Getting ready to build the MPP

[Qiaozhi]

KRinAZ, you have made some mods to the standard MPP. I guess you have tuned the damping resistor. Do you know what value it is.
I'm testing at the moment and although I'm able to sample at 7-8us, I find the value quite low (between 350 -370 ohm).

Jacob

The mods made by KRinAZ are here -> http://www.geotech1.com/forums/showt...586#post206586

[KRinAZ]

If you imagine looking at a cross-section of the coil windings, with the magnetic field lines circling around the cross-section, you can see why the parallel target is strongest at the coil center, and weakest at the edge. And vice-versa when the target is perpendicular to the coil.What are the diameters of the gold and lead test targets?By the way, do you have the names of the Gold and Lead images swapped over? The lead target looks gold in color, and the gold looks lead colored.

OK I guess that makes sense, I never noticed the orientation act that way before with coin targets & such - I'm going to do some more tests with different metals... The test gold and lead pieces are both 1/4" / .635cm wide, the gold thickness if 1/32" / .90mm and the lead is 1/32" / .95mm thick, the gold weighs .22 grams, the lead is .23 grams. And when I first posted I had the names of the photos swapped, I immediately edited the post to fix that but it seems you saw the post with wrong pic names haha - solves that mystery...

[KRinAZ]

Hi KRinAZ That is good results for a such a small target. Qiaozhi my view shows gold color for the nugget. The lead has a dark blue color with the nickel a blue hue. Must be different browsers. I am on Goggle Chrome.Have a good day,Chet

Hi Chet, many thanks!

[KRinAZ]

KRinAZ, you have made some mods to the standard MPP. I guess you have tuned the damping resistor. Do you know what value it is.I'm testing at the moment and although I'm able to sample at 7-8us, I find the value quite low (between 350 -370 ohm).Jacob

Hi Jacob, my damping resistor is actually a resistor network at 2455 ohms. Keep in mind - as Dan pointed out in post #295: "...did you keep the original 1K input resistor on the input to the preamp? If so the actual 'in circuit' critical damping resistance with the 2455 ohm coil resistor in parallel with that 1k input resistor is 710.5 ohms..." I used an o'scope and Qiaozhi's damping tool to find the highest ohms value possible while just barely (but completely) eliminating the ringing, are you doing something similar to this? - Kyle

[Achillion]

Hi Jacob, my damping resistor is actually a resistor network at 2455 ohms. Keep in mind - as Dan pointed out in post #295: "...did you keep the original 1K input resistor on the input to the preamp? If so the actual 'in circuit' critical damping resistance with the 2455 ohm coil resistor in parallel with that 1k input resistor is 710.5 ohms..." I used an o'scope and Qiaozhi's damping tool to find the highest ohms value possible while just barely (but completely) eliminating the ringing, are you doing something similar to this? - Kyle

Yes mate, I have the 1k still in place, added the diode and have the basket weave coil (350uH, 26AWG, 3.4ohm). Coil resister is only about 350 ohm though. I will check tonight and upload a scope image. There is a little overshoot on the pre-amp output but a nice response to small gold. Theoretically this would mean that I have a lot of capacitance sitting somewhere in the coil circuit right? Well, I sorted out quite a bit of issues with my modded MPP but this puzzled me. Cheers,
Jacob

[Qiaozhi]

And when I first posted I had the names of the photos swapped, I immediately edited the post to fix that but it seems you saw the post with wrong pic names haha - solves that mystery...

So I'm not completely losing it after all.

[baum7154]

Hi Dan, nice job on creating the test targets & performing the measurements & on the creation of your new coil. I'm curious what field/detection pattern it has (e.g.cone shaped) and what delay (in uS) you're using, the ohms of your coil, and also the specs of the wire you used for your coil? I'm still thinking my resistance at 3.1 is too high, slowing my coil down (too long of a spike recovery time) - making it miss very short TC targets...we'll see when I build & compare to the next coil...next coil will be identical except with more optimum wire... I agree, the shape of the target & it's orientation to the coil (both air testing and in the ground) make a dramatic difference to detection depth - for all types of detector technologies I've used. So what I'm using are lead bullets & fragments that I recovered from the ground - with oxidization - that are deformed to similar shapes of the natural gold nuggets I have found. I only use soft lead bullets (ones I can easily scratch with my fingernail) - I don't use bullets that are jacketed with any other metal. This way I end up with test lead targets with as close as possible TC's to actual in the ground gold nuggets. As for orientation I always present the largest surface area of the target to the coil - the best case scenario for that target - for measurement purposes. Thanks - Kyle

---------------------------------------------
Hi Kyle, sorry not to reply sooner but was out of internet contact for a week. The 4" X 12.5" 3DSS racetrack coil has an 8.5" long detection 'blade' at its center line and a slightly stronger 'pinpointing' capability at each of the rounded ends. In close proximity to the coil there is a double bump but that transitions to a sharp blade pattern about 2" out from the coil. The lowest delay I can get on my CHANCE PI is 8us. The coil is wound with 24awg 600 volt Teflon insulated stranded silver plated wire. If memory serves it is about 2 ohms. Complete details of this coil and it's construction are in the COILS forum in the CHANCE PI COIL thread.

I assume your soft lead bullet targets do not have any copper plating on them as found on many .22 cal bullets. Lead Bullet flakes often come up in my prospecting areas and the low signal level ones discriminate as Aluminum/Gold on my VDI. Quite a nuisance! Jacketed bullets come up as copper.

Regards,

Dan

[baum7154]

Hmmm, ok thanks Dan. I'm curious if you tried other larger sizes & found a size that was as large as practical? That would establish a ball park range of sizes - 24AWG being the minimum to ???AWG being the maximum - that would be very good to know! It would be quite interesting to wind identical coils with each AWG size in the range & document all the performance specs - SRF, inductance, ohms, damping resistance, spike recovery time, etc.Thanks - Kyle

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The only other wire of exact materials and construction I have tried is the 26awg and it appears to be slightly less sensitive to my gold test targets. I believe this is due to less coil current, however it's SRF and eddy currents, should all be better than the 24awg coil.

Dan

[KRinAZ]

---------------------------------------------Hi Kyle, sorry not to reply sooner but was out of internet contact for a week. The 4" X 12.5" 3DSS racetrack coil has an 8.5" long detection 'blade' at its center line and a slightly stronger 'pinpointing' capability at each of the rounded ends. In close proximity to the coil there is a double bump but that transitions to a sharp blade pattern about 2" out from the coil. The lowest delay I can get on my CHANCE PI is 8us. The coil is wound with 24awg 600 volt Teflon insulated stranded silver plated wire. If memory serves it is about 2 ohms. Complete details of this coil and it's construction are in the COILS forum in the CHANCE PI COIL thread.I assume your soft lead bullet targets do not have any copper plating on them as found on many .22 cal bullets. Lead Bullet flakes often come up in my prospecting areas and the low signal level ones discriminate as Aluminum/Gold on my VDI. Quite a nuisance! Jacketed bullets come up as copper.Regards,Dan

OK thanks Dan for the info on your coil. I forgot about the discussions over in the Chance PI forum - been a year since I was in those, I probably should go back in again & catch up. I'm still in a quandry over what wire size to get for my next coil builds/experiments - I'd like to get sub ohm in resistance like the Minelabs & am considering maybe 22AWG...but wire is so expensive that I don't want to buy more wire that doesn't perform better than my 26AWG...so I haven't bought more yet. I posted in the Coils forum on this but no replies so far... Yes my lead bullet test targets are ones that were never jacketed or coated - just soft lead - with some being the smaller size bird shot (lead, not steel or other metals). I also get a distinctly different target response from my two detectors (the MPP & a Whites Goldmaster VSAT) - neither has a VDI or discrimination or ID - all metal mode all the time - but the difference in target sound between gold/lead and lead with a jacket or any other metal present is unmistakeable - thx - Kyle

[KRinAZ]

So I'm not completely losing it after all.

Hmmm, well, I think we'll need solid confirmation on that before we go assuming anything LOL

[KRinAZ]

Yes mate, I have the 1k still in place, added the diode and have the basket weave coil (350uH, 26AWG, 3.4ohm). Coil resister is only about 350 ohm though. I will check tonight and upload a scope image. There is a little overshoot on the pre-amp output but a nice response to small gold. Theoretically this would mean that I have a lot of capacitance sitting somewhere in the coil circuit right? Well, I sorted out quite a bit of issues with my modded MPP but this puzzled me. Cheers,
Jacob

Hmmm, that's curious Jacob. Sounds like your coil is pretty similar to mine. Did you use the wire to run up the pole to the control box? That's what I did, so it's one continuous length of wire from the control box down to the coil & all the windings, then back up to the control box. when I try to measure the capacitance of my coil at the connector (unplugged) it's too low for my meter to even see it's connected to something. Perhaps you could see if you get a capacitance reading. There's got to be a reason for the low value - 350 ohms is really low for that coil. How long in microseconds does the coil take to recover from the spike? - thx - Kyle

[Achillion]

Hi Kyle, This is the picture of the pre-amp output (second stage). I've added a bit resistance so that there is a smooth decay without ringing at the shortest possible time. This allowed me to ramp up the damping resistor to 610 Ohm which looks a bit better. With a 30us TX pulse duration I measure about 6us before recovering (see scope image). I've separated the pre-amp from the main board and build the pre-amp in the coil housing as some kind of experimental mod.

Since last week the detector produces sound but I'm not really satisfied with the sensitivity. The coke can test gives me about 50cm with the basic MPP settings 58us TX etc.). There may be too much noise in the house but in this test phase I just need to hook my equipment to do some measurements. Further to that I have used all of your mods with respect to SAT and sensitivity adjustment and used some different components in the front end. I noticed that the coil wire I use has only a very thin layer of teflon insulation. When I checked the photo's of your coil build it seems the insulation is a lot thicker. Not sure if that makes a huge difference though.

Thanks for your advise.

[baum7154]

Achillon,

Yes the 600 volt PTFE insulation really does make a significant difference. I have used 250 volt PTFE and the capacitance goes up in the coil to the point I could not operate below 12us. If you could get even higher voltage PTFE insulation it would help a bit more but it is a diminishing return.

regards,

Dan

[baum7154]

OK thanks Dan for the info on your coil. I forgot about the discussions over in the Chance PI forum - been a year since I was in those, I probably should go back in again & catch up. I'm still in a quandry over what wire size to get for my next coil builds/experiments - I'd like to get sub ohm in resistance like the Minelabs & am considering maybe 22AWG...but wire is so expensive that I don't want to buy more wire that doesn't perform better than my 26AWG...so I haven't bought more yet. I posted in the Coils forum on this but no replies so far... Yes my lead bullet test targets are ones that were never jacketed or coated - just soft lead - with some being the smaller size bird shot (lead, not steel or other metals). I also get a distinctly different target response from my two detectors (the MPP & a Whites Goldmaster VSAT) - neither has a VDI or discrimination or ID - all metal mode all the time - but the difference in target sound between gold/lead and lead with a jacket or any other metal present is unmistakeable - thx - Kyle

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Looking at the AWG tables... going to 24awg will give about a 37% reduction in resistance. 22awg will give about a 60% reduction in resistance compared to 26awg. Keep in mind that the larger gauges will increase interwinding capacitance in the coil and twisted feed, increase weight, increase eddy currents, and fill the slots of the coil form more quickly.

That said, I believe 24awg is the best wire choice in optimization of my 3DSS monocoils.

Regarding the audio response between lead/gold and copper jacketed bullets they are unmistakeable on the CHANCE PI too. The longer copper jacket response of the bullet easily overwhelms the short response of lead or gold bits.

Good luck!

Dan

[Achillion]

Achillon,

Yes the 600 volt PTFE insulation really does make a significant difference. I have used 250 volt PTFE and the capacitance goes up in the coil to the point I could not operate below 12us. If you could get even higher voltage PTFE insulation it would help a bit more but it is a diminishing return.

regards,

Dan

Thanks Dan, I will built a coil with the recommended wire in the same way Kyle did and see if it makes a difference. Defenitely worth trying.