A couple coil experiments What do you guys think?

[baum7154]

Hi Dan,
I Picked up a micrometer and the can squares measured .0040 for the pelegrino can and .0050 for the coke can. Still a bit thicker than the ones you measured. So I will stick with the pelegrino one for now. Both squares came from the middle third area of the cans.

I do have a scope and adjusted the damping by switching out resistors.
The coil I set it to was 300 uh and all the other coils are very close so I have not adjusted between coils.
But that's a good point when it came to the 285 uh shielded one I did not dial it in so it may do a bit better. Every time I make a adjustable set up It blows the pot as soon as I go to adjust it.
I just got some more 10 k pots to try again.

Thanks Again guys for all the comments!

e
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Hi Godigit,
Did you get your micrometer from Harbor Freight? They do have some pretty good deals. Just curious if you have checked its accuracy against a feeler gauge because sometimes they need to be zero adjusted by rotating their barrel scale. Just a thought cause I've never seen a .005" sidewall midpoint.

Regarding the critical damping, it is determined by inductance and capacitance or in other words Self Resonant Frequency of the coil, feed line, and detector. The formula PI X SRF X uh = Damping Resistance. This will get you in the ball park for the coil/ feed only. What all this says is that even though three 300uh coils made with different materials or shapes will not use the same damping resistor. So you should damp each separately to optimize performance.

Good Luck,

Dan

[Qiaozhi]

Hi Dan,
I Picked up a micrometer and the can squares measured .0040 for the pelegrino can and .0050 for the coke can. Still a bit thicker than the ones you measured. So I will stick with the pelegrino one for now. Both squares came from the middle third area of the cans.

I do have a scope and adjusted the damping by switching out resistors.
The coil I set it to was 300 uh and all the other coils are very close so I have not adjusted between coils.
But that's a good point when it came to the 285 uh shielded one I did not dial it in so it may do a bit better. Every time I make a adjustable set up It blows the pot as soon as I go to adjust it.
I just got some more 10 k pots to try again.

Thanks Again guys for all the comments!

Are the values of 0.040 and 0.050 in inches or millimetres?
The thickness of the square I cut from a Schweppes Tonic can measures as 0.1mm (0.004").

[godigit1]

Hi Dan,
I Picked up a micrometer and the can squares measured .0040 for the pelegrino can and .0050 for the coke can. Still a bit thicker than the ones you measured. So I will stick with the pelegrino one for now. Both squares came from the middle third area of the cans.

I do have a scope and adjusted the damping by switching out resistors.
The coil I set it to was 300 uh and all the other coils are very close so I have not adjusted between coils.
But that's a good point when it came to the 285 uh shielded one I did not dial it in so it may do a bit better. Every time I make a adjustable set up It blows the pot as soon as I go to adjust it.
I just got some more 10 k pots to try again.

Thanks Again guys for all the comments!

[baum7154]

Hi Godigit,

The 1/4" square of AL can sidewall is indeed a tough little target and a good test for a gold detector. I'm surprised that the cola can sidewall was visually thicker. All the cans I have measured, from the cheapest generic soda water to the well known brands of beer and soda, have sidewalls that are from .0035" to .0042" thickness. In the production of these cans those dimensions are carefully maintained. Perhaps we should specify the middle of the can sidewall to get a bit more thickness consistency. The top and bottom ends of the sidewall are typically thicker than the midpoint of the sidewall. Do you have access to a micrometer or a digital caliper? I'd be curious how thick that cola can sample is.

The top and bottom of any AL can is 2 to 3 times thicker than the side wall and, when used as a target, have PI responses that are about 10 times stronger than the sidewall. Thickness is a key factor is detection of these small targets.

Setting Critical Damping makes a huge difference in a coils ability to detect fast decaying targets. Large, slow decaying targets are much less affected by coil ringing but for a faster target like small gold nuggets it is most important to effectively damp the coil. Do you have access to an oscilloscope? It is required to see the coil ringing and to set the damping.

In my experience a coil without a shield is always faster than when any shield is applied because the shield adds capacitance. For me the beauty of the 3DSS winding is the elimination of the additional shield. Many other coils such as a spiral wound are are extremely fast when unshielded. The problem then is to apply an effective shield across and around that broad winding geometry, with minimum added capacitance that we all know destroys coil speed. This problem is the same for all other two dimensionally wound coils such as basket weave, spider wound, etc. but in varying degrees.

Happy building

Dan

[godigit1]

Hi Dan,
I had a little time last night to test a 1/4 square of can but I did not have a coke can so I used a pelegrino soda can, today I cut a cola can and it was visually thicker.

I used my TDI pro to test all 4 of the coils I made so far (3 x 8" and a 8" x10") and a 8 inch 3D SS. All the coils have a inductance right at 300 except the last one I built. ( the one with the wrap). It came out a little lower, 285 I think maybe the toothpick wide space between the inner 10 wraps lowered it a bit. All the coils are 30 wraps. all 26 awg except one 8 inch 24 awg The 3d SS and the 8x10 7 strand the rest are solid.

I'm able to detect the can piece form 1 1/2" to 2 1/2 with just the right sweep.
I had the TDi set 10 us at 50% gain, to start with the GB off for all the coils. (cant turn it on indoors)
In my hand and holding the foil flat I could barely get it with any of the coils, would not really call it a signal. I placed it on a piece paper and it was a nice little hit.

Of all the coils they were close but the 3DSS trumped as I was able to get the gain/ threshold a bit higher it ran a little quieter then the non shielded coils in my bad test environment where I am now.
To my surprise the coil I did the 3 d wrap on was a little quieter as well, It Appears to be shielded some. Maybe Ill go ahead and try one in the flat version and straighten the tracks a little.

I still have not been able to get any of the coils over real dirt yet and away from all the emi they may work fine not shielded. Maybe not.
Really the 3DSS is a hard coil to beat its a great performer for a home built coil.
But Ill keep trying.

Take it easy.

[baum7154]

Hi

Ok I finally was able to finish the 3D wrap around the vertical wrap coil.
I went 20 wraps then inserted the 3D form between the wraps.

[ATTACH]36080[/ATTACH][ATTACH]36081[/ATTACH]

Then I went 10 wraps of the 3DSS coil wrap around it. I used two forms and put 5 wraps on each so as to avoid the corners overlapping.
This is the same wind as a3DSS but it comes out more like a braided cable look. (A new coil in itself)

[ATTACH]36082[/ATTACH][ATTACH]36083[/ATTACH]

[ATTACH]36084[/ATTACH]

While it came out ok There is probably not enough wraps to call it a effective shield. I think doing any wore wraps negates the benefits of the vertical wrap.
It measured 285 uh at 2.9 ohms .

I probably will not try another of this style as it does not perform as well as the original or my coils. My coils are getting a good 2 extra inches over the 3D design as tested on a barracuda indoors. What was 9 to ten inches is now 12 to 13 inches on a nickel.

Though the coil shows no sensativity to my hand at 7.2 us I will try it on my MPP at 5 us soon, I bought another MPP for the bench as I don't want to keep removing my coil from the one I have.

I will look into shielding methods for my wrap that hopefully will not negate the extra depth.
That is if its even needed. I have to get the coils outdoors and see how they do over ground.

Any advice on how to check if a shield is required other then just the hand thing would be greatly appreciated.

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Hi Godigit,

Did you perform critical damping on each of your coils tested? Are each of the coils within about 25uh of each other? Are each of your coils round and the same diameter? Did you use the same sample delay, pulse rate, and other settings on all of the tested coils? A nickel is a fairly big target. Could you test the coils with a 1/4" square of the sidewall of a soda can? If that does not work a 3/8" square is an easier target.

Good luck,

Dan

[Davor]

Most probably shielding is adequate. Even in full 3DSS only the last few turns count as shield.

[godigit1]

Hi

Ok I finally was able to finish the 3D wrap around the vertical wrap coil.
I went 20 wraps then inserted the 3D form between the wraps.



Then I went 10 wraps of the 3DSS coil wrap around it. I used two forms and put 5 wraps on each so as to avoid the corners overlapping.
This is the same wind as a3DSS but it comes out more like a braided cable look. (A new coil in itself)





While it came out ok There is probably not enough wraps to call it a effective shield. I think doing any wore wraps negates the benefits of the vertical wrap.
It measured 285 uh at 2.9 ohms .

I probably will not try another of this style as it does not perform as well as the original or my coils. My coils are getting a good 2 extra inches over the 3D design as tested on a barracuda indoors. What was 9 to ten inches is now 12 to 13 inches on a nickel.

Though the coil shows no sensativity to my hand at 7.2 us I will try it on my MPP at 5 us soon, I bought another MPP for the bench as I don't want to keep removing my coil from the one I have.

I will look into shielding methods for my wrap that hopefully will not negate the extra depth.
That is if its even needed. I have to get the coils outdoors and see how they do over ground.

Any advice on how to check if a shield is required other then just the hand thing would be greatly appreciated.

[aeberbach]

Wow, great-looking coil.

[Davor]

Nice

[WM6]

It is about homemade plastic "U" profile for experimental (mostly PI) coils. You can get free sample (coil sample too).

To retain low weight coils are wound using Alu wire - of course all is about prototype coils and not shiny commercial design.

Plastic channel for electrical installations is used, cut alongside, epoxy glued together and cut again in pieces needed:





Other materials can be used too in some cases, as par example plastic roof gutters:

[Davor]

Mine is zig-zag (somewhat "basket"-like) wound inside (zig-zag wound turns inside, act as coil former for final two way /sparsely/ wound spiral self shielding turns).

"Plastic profile" to holding lower rod are in fact impregnated wooden Chinese chopsticks ("Lidl" surplus) finally epoxied and painted.

I'm interested in this particular plastic thing:

[godigit1]

Hi Dan,
I should have included the bundle size before the 24 awg coil with 30 wraps is 3/4" high by 5/8. the 26 awg is 3/4 high by 1/2 wide.

Since lunch I have had some time to think about it and I have a way to easily form both coils together.
Ill go my wind for a air core and 3D SS wrap to top it off as suggested .
I have got some more of the 26 awg on hand so Ill use that for starters.
Give me a little time to put it all together should be fun.

Thanks again everyone.

[baum7154]

Hi guys Thanks for the positive comments.
Hi Dan,
On the first example I have the wrap where negative is on both sides of the coil and winds toward the center. maybe doing this method and putting the Positive lead directly in the center may be better instead of under only five wraps. What do you think maybe a shielding ???

I have Graphite shielding and will also make a shielded housing to see if I loose performance.

I would be willing to make and send someone better capable than myself a coil for proper full testing As well as one with 23 wraps for Green to add to your chart if your out there and interested.
Of coarse you could keep them when done.

Thanks Again for all your help.
Digit

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I think if you are not encircling the coil windings with the ground end of the winding, much like winding a toroid core, the self shielding won't be effective. The other thing to remember is to keep the width of the coil bundle compact so as not to spread out the shield over too great of an area leaving bigger holes in it. In other words I think a compact, nearly circular cross section of the air spaced coil is best. I try to keep my 3DSS coils within about 1.1" cross section.

Regarding graphite shielding I do have a 3DSS coil wound with enamel sold wire that is also graphite shielded at about 1300 to 1500 ohms /inch and it suffers from loss of speed and sensitivity. Probably the fault of the solid enamel wire. I would not graphite shield the 3DSS coil unless it is absolutely necessary...try it out first.

Dan

[WM6]

Mine is zig-zag (somewhat "basket"-like) wound inside (zig-zag wound turns inside, act as coil former for final two way /sparsely/ wound spiral self shielding turns).

"Plastic profile" to holding lower rod are in fact impregnated wooden Chinese chopsticks ("Lidl" surplus) finally epoxied and painted.