spiral pi coil

[6666]

Hi 6666,
I was wondering what type of coil this is and wire type is used in the construction if I could ask?
Shielded?
Thanks

Hi clogged up with a cold will try and reply when my head stops spinning

[green]

Hi godigit, interested in what you learn with your experiments. Other than speed(SRF) I don't see much difference if inductance, size and Tx profile are the same with the things I've tried. Haven't tried a 3DSS. Been using figure8 Rx with a Tx surrounding Rx. Less noise and reduces ground signal are the main reasons.

[godigit1]

For what its worth my professional made small gold detecting coils that detect .06 and .1 gram nuggets are very low resistance .6-.7 -.8 ohm,
that would help to discharge the coil quickly.

Hi 6666,
I was wondering what type of coil this is and wire type is used in the construction if I could ask?
Shielded?
Thanks

[godigit1]

I have thought about winding the coil with 4 or 5 strands of AWG32 or AWG34 also. I've been using AWG28 because I bought a large spool surplus long time ago. Four strands of AWG34 would be the same as one strand of the AWG28. AWG28 has a time constant of about .3us, not sure four strands of AWG34 would be that much better than one strand of AWG28.

Why wind a spiral coil? The only reason I can think of is fast coil with magnet wire. I have read you can wind a fast bundle coil or 3DSS coil with Teflon coated wire but not magnet wire, haven't tried it. A fast coil is needed for short time constant targets, less than 5us maybe 2us? 6666 replied detecting .06(17/gram) and .1gram(10/gram) nuggets reply#37. I tested some 8 mesh nuggets(7/gram), time constant varied between 1 and 2us. Thinking 6666's nuggets are less than 1us time constant.

Coil has resistance, inductance, self resonant frequency and size. Anything else? How low resistance do we need? Made a chart for fast and slow circuit. Lead capacitance is at least 1pf/inch, 30pf minimum. Fast circuit needs MUR460 diode in series with coil. Slow circuit probably works for coin targets including a US nickel(TC=10usec).

Some thoughts, Any other thoughts or corrections?

I was thinking that the multiple strands just 4 to 6 at 30 awg max for complete penetration of skin effect would be a good compromise between solid or litz wires like the 46/42. I think all the extra strands are not needed at our frequency ranges.
I think all the extra strands may add more innerwire capacitance than a minimal wire bundle.
32 or 34 as you say may be better, I picked up some 30 for real cheap and some 32 when it was on sale at frys so I figured why not.
By the way Jamco has 30 in 850 foot rolls for 12 bucks of 75 foot for $2.

I think Ive seen on the forum where members have done a 3dss out of magnet wire. Im sure it would work but maybe not perform as well as a wire with thicker insulation. My first 3dss was 30 awg but kynar and even with that I added a layer of teflon tape between wraps for extra spacing.

Im thinking another important factor in a fast coil is form material.
the perfect form is no form so thats where a bundle gets it benefit as it is its own form but the extra capacitance slows it down a bit.
Ive tried making formless coils with just foam. now im thinking that is still the way to go but with different types of foams.
Not only for their dialectric values but for their polar poperties.
This article covers it very briefly but gives a good explenation of the electrical properties of the polymers.

https://passive-components.eu/what-is-dielectric-constant-of-plastic-materials/

Under polar plactics Vs non polar, short and sweet
Looking at this Ive come to the conclusion we dont want pvc or abs as a form as in most the 3dss coils Ive made. It is good for the shells but the form should be non polar for best dialectric performance.
An example of this in professional manyfacturing would be the big foot coil a, A bundle coil arrangement held to shape and coushioned with a nonpolar plastic. (polyethalene) by the way when expanded closed cell has a better base dialectric than teflon. And then the big foot is packaged in a polar plastic PVC. I think there was more to material selection going on here than meets the eye as well a manufacturing processes like dipping and baking the coils.
I think this is why pro coils are hard to beat. all the best materials enginerred by the best minds with the best production methods.

Resistance in the coil is a big question for me. Im still trying to fully understand, I see allot of the pro coils with very low resistances.
But for the coil to charge fast you want a certain amount of resistance ? I think ?
So by playing with multiple strands I thought I would maybe be able to find a sweet spot. Plus wanted to try some twisted pair stuff.
See how it all effects the Srf of single repeatable coil design.
Still trying to find a design i want to use. Leaning toward a figure 8 as its tests well indoors.

[green]

Hi Green,
Very interesting information.
I was just starting to wind a combination spiral bundle coil and the spiral portion I had designed has 1/8 spacing between each wrap.
I was thinking this might help resonance through less capacitance. Looks like that may not be the case.

This coil im winding is kinda a bear Im on my second form and just redrew a new pattern last night. Now Im thinking a complete redesign is in order before I actually build.
one of the layouts to give you a idea of what im talking about.[ATTACH]46892[/ATTACH]


I have been thinking as you said the single wire magnet wire spiral coil could be very fast but also thinking a small # of strands bundled to reduce resistance. Ie 3 to 4 strands 30 or 32 awg. Just enough to take of the full benefit of bundling the wire .
#30 awg
1 strand .338557 Ohm /m
2 - .16928
3- .11285
4- .084646 = comparable 24 awg
5- .067706
6- .056425
7-. .04836
8- .04325
After the 4th strand the major benefit to reduction of resistance per strand falls off compared to gain in wire diameter for the 30 awg wire.
A estimated coil of 300uh single strand 30 awg aprox 50 feet of wire = 5.15ohms
A 4 wire bundle the same length =1.29 ohms
8 wire =.659
So that being said where do we loose the benefit of resistance reduction to the larger diameter wire and skin effect?

A coilteck joey dd has
498uh tx with 5.4 ohms
And a 285uh rx with .4 ohms.
this coil is not a litz wire coil at least i dont think so. so It looks like the wire diameter would have to be somewhere around 20 awg to get that value resistance. Im sure its bundle wound.

Thanks Again

I have thought about winding the coil with 4 or 5 strands of AWG32 or AWG34 also. I've been using AWG28 because I bought a large spool surplus long time ago. Four strands of AWG34 would be the same as one strand of the AWG28. AWG28 has a time constant of about .3us, not sure four strands of AWG34 would be that much better than one strand of AWG28.

Why wind a spiral coil? The only reason I can think of is fast coil with magnet wire. I have read you can wind a fast bundle coil or 3DSS coil with Teflon coated wire but not magnet wire, haven't tried it. A fast coil is needed for short time constant targets, less than 5us maybe 2us? 6666 replied detecting .06(17/gram) and .1gram(10/gram) nuggets reply#37. I tested some 8 mesh nuggets(7/gram), time constant varied between 1 and 2us. Thinking 6666's nuggets are less than 1us time constant.

Coil has resistance, inductance, self resonant frequency and size. Anything else? How low resistance do we need? Made a chart for fast and slow circuit. Lead capacitance is at least 1pf/inch, 30pf minimum. Fast circuit needs MUR460 diode in series with coil. Slow circuit probably works for coin targets including a US nickel(TC=10usec).

Some thoughts, Any other thoughts or corrections?

[godigit1]

Hi Godigit1,

I'm pretty sure you have the TX and RX figures back to front. TX should be 285uH & 0.4ohms and RX should be 498 uH & 5.4 ohms.

AuTitch

My Bad, been a while since Ive hooked that coil up.
Thanks

[autitch]

Hi Green,
Very interesting information.
I was just starting to wind a combination spiral bundle coil and the spiral portion I had designed has 1/8 spacing between each wrap.
I was thinking this might help resonance through less capacitance. Looks like that may not be the case.

This coil im winding is kinda a bear Im on my second form and just redrew a new pattern last night. Now Im thinking a complete redesign is in order before I actually build.
one of the layouts to give you a idea of what im talking about.[ATTACH]46892[/ATTACH]


I have been thinking as you said the single wire magnet wire spiral coil could be very fast but also thinking a small # of strands bundled to reduce resistance. Ie 3 to 4 strands 30 or 32 awg. Just enough to take of the full benefit of bundling the wire .
#30 awg
1 strand .338557 Ohm /m
2 - .16928
3- .11285
4- .084646 = comparable 24 awg
5- .067706
6- .056425
7-. .04836
8- .04325
After the 4th strand the major benefit to reduction of resistance per strand falls off compared to gain in wire diameter for the 30 awg wire.
A estimated coil of 300uh single strand 30 awg aprox 50 feet of wire = 5.15ohms
A 4 wire bundle the same length =1.29 ohms
8 wire =.659
So that being said where do we loose the benefit of resistance reduction to the larger diameter wire and skin effect?

A coilteck joey dd has
498uh tx with 5.4 ohms
And a 285uh rx with .4 ohms.
this coil is not a litz wire coil at least i dont think so. so It looks like the wire diameter would have to be somewhere around 20 awg to get that value resistance. Im sure its bundle wound.

Thanks Again

Hi Godigit1,

I'm pretty sure you have the TX and RX figures back to front. TX should be 285uH & 0.4ohms and RX should be 498 uH & 5.4 ohms.

AuTitch

[godigit1]

Couple tests with spiral and bundle coil reply #26. Some thoughts, maybe correct maybe not. Spiral coil is best wound with zero spacing between wraps. (1.5 inch foil target)bundle coil has higher signal at same delay time because of higher inductance for same number of turns. Spiral coil has higher circuit resonance allowing less delay, higher signal if sampled sooner. Bundle wound with Teflon coating wound have higher circuit resonance, thicker insulation calculates inductance similar to spiral coil. Thinking magnet wire spiral coil would be similar to Teflon bundle coil. (copper plate target)shows increase in needed delay if MUR460 is eliminated, less than 5us with and little over 10us without. Signal same with and without MUR460 because Tx current is in a control loop(160us, constant rate, 1A peak). Two stage amplifier, gain of 15/stage for total of 225. Still wondering why professional coil might be better? Spice simulation seems to be similar to comparison test.

Used 1inch foil target(TC=1.5us) instead of 16mm square foil(TC=1us)spice model to get a higher signal for testing. Square target cut from regular strength aluminum foil(.063us TC/mm)*number of layers. About 16mm for 1us target. Square target cut from aluminum coke can side(.225us TC/mm). About 4.5mm for 1us target.

Hi Green,
Very interesting information.
I was just starting to wind a combination spiral bundle coil and the spiral portion I had designed has 1/8 spacing between each wrap.
I was thinking this might help resonance through less capacitance. Looks like that may not be the case.

This coil im winding is kinda a bear Im on my second form and just redrew a new pattern last night. Now Im thinking a complete redesign is in order before I actually build.
one of the layouts to give you a idea of what im talking about.


I have been thinking as you said the single wire magnet wire spiral coil could be very fast but also thinking a small # of strands bundled to reduce resistance. Ie 3 to 4 strands 30 or 32 awg. Just enough to take of the full benefit of bundling the wire .
#30 awg
1 strand .338557 Ohm /m
2 - .16928
3- .11285
4- .084646 = comparable 24 awg
5- .067706
6- .056425
7-. .04836
8- .04325
After the 4th strand the major benefit to reduction of resistance per strand falls off compared to gain in wire diameter for the 30 awg wire.
A estimated coil of 300uh single strand 30 awg aprox 50 feet of wire = 5.15ohms
A 4 wire bundle the same length =1.29 ohms
8 wire =.659
So that being said where do we loose the benefit of resistance reduction to the larger diameter wire and skin effect?

A coilteck joey dd has
498uh tx with 5.4 ohms
And a 285uh rx with .4 ohms.
this coil is not a litz wire coil at least i dont think so. so It looks like the wire diameter would have to be somewhere around 20 awg to get that value resistance. Im sure its bundle wound.

Thanks Again

[green]

For what its worth my professional made small gold detecting coils that detect .06 and .1 gram nuggets are very low resistance .6-.7 -.8 ohm,
that would help to discharge the coil quickly.

My thought. Tx resistance doesn't effect coil discharge time. Low resistance means higher current with the same inductance and more efficient if coil energy is dumped back into the battery. Any other reasons to use low resistance Tx?

[6666]

thanks bruce, still no chip delivered, but i know how long post can take from there to here, my grandma's sister(aunt emily) used to live in Adelaide and every year she sent me 25 dollars in a xmas card, and it never came till new year lol.
in fact the only year i got it at christmas was 1987 and they came home for xmas that year!.
it will turn up.

The chip hopefully will arrive soon, the post can be very slow sometimes

[sinclairuser]

This one ?[ATTACH]46882[/ATTACH]

thanks bruce, still no chip delivered, but i know how long post can take from there to here, my grandma's sister(aunt emily) used to live in Adelaide and every year she sent me 25 dollars in a xmas card, and it never came till new year lol.
in fact the only year i got it at christmas was 1987 and they came home for xmas that year!.
it will turn up.

[6666]

Still wondering why professional coil might be better?

For what its worth my professional made small gold detecting coils that detect .06 and .1 gram nuggets are very low resistance .6-.7 -.8 ohm,
that would help to discharge the coil quickly.

[6666]

thanks for your tests green, i have started winding a basket coil for testing, the flat spiral that was in the photo earlier in the thread did not have a big enough hole in the centre(thanks carl and 6666), once i unwound some of it and wound those turns on the outside of the coil i got usable inductance and ohm readings.

does anyone have an english coil build guide for the chance type basket coil?, i found one in russian and a few scraps nothing definitive.
thanks,
aly.

This one ?

[6666]

Good info Green, thanks

[sinclairuser]

thanks for your tests green, i have started winding a basket coil for testing, the flat spiral that was in the photo earlier in the thread did not have a big enough hole in the centre(thanks carl and 6666), once i unwound some of it and wound those turns on the outside of the coil i got usable inductance and ohm readings.

does anyone have an english coil build guide for the chance type basket coil?, i found one in russian and a few scraps nothing definitive.
thanks,
aly.