Magnet wire coil

[Chet]

Hi Green, Is it possible that the inductance/coil capacitance ratios are different? The same resonance frequencies achieved by different ratios might require different value damping resistors. The 1k damping value may be too high for either coil. I normally use a pot and adjust for the best wave shape. Most always the best value is less than 1k.

[green]

Made a single layer vertical 3-1 to match 3-1 spider web ZZ. 22 turns about 230 uh. Both have a circuit resonance around 2 Mhz with R damping removed (not shielded). Got some overshoot with the vertical coil shielded, so I removed the shield from both. Used 1k for R damping (R5) with the bench circuit and LT spice. With Lt spice the circuit critical damps with 1470 ohms. I've thought I found why the overshoot before but it keeps showing up. The zeners were added in LT spice so the circuit would clamp the same as IRF740 in the bench circuit. Any idea why two coils with about the same specs would act so much different?

[Chet]

Hi Green, Your charts show that targets on edge really reduce the depth of detection. Also explains why pinpointing is ineffective on many targets that are not flat.
Thank you again,

[green]

Charted the outside offset signal.

[baum7154]

Yes but the speed of it was not as good as the PTFE insul wire and I could not get it to run at a minimum sample delay of 8us as with the PTFE wire.

[green]

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Is the 3:1 coil you finished wound on the toothpicks as in the original post of this thread or is it the self shielding spider wound coil I posted in Chance PI Coil post 134?

It's wound on the toothpicks with spacers so I can remove the coil from the cardboard form. Without the spacers the coil stuck to the form when I coated it,so it wouldn't come apart when removing it from the form. Chet posted a site in reply #11 that had a coil that looked like the winding called a spiderweb coil. Want to try one like yours with the magnet wire. In one of your posts I think you said you tried one with enamel coated wire. Did it self shield?

[baum7154]

Finished the rectangle coil. Wanted to run target offset from center chart. Charted offset for 200 mm round coil first. Does it make sense before I chart rectangle coil?

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Is the 3:1 coil you finished wound on the toothpicks as in the original post of this thread or is it the self shielding spider wound coil I posted in Chance PI Coil post 134?

[Chet]

Hi Green, thank you for the charts and information, a lot to ponder over. Somewhere I had seen information where the maximum target time constant was determined by moving the sample gate out until the target was no longer discernible. Then possibly dividing by 5 might be close to a usable target time constant factor. Looking at your charts this method doesn't look too good. I like the distance results that you got with the graphite shielded coils.

[green]

Hi Green, thank you for another fine demonstration. The numbers seem to meet expectations except the measurement with the rectangular coil with the nickel at 2”. Possibly some kind of loading, inductance change, or opamp saturation is occurring.
Advantages of the rectangular coil are to have better pinpointing from side to side movement and the ability to slide in between rocks and bushes plus a small gain in forward ground footprint. When sliding in between rocks and bushes the center sensitive area of the rectangular coil will most likely pass over the target.
A couple of questions; I presume that the measurements were taken without the graphite shields. Have you taken any measurements with the coils shielded? The time constants of 2.6us for the lead shot and 10us for the nickel are they in a table somewhere that I can access?
Thank you,

Hi Chet, I think the low reading at two inches and 125mm with the nickel is because the coin is even the coil end. The measurements were made with the coils shielded with EZ-SLIDE graphite coating purchased at Tractor Supply, suggested by Silver Dollar. Including some charts of many I've posted in the past. The coil on time, target orientation, and what else effects time constant. I have gotten some different numbers than posted in some of the other threads. The reason I've posted the charted decay instead of a number most of the time. The aluminum foil is regular Reynolds foil, not sure if it's .001 inch thick. I want to plot the the nickel response with the coin at 90 degrees to the coils at different distances. Probably need to plot some points past the coil edge with the coin flat and 90 degrees to the coil.

[Chet]

Hi Green, thank you for another fine demonstration. The numbers seem to meet expectations except the measurement with the rectangular coil with the nickel at 2”. Possibly some kind of loading, inductance change, or opamp saturation is occurring.
Advantages of the rectangular coil are to have better pinpointing from side to side movement and the ability to slide in between rocks and bushes plus a small gain in forward ground footprint. When sliding in between rocks and bushes the center sensitive area of the rectangular coil will most likely pass over the target.
A couple of questions; I presume that the measurements were taken without the graphite shields. Have you taken any measurements with the coils shielded? The time constants of 2.6us for the lead shot and 10us for the nickel are they in a table somewhere that I can access?
Thank you,

[green]

Charted the rectangle coil. I cut the form 3 to 1 to give the same circumference as the round form. But then I added a 5mm bead to each toothpick so I could coat the coil and then remove the toothpicks. The rectangle isn't quite 3 to 1. The beads were added to both the round and rectangle coils, so they are close to the same circumference. Measurements were made from the center of the winding. The round looks a little better. Would need to try them on the ground to see if the rectangle might be better. Maybe it would pin point better.

[Chet]

Hi Green, great work as usual, very impressive results. The data looks good for the offsets that you used. Do you have a feel for how discernible the measurements at 12" will be compared to noise after processed through the rest of the circuity and the audio output? I look forward to your measurements on the rectangular coil.
I have two Big Foot coil housings from Bill Hays; a 15" x 3 1/2" and 18" x 5 1/2". Plan is to fill them with coils wound per post #134 of the Chance PI Coil thread.

[baum7154]

More info is always good! This is especially true since we are trying to define the field shapes/differences between round and the 3:1 coils.

Thanks

Dan

[green]

Finished the rectangle coil. Wanted to run target offset from center chart. Charted offset for 200 mm round coil first. Does it make sense before I chart rectangle coil?

[baum7154]

I followed this guide but used two rows of holes. It should have been enough room but I had to
add some more in a third row to finish the wind;

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The attachment is very good and yet they miss or ignore one of the best features that only this coil geometry can offer…self shielding! There is no mention made of it or to be sure to put the outermost winding lead at system ground in order to realize the self shielding benefit. This eliminates the need for further shielding of the coil. Also the use of 600v Teflon/PTFE insulated wire instead of magnet wire greatly reduces the inter turn capacitance of this coil type if one needs to sample at very short intervals for gold, etc. The Teflon/PTFE does require a form with deeper slots because it typically is about .038" O.D. so their housing will not accommodate the larger form.

Regards,

Dan