Down the rabbit hole with oOo pi coil

[6666]

Thanks Chet, very cool info.

[Carl-NC]

There is a rectangular Ferrite housing surrounding the printed circuit board, for EMI out/in?

It's for magnetic shielding. Variable ground (or just unlevel sweeping) causes a variation in the magnetic field seen by the coil. This can induce eddy currents in even a small solder blob which then responds like a target. Encasing the solder blob in ferrite prevents this. Minelab has a patent on this technique.

[Chet]

Carl, thank you that is good information,
Chet

[Altra]

Great info Chet. I think there is a security chip up in the connector. Did you notice this?



Add: This has some photos to go with Chets info

https://www.geotech1.com/forums/show...-homebrew-coil

[Chet]

Great info Chet. I think there is a security chip up in the connector. Did you notice this?
Yes, I purchased this defective coil to use the security chip and Minelab cable with a Russian coil that comes without the security chip.


Add: This has some photos to go with Chets info

https://www.geotech1.com/forums/show...-homebrew-coil

Hi Altra

Yes, I purchased this defective coil just to cut the Minelab cable off and use it with a Russian coil that comes without the security chip.
I am very pleased with the Russian coils performance and reduced weight. I will probably not use the original Minelab coils again.
For those curious, the Minelab coil is very well built with much epoxy used. In this case one short span (a few mm between glued points) of the very small receiver wire was broken by heat expansion from the hot California sun.

Have a good day,
Chet

[6666]

In this case one short span (a few mm between glued points) of the very small receiver wire was broken by heat expansion from the hot California sun.

Thats interesting as this should be happening here in Australia as well, we have a very hot sun here in summer, I wonder how old that coil was ?

cut the Minelab cable off and use it with a Russian coil that comes without the security chip

I wonder why they are selling them if they dont work without the chip

[Chet]

Hi 6666


The coil was painted black for some ??? reason; which was surely a contributing factor.


The Russian coils are sold with a standard 5 pin GPX type connector attached. Instructions are provided on where to cut and use the Minelab 7 pin connector, security chip and cord and how to solder in a mating 5 pin connector. This is how they get around any legal hassle with Minelab.


Have a good day,
Chet

[Chet]

It's for magnetic shielding. Variable ground (or just unlevel sweeping) causes a variation in the magnetic field seen by the coil. This can induce eddy currents in even a small solder blob which then responds like a target. Encasing the solder blob in ferrite prevents this. Minelab has a patent on this technique.

Hi Carl


With much patience I read through Minelab’s patent application and US patent on the use of the ferrites to shield the solder joints and printed circuit board inside of the coil housing. I have more than 50 years in electronics; engineering, training, and report writing. It is amazing how difficult it is to get to the meaning of some of Minelab’s patents.


The Russian coils are sold with a standard 5 pin GPX type connector attached. Instructions are provided on where to cut and use the Minelab 7 pin connector, security chip and cord and how to solder in a mating 5 pin connector. This is how they get around any legal hassle with Minelab.


Before cutting the cord on the Minelab cable there was a concern about installing a male and female connector at 24 inches above the coil which would be within the detection range of the coil. To alleviate this concern a male and female connector where taped to the top of the 14 inch Minelab coil. A 0.06 gram test nugget was placed under 1 inch of Northern Nevada desert sand and gravel. The detector response was the same as without the connectors. The test was repeated with a standard aluminum can taped to the coil shaft at 24 inches above the coil. There was no obvious degradation in either test.


Supposition; The Northern Nevada desert has mild mineralization as compared to the Australian gold country.


For do-it-yourself coil builds consider what kind of ground will be encountered. Install ferrites to shield the solder joints or locate the solder joints as high as possible to reduce hot ground magnetization effects.


Have a good day,
Chet

[6666]

Hi 6666


The coil was painted black for some ??? reason; which was surely a contributing factor.


The Russian coils are sold with a standard 5 pin GPX type connector attached. Instructions are provided on where to cut and use the Minelab 7 pin connector, security chip and cord and how to solder in a mating 5 pin connector. This is how they get around any legal hassle with Minelab.


Have a good day,
Chet

Thanks, that makes sense now on how they do it, did the coil you opened have the pressure relief valve on it ?

[green]

It's for magnetic shielding. Variable ground (or just unlevel sweeping) causes a variation in the magnetic field seen by the coil. This can induce eddy currents in even a small solder blob which then responds like a target. Encasing the solder blob in ferrite prevents this. Minelab has a patent on this technique.

Wondering if there was a way to simulate effect on the bench I placed a coin near the coil and tried exciting it with a ceramic magnet. No effect, what am I missing or is there a way to show effect on the bench?

[dbanner]

Wondering if there was a way to simulate effect on the bench I placed a coin near the coil and tried exciting it with a ceramic magnet. No effect, what am I missing or is there a way to show effect on the bench?

The patent Carl is referring to is a patent application US20130057286A1.
I don't know if a patent was actually granted by the US patent office. But it reads like piece of scientific witchery!

The magnetic field mentioned by Carl seems to be the response magnetic field of the soil(from the magnetic field transmitted by the tx coil), which then itself generates the Eddy currents in the solder blob. The response magnetic field changes in intensity caused by uneven or changing ground or uneven sweep. The blob is now visible to the RX coil.
Don't know how you'd go about simulating that with a coin and a magnet. Lest not forget these magnetic fields expand and collapse on the order of few micro seconds.

[green]

Another try. Scoped integrator out. Target(US nickel)spaced 3.25 inches from center of one of 8inch fig8 coils. Used ferrite core to couple Tx signal to target(US nickel). No visible change in integrator out signal when bead touches coil(no target, core only).
Was wondering if fig8 coil could cancel coupled signal. Looks difficult with nickel, have to try with soldered joints and damping resistors when I wire the fig8 coils I'm working on. Thinking it simulates effect described in the patent?

[green]

Was wondering why the ferrite core works and ferrite magnet didn't. Tried ferrite magnet(1/2inch diameter) with nickel at 3.25inches. Easily detectable(20 to 30mV)change at integrator out depending on magnet orientation. Tried with US quarter as target reply #25 at 3.25inches. Very little change at integrator out.