Is there an inductance formula for DD coil, is there a "Wheeler" equivalent?

[majesus]

Hi,


I'm building my first E-trac coil. I took some measurements of the Minelab stock coil with my RLC meter. Here is what I got when measuring the coil's output pins 1 thru 5:

1. pins 1 & 2 Resistance = 1.1ohm, Inductance = .54mh
2. Pins 3 & 4 Resistance = 5.8ohm, Inductance .69mH
3. Pin 5 Resistance = open with all other pins (drain pin)

I'm using Barry's Inductor Simulation to determine what guage of Magnet wire to use.
http://www.coilgun.info/mark2/inductorsim.htm

However I want to build a D-coil. How to componsate for the change in geometer from a perfect circular shaped inductor to a D shape coil? Is the a Wheeler's equilavent formula to model the following parameters (X,Y,Z) as shown in the attachment?




Thanks,
Majesus

[porkluvr]

I once asked the same question. The answer I got was something to the effect, measure the overall length of wire in one winding of one D section and substitute that number for circumference in the formula:

diameter = circumference/pi.

Then, plug diameter into your favorite coil calculator. (Why didn't I think of that??)

It might be simplified but you will be in the ballpark. Somebody with more time and motivation than I could maybe work out a useful 'k' factor to apply to the result.

[majesus]

Simple enough.

I can model the coil in Autocad to see how I like the inner geometer and then determine the circumference from it.

Then use the following calc to get my AWG and outer dimensions.
http://www.circuits.dk/calculator_mu...er_aircore.htm

Gives me a starting point.
Thanks

[Skippy]

I did some experiments along these lines last year. I made a circular coil, measured its inductance, and then re-shaped it to a rectangle, with approx 2:1 aspect ratio. The coil L was remeasured.
Details were: 100 turns of 0.25mm diam ECW.
Circular coil, diameter 48mm: L =980 microhenry.
Reshaped coil, 55mm x 28mm: L = 880 microhenry.
Note: the rectangle shape has rounded corners, bend radius approx 6mm.
So L is reduced by 10 percent in this case. I think the reduction for a D coil will be less, probably 5 to 8 percent.
Hope this helps.

[Skippy]

I've just thought some more about this, and have worked out the theoretical size of a D coil made from my example coil (above). It's actually very similar to my rectangular coil, so maybe an L reduction of about 10 percent is a fair approximation when going from a circular to D shape with the same circumference.
Maybe you could wind a crude test coil with less turns, for example, on your former, then measure its L and use this data to guide your final coil design.

[majesus]

Now that I can ball park the AWG I need, I was going to wind it, strip just a little of the insulation to measure while on the JIG. If I need more turns, revarnish the nick and keep winding.

5 to 10% from a circular coil to a 2:1 D coil. Hmmm, I'll take some measurements once I CAD the geometery and compute the diameter as per porkluvr's suggestion and calc the inductance using an inductor calc.

See if the inductance is 5 to 10% different.

[mikebg]

Re: Minelab stock coil

Majesus, do you have a signal generator and oscilloscope to measure the resonant frequencies of windings in your Minelab stock coil? What is the overall diameter and weight of Minelab coil?

[Skippy]

The other thing you could do, of course, is when you wind your coil, bring out a couple of taps near the end. This will allow you to fine-tune after winding and varnishing,etc. Example wind with 40+2+2, this allows 'ideal' +/- 10%. If you isolated the extra winds, you could join them back in reverse, ie. winding 40 & 2+2 would allow 36,38,40,42,44 T coil. Hope this helps.