Single layer Spiral coil

[mario]

Hi, is there a right approach to winding spiral coils. In my winding, there's an area where the wire entering the winding overlaps with the spiral body. And I can't figure out a way to make the wire not overlap the spiral body. 1) is this area a problem? won't the detector constantly be set off since it's metal wire. and this over lapping area is in the coils field of view?

2) whats the proper way of winding spiral coils?

3) are spiral coils that take up the whole coil diameter deeper and sensitive, than traditional "ring" coils? My Minelab sucks when it comes to the really small gold, so I'm trying to know if spiral coils are the way to go. I'm hoping to get at least 9" depth for a BB size object.

By the way, I'm working on a 8 inch coil for my Mine lab SD 2100v2. using spiral coil calculator here, I was able to target a 310uH Inductance using 45 turns. I know I need to target .5 Ohms. Also, I'm using single threaded Tin plated 20 AWG copper wire. I couldn't find any teflon insulated wire so the insulation of this wire is PVC. The full specs of the wire are found here. I tried finding multi stranded tin plated wire but was not successfull at 20 AWG. I read that when making small coils you need tin plated stranded wire because the eddy webs are much stronger and therefore the coil much sensitive.

Can any one please answer me the questions above?

Thanks in advance.

[IBGold]

Hi I would try tinned copper with polyolefin insulation 32/.2mm strands (Marine Wire) that is what I used to wind mine start from the inside and wind out using super glue to hold turns together you could also use 660/46 Litz wire which is available on EBay in your country probably your best option I had no problem with the outer strand crossing the coil although I did space it 5 mm from the coil as long as the strands are .2mm or less the coil will not see eddy currents in the wire shielding is the tricky bit.
Regards, Ian.

[WesP]

See if this is what your looking for. Gary can wind you a custom coil just about any size.

http://www3.telus.net/chemelec/Proje...g/Coil-Jig.htm

[porkluvr]

Just make your connection to the coil as direct as possible. There is no use tearing your hair out over that.

I used the calculator from the URL, and also the flat spiral coil calculator in MiscEl. Using numbers from your datasheet, both calculators agreed that you could only get about 250uH with 45 turns and an outer diameter of 8". One of us probably used incorrect wire overall diameter od.

But I am not here to argue the accuracy or inaccuracy of your coil calculator or mine or the methodology.
Here is my issue:

The choice of stranded vs solid and the choice of tin vs silvered wire, are relatively minor points compared to the choice of PVC insulation vs Teflon. This is because of PVC having about 1.5x (or more) greater Dielectric Constant. Your wire datasheet does not list DC, but it will likely be 3 or more. The DC of Teflon comes in at 2.0.

If you go to the trouble of hauling a heavy, large gauge coil, you should not waste the effort by using PVC insulation material. It might not matter for other types of targets, but looking for BB sized nuggets you must avoid adding unnecessary interwinding capacitance. Using PVC is wire is shooting yourself in the foot. Some people say that winding a spiral coil might be inferior to winding a bundle, but I'm not going there.

I wish they had a better selection but Surplus Sales of Nebraska lists solid 20AWG Teflon insulated wire on their website. Solid, silvered, but good enough.

I would prioritize the wire's insulation material over whether or it is silvered or stranded but that is just my opinion.

[mario]

Thank you for the suggestions. I appreciate the attention given to shielding. I have to agree with the issue of tinned plated vs. silver plated wire being minor to the thicker pvc shielding. I purchased the wire assuming that because it had a .41mm thickness
(compared to the listed .254mm teflon), the thin pvc shielding would make the wire have a similar dielectric constant. However, it doesnt, so I shouldnt even consider using pvc. I'm going to see if i get the litz 660/46 or the silver plated 20 awg at surplus of Nebraska. Having said that, what are your thoughts on using pvc shielded wire on large coils? I would prefer not having to waste this fine wire. Should I avoid pvc altogether?
Last question, what about using the flat spiral design on large coils 16inch+? Has any one experimented and obtained results regarding advantages/disadvantages in depth/sensitivity?

[porkluvr]

I like IBGold's suggestion for polyolefin wire. That has got to be cheaper and easier to glue than Teflon. It also has a low DC, although not quite as low as Teflon's. But polyolefin insulation can be "puffed" to make DC lower than Teflon.

I had some time to to ponder the discrepancy between the inductance values you and I calculated with the wire you specified.

With the calculator you used, I believe you must take insulation thickness, and double that number to obtain minimum wire spacing. If you make a calculation based on having only a single insulation thickness, your measured result will be much lower than what you calculated.

The MiscEl spiral calculator has fields for entering wire od and copper od and takes into account insulation walls on both sides of the conductor.

You can possibly use PVC wire if you intend to make a large loop for relic hunting where a fast response is not necessary. You might want to make some tests with the wires tied together but not potted to check this out.

Last question, what about using the flat spiral design on large coils 16inch+? Has any one experimented and obtained results regarding advantages/disadvantages in depth/sensitivity?

This one is still open. Somebody?

[mario]

I like IBGold's suggestion for polyolefin wire. That has got to be cheaper and easier to glue than Teflon. It also has a low DC, although not quite as low as Teflon's. But polyolefin insulation can be "puffed" to make DC lower than Teflon.

I had some time to to ponder the discrepancy between the inductance values you and I calculated with the wire you specified.

With the calculator you used, I believe you must take insulation thickness, and double that number to obtain minimum wire spacing. If you make a calculation based on having only a single insulation thickness, your measured result will be much lower than what you calculated.

The MiscEl spiral calculator has fields for entering wire od and copper od and takes into account insulation walls on both sides of the conductor.

You can possibly use PVC wire if you intend to make a large loop for relic hunting where a fast response is not necessary. You might want to make some tests with the wires tied together but not potted to check this out.



This one is still open. Somebody?

The discrepancy is largely due to the spacing as you originally suggested. I entered a generous number, taking under consideration an imperfect scenario. Anyway, that point is now mute. I did some calculations and realised using litz 660/46 wire (18 AWG equivalent - please correct me if im wrong about it being 18 AWG) will get me very close to the .300 uH, .500 OHM target using coils up to 18 inches (mainly coil bundles, not spirals).

Here is an interesting question i have yet to see posted: For discrepancies in ohms between your coil and target coil, is it ok to use a resistor to bring your coil up to specification? will it degrade coil performance? Idont think any body has posted anything about the use of resistors in regards to spec compliance and their effect on performance.

[chemelec]

Just a Few Comments:

Due to Insulation Thickness, Making a Coil with PVC Coated wire will become Quite Large and Heavy to get the required inductance.

Same goes for Teflon coated wire, but Teflon coated wire will be a faster coil than the PVC. For Better Bonding it is Easier if the Teflon is Acid Etched beforehand.
Litz wire will also make a Fast Coil, but again it requires more turns due to insulation thickness.

I Find that Soild 18 AWG adds a considerable amount of Interwire Capacitance, as compared to 20 or 22 AWG.

A Typical Coil that I wound with 22 AWG Enamelled wire is 6" ID and 8" OD.

The 22 AWG was Calculated as .025" and spacing of .002" to allow for a tiny gap and insulation thickness.
Calculated inductance was 408.223 uH.
Actual Measured inductance is 408.8 uH.
Pretty Darn Close to Calculated value.
(DC Resistance is 1.07 Ohms, and Free Air Resonance is 1.88 Mhz)

Kinda Wish I had some 22 AWG Enameled "Silver coated" wire to see how well it would work out, as a comparison.
Should be LOWER Resistance!

[IBGold]

Hi all,
The silver plated wire is no good in Minelab PI machines as the wire is seen as a solid wire and will support eddy currents I made this mistake when I first started making coils and bought a large amount of 16 AWG and 20 AWG silver Teflon stranded wire and it was no good so if someone wants some it is sitting here wasting space only trouble is I am in S.Australia, the enameled silver wire would be great but 17 AWG would be optimum let me know if you find some.
Yes the 660/46 is close to 18 AWG but for correct inductance and usable resistance OK for coils to about 14" max.
Regards, Ian.

[mikebg]

Gary's winding form

Improvement of Gary's form. Eight more openings are made to glue more surface at winding large coils. They are smaller as seen in the attached image.

[chemelec]

Improvement of Gary's form. Eight more openings are made to glue more surface at winding large coils. They are smaller as seen in the attached image.

That Aluminum Plate WAS one of my Earlier Jigs.
But it WARPS too Much, Especially as you wind larger coils.
I would think that your Extra Grooves will also cause MORE Warpage.

My Newer Jig I now use is made of 2 Plates of 3/4 Inch Lexan, 14 1/2 inches in Dia. and it will wind a coil Up to 13 inch Dia.
And it uses Special Designed Center Piece to offset warping, So No need for Extra Grooves.

I use My Mill Machine to hold this Jig, and thats the Biggest diameter it can hold.

But this is OK, As Shipping Coils larger than 13 Inches becomes a Problem anyway.

[mikebg]

Gary, I designed a so complex form for winding flat spiral, that it's even hard to describe the structure and procedures in use.

The main metallic disc here is denoted as position 1. He did not warp because the metal is thick and there are not any slots, only small openings for leads of wire. Position 7 is the opposite disc with less diameter because after winding, sectors 2 should be tighten with four clamps against warping. The eight sectors pos. 2 and disc 7 may not be made of metal.
Reel is formed when on disc 1 is placed disc 4 for distance because is thick as wire. Then through an opening in 1 is inserted wire lead and tighten the eight sectors pos. 2 each with a screw pos. 3 with hidden head. Then with four screws pos. 6 is tighten the opposite disc pos 7. After winding are used 4 small clamps to prevent warp of four diametrically opposite sectors, for example as indicated by numbers 1, 3, 5 and 7. Remove the four screws pos. 6 and the disc 7. Then remove the screws that hold the sectors with numbers 2, 4, 6 and 8 (which are not tightened with clamps). This opens four diametrically opposed sectors of coil to enable covering wire with very thin sheet glue. After glue hardens, removed sectors are inserted and tighten with clamps, and removed sectors 1, 3, 5 and 7.

[chemelec]

Gary, I designed a so complex form for winding flat spiral, that it's even hard to describe the structure and procedures in use.

The main metallic disc here is denoted as position 1. He did not warp because the metal is thick and there are not any slots, only small openings for leads of wire. Position 7 is the opposite disc with less diameter because after winding, sectors 2 should be tighten with four clamps against warping. The eight sectors pos. 2 and disc 7 may not be made of metal.
Reel is formed when on disc 1 is placed disc 4 for distance because is thick as wire. Then through an opening in 1 is inserted wire lead and tighten the eight sectors pos. 2 each with a screw pos. 3 with hidden head. Then with four screws pos. 6 is tighten the opposite disc pos 7. After winding are used 4 small clamps to prevent warp of four diametrically opposite sectors, for example as indicated by numbers 1, 3, 5 and 7. Remove the four screws pos. 6 and the disc 7. Then remove the screws that hold the sectors with numbers 2, 4, 6 and 8 (which are not tightened with clamps). This opens four diametrically opposed sectors of coil to enable covering wire with very thin sheet glue. After glue hardens, removed sectors are inserted and tighten with clamps, and removed sectors 1, 3, 5 and 7.

Looks like a Lot More Complicated and Slower of a procedure to wind a coil, than on my New Jig.
Sorry No Picture at present time. Have Company here from England!

[mario]

NEW UPDATE!

Ok gentlemen, I have been working on this project for a while now. I decided to take some of your advice and asked Gary to make me a coil with the requirements needed for my Minelab. He did a really great job.


As you can see from the picture above, Gary's rig is simply amazing. It was a perfect circle. Everything was on track until I received my coil housing from Hay's electronics. Unfortunately I made the assumption that the coil housing was a precise 8 inch coil housing, so I asked Gary to make a coil 2mm short of 8 inches. I therefore provided him the wrong outer diameter, which really sucked because I was not able to fit the coil inside the coil housing. When I finally got my coil housing, It turned out the coil housing was more like 4mm short of 8 inches, rendering my coil housing useless. I had two options, make a coil housing out of Resin OR make another coil meeting the coil housing specifications.

I decided to make a coil.



As you can see from the image above, my first coil was not that particularly tight. I realized that I had to come up with a set up to tighten the windings, which brings me to my next photo


I used a special U shaped vice to keep the two tile slabs together. Unfortunately, since I did not use a strong tile material such as granite, the slab broke into two pieces as I tighten the vice. I cut the tile slab with a tile cutter. For spacing I used 8 dime coins. They accommodated the wire thickness perfectly.

And here's a picture of my final coil. I used litz wire 330/46. I was able to get 301 uH, with a .9 Ohm Resistance. Unfortunately, the resistance was a little higher than I would have liked but couldn't do anything about it. I suppose I needed thicker litz...



Here's a picture of my final coil inside the housing:

[mario]

After testing the unit I realized my coil needed shielding. As I lower the coil, its very noisy. I need to shield the windings with scotch 24 tape. However, since the coil is spiral in design, scotch tape will not be the most appropriate material. Does anybody have suggestions? could I use epoxy as shielding?

I noticed Gary's coil had a thin epoxy layer. Can this layer lower the noise effect of lowering the coil?


Also, I have another question. I measured the resistance (at connector) to be 1.6 OHM. I know Minelab specifications call for .5 OHM. Is the higher Resistance bad for sensitivity?

Thanks for any suggestions