi,
I think there are at least two ways to get your wire length. 1. the complicated (for me) calculus method. 2. the area method. I also think these two alternatives give the same answer. Using method 2. Calculate the area of the coil [D^2d^2]*Pi/4=[2^20.5^2]*3.14/2=2.945 sqin. Now take the area and divide by the wire thickness. A/Thk=4.7/0.0078=378inches. Allow for the thickness of the insulation. Now it's easy to arrive at the wire length used to form any thickness of coil. Just multiply by the number of turns per layer. Caution, when winding this or any coil, it is more important to get the number of turns consistent than the wire length, but good technique should give fairly close tolerance on length.
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regards Andrew T. 
27th May 2007, 02:32 PM  #8 
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA

Well I'm curious. It appears you want to make a "pancake" coil similar to what they used on ancient radios. They used to interweave them on radial spokes. Is this for a crossover, or something completely different? IMO, nobody makes these because they're so difficult, and I don't know if the properties are better or worse or the same as the usual multilayer design, but it should be really interesting to find out.

27th May 2007, 02:53 PM  #9 
Warp Engineer
On Holiday

I would imagine that they are a less perfect inductor than the standard multilayer type.
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 Dan 
27th May 2007, 03:58 PM  #10 
diyAudio Member
Join Date: Jul 2003
Location: Newcastle, Australia

See the pic for some numbers.

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