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GPZ 7000 Tear-down


phrunt

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In the case of the smaller coils lacking space for the windings, is it possible to get rid of some windings and then raising the coil inductance back to 300mH via the inclusion of a small bit of ferrite or even iron to boost it back to spec? Maybe using the ferrite for shielding in some way too such as powdered ferrite paint instead of powdered graphite paint (2 birds, one stone)? Permeability of ferrite is like 1500x that of air, foam, etc so it shouldn't take much to raise the total coil inductance - at least in terms of the inductance the detector sees.

If that works, would adding materials with varying degrees of magnetic permeability to increase inductance be a potential experiment to reduce winding weights and thus total coil weights too? Or would those materials themselves have a negative effect on the detector operation/ground balance/etc? I wonder if in some way this is one reason the ferrite was included with the GPZ due to some kind of design considerations like this?

Slightly smaller diameter wire could be used to increase the coil resistance if reducing the turns/wire length threw it out of a critically damped state and introduced ringing? Or I assume they are designed to be critically damped anyways, but I guess I'm not sure.

I've never built coils for detectors, but I find it interesting to think of experiments to try to potentially improve upon them. It's educational to see some discussion of it here.

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Iron will not work because it retains a magnetic charge and delay in current changes. 
     I have wound some ferrite coils approximately 1” x 10” for the GPX 5000. It was an experiment to help a friend develop a probe that would go into drilled holes in a gold mine. My friend did finish one on his own. I never heard of any success in finding gold pockets with it. It did a great job in reducing the wire gauge and number of single layer windings. Instead of the long rod a thick round shaped core could be used. But the weight of the ferrite core material far exceeds the normal weight of standard windings. 
 

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I don't mean a solid ferrite core, I just mean adding some small amount of ferrite to make up for the inductance loss resulting from subtracting some coil turns. Maybe even just using ferrite powder in a paint as shielding instead of graphite paint could be enough, no idea. 

A little bit of highly permeable material can raise the inductance quite a lot.  Permeability of air = 1. Permeability of ferrite can be around 1500. 

So for a solenoid inductance would be something like this:

image.png.c7583178f01aa96ca617a54c938d2835.png

So adding a ferrite core would increase the inductance 1500 times over a foam/air core, which is way too much because we'd need to subtract a substantial amount of windings to get back down to 300mH.

But if you just added a small bit of ferrite so that the entire coil "sees" an average permeability increase of maybe 25 (since much of the foam or air will still be permeability = 1, the total permeability will depend on the size of the ferrite), then you could remove quite a lot of copper. The ferrite might actually weigh less than the copper did, and the coil could be lighter? Less turns will mean less resistance, so the remaining turns could be downsized in gauge diameter to increase resistance again, and this too would save weight. 

Just a quick example: Foam core, 1 cm thickness, 10cm coil radius (8 inch wide) means you need 275 turns of copper to equal roughly 300mH.

Conversely, add a bit of ferrite in some way or form to increase permeability to 25, at least in terms of what the detector sees (this would be a small amount of ferrite) and now that same coil dimension only needs 55 turns of copper.

(I have no clue if this is close or far from actual ML coil turns, I'm just using a generic example with the formula above)

In this example the ferrite would probably weigh less than the 220 turns of copper removed from the coil since the ferrite permeability is 1500, you wouldn't need to cover the entire core to reach an average permeability of 25.

No idea if that would work or if the hysteresis curve of the ferrite would screw the detector sensitivity up. Curious though. A high permeability material with a slim hysteresis curve might exist? 

 

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    Your ideas may have some merit but I am kind a nuts and bolts, do it your self-person. This would not be an easy experiment.
     One mistake in your calculations is the coil inductance of 300 milihenries should be 300 microhenries.  So an 8 inch coil would require around 33 turns of 1.86mm Litz wire. Attached is a calculation for an 8 inch mono coil. It uses 65 feet of wire with a weigh of 0.55 pounds.
 
From: http://electronbunker.ca/eb/InductanceCalcML.html


 

8 inch mono coil windings.JPG

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Well all this explains to me why one of the X-coils guys is a mathematician.  🙂

 

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I wasn't really shooting for any kind of accurate number, I just wanted to show the physical relationship between coil inductance, material permeability and number of turns in a quick example. Just to show one might be able to replace some amount of copper wire with a lesser weight of ferrite. No clue though.

I guess it would really need to be something done via experiment, or done in EM finite element analysis modelling software though to know if it worked. Hysteresis as well as localized magnetic flux concentration around the ferrite might make this infeasible in a PI, GPZ maybe not? The flux concentration effect might be interesting to experiment with too though on it's own for various reasons too. 

I guess I'll chunk it onto the log pile of ideas I'll never get around to actually trying. 😄 One of these days I'd like to get back into electronics again, just no time now. 

 

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On 5/3/2023 at 10:07 AM, phrunt said:

I recall X-coils made @davsgold a drag coil for the GPZ, well I think they did, perhaps it was just a plan that never went ahead I don't remember. 

 

I did at the time discuss the idea with X-Coils for a big drag coil for the GPZ7000 but it never happened

cheers dave

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A problem with drag coils is that the bigger you build the more that the problem develops, being that the ground balance varies from one side of the coil to the other. 

Unless you have a detector that can handle such electronic variations and still give accurate readings you will find 'sledding' (drag coils) challenging.

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