Has anyone ever tried a different coil wire??

[Huizer]

Hi,
just wondering if anyone has ever tried different wire for the coil. Like I know that copper is the prefered choice for all coils and manufacturing, but what would happen if you tried silver wire or aluminum wire or insulated steel wire??
I am just coming up with questions to wile away the last few minutes of the day.
Later, Dan.

[chemelec]

Probably very little difference in performance on most detectors.

On a PI detector, Silver would be better because of lower resistance, but just try to buy Enameled Silver wire. Not very common and if its available, it would be Expensive.

[GTK]

Silver wire 0.508mm costs 330-350$ for 80-100 meters. Yup to expensive even to try. Minelab use aluminium wire (0.5-0.6)

[bbsailor]

Wire Insulation

The kind of wire you use depends on two primary PI design factors.
1. The speed of the main pulse generator. Lower frequencies in the range of about hundred to a few hundred Hz typically need to use thicker wire to get a strong magnetic field to penetrate the ground at the maximum distance. These machines are using more of the "brute force" method to optimize the PI machine performance. Almost any type of wire can be used as long as the MOSFET is not getting too hot. Higher frequency PI machines in the 500 to a few thousand Hz, such as the Hammerhead, typically should use coils with little higher resistance (about 5 ohms) and MOSFETS with a higher on-resistance (1 to 4 ohms) to keep the coil current lower so that the MOSFET does not overheat and the batteries last a reasonable amount of time.

2. What type of targets you are seeking. This relates to the size, depth and metal composition of the targets you seek. No single PI machine or coil size can be optimized to find everything although it may find a good range of targets. That is why if you are seeking coins, with longer target responses, you can use a higher power machine as indicated above. However, if you seek gold nuggets in stream beds or gold jewelry on the beach, you need to be operating your machine at earlier sampling times to obtain a strong response from the gold targets. Here is where sampling at 10uS or earlier make more difference that just trying to pump more power into the ground.

Once you decide that a fast target response is what you want, many factors come into play. The insulation on the wire can have an impact along with other considerations. Teflon has the lowest dielectric constant of makes a coil with the least amount of capacitance and this contributes to making a faster coil along with other factors. Steel wire is not a good wire as the wire itself could become a target. Thin wire such as AWG30 Teflon (available on eBay) makes a small wire bundle that can be easily shielded with minimal additional capacitance added. This additional capacitance would tend to make a slower coil. The shield spacer dielectric, wire insulation dielectric, damping resistor value, MOSFET output capacitance, coax type and length all contribute to the coil speed necessary to detect fast decaying targets like gold.

Making a fast coil requires taking an inventory of everything that contributes capacitance to the coil, the coil circuit in the PI machine, and the type and length of the coax used to connect the coil to the PI control box. Many small optimization factors are cumulative and come into play when trying to squeeze the most speed out of your PI coil or PI machine.

No single thing, like changing the wire, will have a significant impact on PI performance. Start with deciding on what you are seeking first, then use the search feature on this forum to find ways to optimize your PI machine and/or coil to match your target. There is a wealth of information available on this forum.

bbsailor

[Tinkerer]

Coil shielding

bbsailor,

like always, your posts help a lot. I have a question about the shielding of the coils. When the shielding is very close, there is a lot of capacitance. with a good dielectric spacer the capacitance is reduced. But , is there an ideal amount of space? Or is more better? I have been using about 4 to 5 mm, would more be better?

Tinkerer

[bbsailor]

Tinkerer,

Go to the following web site and see for yourself. http://www.mogami.com/e/cad/electrical.html

Enter the size of your coil wire bundle as the diameter of the center conductor. Use the thickness of your shield spacer as reflected in it's outside diameter as the shield diemeter input. Then select the spacer type and see the effect of the spacer dielectric on the capacitance. The answers will be in pf/meter. Convert that answer to the ratio of 1 meter to your coil circumference to see the actual predicted capacitance that your shield adds to your coil. This answer may be about 40% off the actual measured value but the "what-if" variables work in the same direction as reality. This is very instructive for coil design experiments.

Distributed capacitance is a very complex and difficult calculation to accurately make but it is very easy to measure. Just clip one LC meter lead to the shield and the other to one of the coil leads. You wil get a slightly different reading by a few pfs depending on which coil lead you choose. This is due to how the wire runs in your wire bundle.

Measure the coil's self resonance frequency with no shield, then add the shield and attach one coil lead to the shield and measure the resonant frequency again. With the shield, the coil's self resonance will be lower, but how much lower? Not by the full value of the coil-to-shield capacitance that you previously measured. If the coil to shield capacitance is 120pf, for example, the lower coil self resonance will only be the equivalent of the shield imposing about 20% or 24pf on the coil resonance. Measure your coil and post the result on this forum.

Thicker shield spacing results in lower capacitance. Using a low dielectric spacer like polyethylene (PE) is better than PVC. Try two layers. The limit is the amount of space in your coil housing.

Using thinner coil wire like AWG 30 Teflon insulated wire that is .024 OD will make a .12 diameter wire bundle with 18 or 19 turns. For a coil with 18 to 20 turns, multiply the OD of a single strand of wire by 5 to obtain the bundle diameter. If you are using AWG22, your wire bundle may be about .3" and this will make a coil that measures hundreds of pfs higher due to the increased surface area.

Go on the web and look up "Scotch 24" shielding mesh (1" wide"). It will make a good shield that is not detected low delays. It can be used on the outside of coil spacers that are .25" to .375" OD by wrapping around the OD circumference of the coil and the folding over and securing with electrical tape. For thicker bundles just spiral wrap the shield around the entire coil, leaving a gap at the place where the wire leads enter and exit the coil.

Rule of thumb for a fast coil; you want the final coil self resonance, with shield and coax to only be about 40% lower than the coil's self resonancee alone.

The Hammerhead would work at lower delays at higher PPS using a thinner wire bundle to limit the peak current at about 2 amps (coil and MOSFET on-resistance of about 6 ohms) or a little lower. If you seek coins or deep relics, then going to a thicker wire may help a bit but at the expense of less battery time and the possibility of the MOSFET needing a heat sink.

Let us know how your experimentatiuon works out.

bbsailor

[Tinkerer]

coil winding and shielding

Bbsailor,

Thanks for the very useful information.
I got good results experimenting with exotic coil windings that are mostly of the flat type, planar, spider wound, basket wound etc. Sometimes I have used PVC insulated wire that I will avoid in the future thanks to the information found on the mogami website that you kindly supplied. I see there that PVC insulation is very much temperature dependent and feel therefore that it is a bad choice for a coil that might get quite warm when working in the sun.
For shield spacers I have chosen materials that consist mostly of air, like styro-foam etc.
For shielding I have used a compound formulated for the manufacturing of carbon film resistors (old recipe), with a drain wire embedded.

The results have been very encouraging:

For a I000uH coil with 32cm diameter I got a sampling delay of 8uS
For a 600uH coil with a diameter of 32 cm, I got a sampling delay of 6uS
Coils and cable shielded.

I have used a continuous coating of about 10 to 30 microns of the shielding compound. I wonder if I should cut a gap in the shield. How do I know if it is needed?

I have also seen that you break up the shielding by cutting patterns. How do I look for a difference in the results between gaps, patterns and full continuous shielding?

I don’t know if there are coil shells on the market for my weird coils, but I hope that if my coils are really good, somebody will make the shells that fit.

Thanks for your help

Tinkerer

[bbsailor]

Tinkerer,

Excellent results.

Please provide the self resonance of each of your coils below?

For a I000uH coil with 32cm diameter I got a sampling delay of 8uS
For a 600uH coil with a diameter of 32 cm, I got a sampling delay of 6uS
Coils and cable shielded.

What is the type and length of your coax?

Are your measurements made through the coax or at the coil?

I have used a continuous coating of about 10 to 30 microns of the shielding compound. I wonder if I should cut a gap in the shield. How do I know if it is needed?

Try making a identical shell without the wire coil and see if the shell is detected at yout lowest delay on each coil. If not, all is well. If it is detected, then try breaking up the conductive coating as described and shown by Reg in another post with a graphic. What you don't want is for eddy currents to be formed on the shield itself.

I have also seen that you break up the shielding by cutting patterns. How do I look for a difference in the results between gaps, patterns and full continuous shielding?

See above.

I don’t know if there are coil shells on the market for my weird coils, but I hope that if my coils are really good, somebody will make the shells that fit.

Please post a photo of your coil and coil shell. Be creative with finding coil shells such as using Frisbees, plastic can or bucket lids, or anything that can securly hold your coil without flexing too much to cause a false signal.

You have just discovered the practical aspects of making a PI coil. While a bundle wound coil may be a little slower than the ones that you made, they can more easily be shielded and fit commercial coil housings (Hays Electronics).

Here is a fast way to see how close a coil is to locking up (meaning reaching it's fastest speed). Obtain an air variable capacitor such as an AM tuning cap, the 365pf type. Connect this across the coil connection and increase the variable capacitance until the coil stops responding (locks-up) to your target being waved under the coil. Measure the capacitance of this variable cap and this will tell you how much more wire (additional capacitance for each wind) you can add to the coil to make it a little more sensitive (reserve sensitivity). This will also show you if you can reduce the delay somewhat. You must continuously adjust the damping resistor with each change of capacitance to see the optimization points. You can also do this by adding a longer coax (about 2 meters) to the point where the coil locks-up. When it does lock-up, cut off one foot at a time until the coil starts working again. Measure the total capacitance of all the cut off pieces and this will tell you how much reserve sensitivity is in your coils.

I doubt that you will do much better than what you have already achieved but it is good to know how close to the wall you are. Optimum design is a series of successive approximations. Good design is also a matter of knowing which variables have room left to be optimized or whether a variable is already at the wall.

I would like to see you answers to my questions. My intuitive guess is that the difference between your two coils total capacitance is about 20 to 30 pf?

bbsailor

[Tinkerer]

coax for coil

Hi bbsailor,

thanks for the help. I will run the tests you recommend and look for the info you are asking for.
For now, about the coax: I could not find a coax that lets me make really short delays, so I normally use a two counductor shielded cable. However, I got worried about this cable maybe causing too much attenuationwhich could explain the seemingly good results.
So for this specific test, I took 2 plain cables and then built a shield around them, with ample space. I rekon this should be a system that gives the minimum possible of attenuation at the frequency we use for PI's.

the 2 conductor shielded cable has a little more capacitance but comes very close to the same results.

More tomorrow

Tinkerer

[Tinkerer]

High inductance coils


Bbsailor

The two coils are of very different design. The one is a planar wound coil, meaning it is basically 2 directional, the other one is wound in a way that I can only call “space wound”, meaning it has with and height. It is wound on the principle that the wires should be as little as possible parallel to each other to reduce capacitance. I posted a picture of that coil, still in its former, but at that time with less turns and less inductance, on a post titled something like “the importance of shielding a coil”
Both coils are wound with the same magnet wire I think it is #24 or 26. (I cant get Teflon here) I used the same shield and the same cable on both coils. As yet I have no shell for the coils, I will make a shell out of fiberglass for the first few coils, once I have decided which one to use.

The two coils respond very different to a Nickel or a small nugget. I use the same coil amp for both but I can see that the AGS kicks in hard on the 1000uH coil to limit the saturation.

I will try your recommendation with the tuning capacitor. For the time being I normally use about 7ft of 2 conductor shielded cable that should give me some reserve that I can cut.

I made these specific coils, looking for the limits. As yet I have not chosen anything final. Seeing that I can get an early sample with high inductance, should I go this way? Or should I try to get more gain and a little longer delay? Should increase the power? Should I take advantage of the high inductance and reduce the power?

Tinkerer

[Unregistered]

Hi,
just wondering if anyone has ever tried different wire for the coil. Like I know that copper is the prefered choice for all coils and manufacturing, but what would happen if you tried silver wire or aluminum wire or insulated steel wire??
I am just coming up with questions to wile away the last few minutes of the day.
Later, Dan.

Hi Dan,

As you say, copper is the preferred choice. If it is stranded wire, use tin plated copper, as the plating breaks up the eddy current paths between strands. I have noticed a considerable difference between copper stranded and tin plated copper stranded. The tin plated makes a faster coil. You would have to be careful with cross section eddy currents with silver wire due to its higher conductivity. The advantage of aluminium wire is lightness, but it is hard to solder to. Steel wire is definitely a no-no, as it will give a very strong magnetic lag signal.

PVC insulation causes problems if you are pumping lots of amps into the coil. I have seen the response curve change quite dramatically as the coil warms up. Use Teflon insulated wire if you are going for higher TX power.

Someone once suggested to me that a coil wound with gold wire, would be more responsive to gold targets. I haven't been able to verify this yet, as nothing suitable has come up on Ebay.

Eric.