GOLD coil hunter

[Tibuck19]

Don't take KT315's [sometimes abrupt] comments personally. It's mainly an issue with the language barrier.

Just realised that myself, We sometimes forget were are all worlds apart with different ways of talking and explaining. I do believe the coil that was causing overheating on the mosfets was one of lower inductive. Honestly?your own personal option, are basket coils really worth the hassle, Judging from KT answer, I would think not, but i'm so inexperience , that it I am left with doubts on what see and read online.

[Tibuck19]

You will need an LCR meter to measure the uH.

with the spider or 3dss coil (like I pictured earlier) it has 6 layers of 5 turns for each layer, x6 = 30 turns give or take, I measure after winding more than needed then removing a layer at a time, not cutting the extra but leaving it connected in one length until I am close to the 300uH I am looking for. usually it ends up close or in a range from 300 to 315uH and around 27/30 turns depending on wire used which in my case is perfect.

edit - looks like a couple replies before I finished typing...lol...

Basically you Over do it on the Uh and cut working your way down to your target Uh. BTW all replays can help trigger thoughts loll ( reference to your edit)

[kt315]

really perfect coil. FASTEST. no polystirol, no glass. nothing. ONLY AIR.

[Qiaozhi]

I do believe the coil that was causing overheating on the mosfets was one of lower inductive. Honestly?your own personal option, are basket coils really worth the hassle, Judging from KT answer, I would think not, but i'm so inexperience , that it I am left with doubts on what see and read online.

There are no fast commercial coils that are basket weave. But that's probably due to being time consuming to produce, and consequently expensive. It's easier if you can get away with a simpler construction. Have you read this article? -> http://www.geotech1.com/cgi-bin/page...oils/index.dat

[Tibuck19]

It looks like a heating element from a glass top range AKA Cooking oven, Is this a coil you made? Correct me if I am wrong which I am sure I am, I thought a slow coil was needed to detect Gold like tiny nuggets ? Also im guessing because this is a REALLY FAST coil it does not need insulation wrappings?

[kt315]

http://www.metaldetectors101.com/surfpi-4.html

[Qiaozhi]

Correct me if I am wrong which I am sure I am, I thought a slow coil was needed to detect Gold like tiny nuggets ?

You have that wrong.
It's all to do with the time-constant of the coil (plus other parasitics that reduce the decay time). You want the current in the coil to reduce to near zero in as short a time as possible. i.e. way before the eddy currents in the target have died away. If you have a slow coil, the target response will have disappeared before you are able to sample.
Have you read ITMD? -> Inside the METAL DETECTOR - Second Edition - Published 2015

[geoscash1]

Basically you Over do it on the Uh and cut working your way down to your target Uh. BTW all replays can help trigger thoughts loll ( reference to your edit)

No, don't cut the wire, just leave the excess until you measure what you want then cut. If you remove a loop and cut and your uH is lower than you wanted then your screwed or will have to solder it back together and you don't want any solder in the coil head itself.

fast coil for gold, slow coil for larger items is the base rule of thumb, at least that's how I understand it after reading a million threads and posts here...lol

shielding reduces sensitivity to small gold but helps in mineralized ground somewhat. When the delay is set below 7/8us then it can become erratic.


It all works together in some way shape or form. A good fast coil with the proper damping resistor, board set at the right delay (low, around 6 to 8 us) , sensitivity and threshold settings will yield what you are searching for.. Gold.




edit -- Qiaozhi beat me to answer again...lol

[Tibuck19]

Ok, I get what you mean, For instance with those stainless rivets we were talking about the other day, the Eddy currents dies away faster then it would with Iron thus making harder to detect. hahahaha " Eddy currents "here I thought they were just currents in oceans the just seem to be everywhere huh! lol. So For gold that has a faster decay then most metals I will need a fast coil to catch those eddies, and smaller the nugget the faster ill need the coil to collapse to zero, did I understand that right?, No I have not read that book but im guessing it would be worth having along with a oscilloscope that I have yet to puck up.

[bbsailor]

There are no fast commercial coils that are basket weave. But that's probably due to being time consuming to produce, and consequently expensive. It's easier if you can get away with a simpler construction. Have you read this article? -> http://www.geotech1.com/cgi-bin/page...oils/index.dat

Qiaozhi and all coil builders,

Building self shielded coils this basket way does lead to higher coil self resonance which translates into a potentially faster coil only as long as the PI delay circuit allows you to low sample to the coil's full potential.

If you apply a shield to this basket weave, you will have more coil to shield capacitance and then you give up most of the benefits of the basket weave over more traditional bundle wound coils. If you bundle wind with Teflon insulation you will do much better than using PVC insulation and then if you use thicker 600 V Teflon insulation the turns will be farther apart but you may need to add an additional turn or two because coil wire farther apart produces less inductance.

My hunt for minimizing the coil to shield capacitance was to use a fine wire mesh Scotch 24 to place around the coil bundle because it has less surface area than a solid shield like a thin foil. Only apply one layer of Scotch 24 around the coil bundle. The key trick to using this wire mesh is to place it a few mm away from the coil by using a low dielectric spacer and then when wrapping the wire mesh around the coil, not to electrically form a resistive contact around the coil cross section circumference. I use electrical tape on any overlap area to prevent any electrical contact and this prevents eddy currents from being generated in the wire mesh shield. The wire mesh get attached to the coil shield or ground connection. Make sure that there is a gap near where the coil wires come out of the coil loop bundle.

Bottom line: the less coil and TX circuit capacitance the potentially faster you can sample. This translates into a higher value of damping resistor and a potential faster stimulation of very small, low TC targets.

As you get down near 5 uS delay you are seeking very small gold targets and then your next barrier to sampling any faster is the coil wire itself. You do not want to use any wire type where the wire is holding any eddy currents near the time you are sampling. Playing in dalays in this low range requires an oscilloscope to fine tune your circuits.

I hope this helps.

Joseph J. Rogowski

[geoscash1]

Ok, I get what you mean, For instance with those stainless rivets we were talking about the other day, the Eddy currents dies away faster then it would with Iron thus making harder to detect. hahahaha " Eddy currents "here I thought they were just currents in oceans the just seem to be everywhere huh! lol. So For gold that has a faster decay then most metals I will need a fast coil to catch those eddies, and smaller the nugget the faster ill need the coil to collapse to zero, did I understand that right?, No I have not read that book but im guessing it would be worth having along with a oscilloscope that I have yet to puck up.

Correct...

[Tibuck19]

Qiaozhi and all coil builders,

Building self shielded coils this basket way does lead to higher coil self resonance which translates into a potentially faster coil only as long as the PI delay circuit allows you to low sample to the coil's full potential.

If you apply a shield to this basket weave, you will have more coil to shield capacitance and then you give up most of the benefits of the basket weave over more traditional bundle wound coils. If you bundle wind with Teflon insulation you will do much better than using PVC insulation and then if you use thicker 600 V Teflon insulation the turns will be farther apart but you may need to add an additional turn or two because coil wire farther apart produces less inductance.

My hunt for minimizing the coil to shield capacitance was to use a fine wire mesh Scotch 24 to place around the coil bundle because it has less surface area than a solid shield like a thin foil. Only apply one layer of Scotch 24 around the coil bundle. The key trick to using this wire mesh is to place it a few mm away from the coil by using a low dielectric spacer and then when wrapping the wire mesh around the coil, not to electrically form a resistive contact around the coil cross section circumference. I use electrical tape on any overlap area to prevent any electrical contact and this prevents eddy currents from being generated in the wire mesh shield. The wire mesh get attached to the coil shield or ground connection. Make sure that there is a gap near where the coil wires come out of the coil loop bundle.

Bottom line: the less coil and TX circuit capacitance the potentially faster you can sample. This translates into a higher value of damping resistor and a potential faster stimulation of very small, low TC targets.

As you get down near 5 uS delay you are seeking very small gold targets and then your next barrier to sampling any faster is the coil wire itself. You do not want to use any wire type where the wire is holding any eddy currents near the time you are sampling. Playing in dalays in this low range requires an oscilloscope to fine tune your circuits.

I hope this helps.

Joseph J. Rogowski

Good evening Joseph, or would you rather Mr Rogowki? I have read your paper on making a fast coil, First off, Thank you for taking the time to write a well written understandable article, and used it along with some help from the Geotect forums, which is what essentially help me make a decent coin coil. there is a part in it that losses me completely, also I see it coming up a lot here in the forums, which is Damping resistor, as mention I have yet to get an oscilloscope and from what I gather, I would need on to help figure out what resistor I need to place after the circuit and before the coil. Is that right or do I really don't understand and need to buy the book Qiaozhi suggested.

[Qiaozhi]

As you get down near 5 uS delay you are seeking very small gold targets and then your next barrier to sampling any faster is the coil wire itself. You do not want to use any wire type where the wire is holding any eddy currents near the time you are sampling. Playing in dalays in this low range requires an oscilloscope to fine tune your circuits.

And ... at such short sample delays you will start to pick up the ground itself, especially in areas where gold nuggets tend to be found. This requires the detector to have some form of ground balance, which none of the PI designs on this forum currently have.

[6666]

Talking about gold coils, Has anybody Xray'd a coil from a sdc2300 ? I would like to know the construction of that gold coil, a friend of mine has one, and she has found some very VERY small nuggets ,
it has an 8 inch cover on the coil, but just how big it is inside and the type of winding is a mystery.

[bbsailor]

Good evening Joseph, or would you rather Mr Rogowki? I have read your paper on making a fast coil, First off, Thank you for taking the time to write a well written understandable article, and used it along with some help from the Geotect forums, which is what essentially help me make a decent coin coil. there is a part in it that losses me completely, also I see it coming up a lot here in the forums, which is Damping resistor, as mention I have yet to get an oscilloscope and from what I gather, I would need on to help figure out what resistor I need to place after the circuit and before the coil. Is that right or do I really don't understand and need to buy the book Qiaozhi suggested.

Tibuck19,

You live in a great international city. After visiting a company called Matrox in Dorvall on many business trips, I really enjoyed Spring and Summer in Montreal, the outdoor dining and wine drinking.

The key to making progress with electronics technology is to gain an understanding of the fundamental electronics principals and experiment to see how these principals interact. To experiment you need a few general tools that are a good investment, even if used and in good working order.

1. A digital volt meter
2, A signal generator that goes up to 2MHz or higher.
3. A 20 MHz bandwidth oscilloscope with 10x probes to reduce the load on sensitive circuits being measured or observed.

For pulse induction metal detectors you need to understand time constants both for the coil charge and discharge time constants as well as target time-constants to appreciate how much energy is being absorbed into the target metal mass as well as how much time that energy will last while discharging itself. It is only during the target discharge time that the Receive mode of the PI detector is capable of detecting a target. Small targets release their stored energy very quickly that is why the shorter the delay, the smaller targets you can detect.

Qiaozhi is right about short delays being more sensitive to mineralized ground typically found where gold is found. Using smaller coils and probes is one way to minimize mineralized ground. Ground balancing samples two ground signals and treats one as positive and the other as negative and cancels out the slowly moving ground signals while letting the faster moving target signal to sound a target.

The ITMD book will get you involved with building your own circuits, measure and observe various circuits, make changes and modifications to optimize for your favorite targets and hunting environment. Learning by doing is a highly effective and efficient way to learn. However, it takes published knowledge, tools to observe, and motivation to stay involved.

You have found the best Internet metal detecting forum to ask questions and obtain directions to make progress in your chosen metal detection directions!

Joseph J. Rogowski