Getting ready to build the MPP

[Chet]

Hi KRinAZ, using the printer---neat trick. The rest of the steps are very clear.
Thank you,

[KRinAZ]

Hi KRinAZ, nice job. How did you build your template so accurate?
Hi Chet, thx!, the template is a printout of the coil form PDF - Coils/Chance PI Coil.../post#7/Plantilla Coil.PDF - I played with printing at different enlargements in my printer settings until the outer was exactly 8", then;
1) x'd the exact center
2) Used a compass with pencil to draw my inner diameter of 5 7/8"
3)Re-drew over 2 of the outer and 2 of the inner existing wire cutout lines as .375" long - opposite sides of each other - to establish where the rings of the insides of the wire cutouts are
4)Re-drew each wire cutout line from the template edge (inside or outside as appropriate) to the ring that is .375" in from it.
This made it easy to make all the wire cutout lines in the right place and the right length.
If desired I can make another template and photo each of the steps.

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[Silver Dollar]

I used some Lits on a quick and dirty coil here; http://www.geotech1.com/forums/showt...over-Litz-Coil

Preliminary testsing shows a faster response than other wires maybe it's just the case that it cannot see itself?

[Chet]

Hi KRinAZ, nice job. How did you build your template so accurate?

[KRinAZ]

I finally got some time today to work on this project!

I started building the MPP - I'm up to Step 4 (the Tx Circuit) almost completed - power supplies have good voltages, Tx oscillator works as expected with correct freq & pulse width & waveform, and the Tx circuit looks good - it's being driven with signal, the only thing I need to complete this step is a search coil. On that note...

I started building the basket weave coil - got the form made today - just cut this one by hand so it's far from a nice clean jig produced one but should work. This was a tedious task to complete - so I will continue soon and begin figuring out how to wire it up. For this search coil I'm going to use 26AWG Teflon coated 600V wire and see how it does on capacitance & speed, next one I make may get Litz wire...

Some photos:

Coil form template being transferred to Lexan

Ready to start cutting

Tools used to cut wire slots - I used a jig saw to cut the inner and outer circles, cleaned them up with sharp fine tooth files, that's a 1/8" bit on the Dremel

Coil form is ready to get wired up

Steps 1-4 components installed, waiting on coil to finish this step. Interesting to note - the power supplies and transmit circuits installed - obviously most of the board is the received signal processing circuitry

How I routed/placed the Cxx capacitor (part of Step 3)

I should have said this in the initial post of this thread...many thanks to Qiaozhi for creating this project and making it available to us all! - and many thanks to silverdog for putting together a really nice kit for this project. I think the artwork is great on the PCB so thanks to both of you for that as well...

I really look forward to getting this detector up and running - it's already a fun project.

[KRinAZ]

Thank you very much Davor & Chet for bringing me up to speed on Litz wire. Thanks Chet for that guide on specific Litz for PI coils - much appreciated - I now have a clue on what to get & watch for a deal on...

[Chet]

Hi KRinAZ
Litz wire for a metal detector fast coil should be made up of many individual small (44-48 AWG) Polyurethane-Nylon coated wires. Served means that the entire Litz bundle of small wires is wrapped with a nylon textile or yarn for added strength and protection.


Litz wire is designed to minimize the power losses exhibited in solid conductors due to "skin effect". Skin Effect is the tendency of radio frequency current to be concentrated at the surface of the conductor. Litz wire reduces this effect by increasing the amount of surface area without significantly increasing the size of the conductor.


Litz wire composed of many strands of finer wires is best for fast coils. Polyurethane-Nylon coating is the best for soldering because doesn’t require pre-stripping before soldering. It’s hard to strip enamel coating off of 330 individual 44-48 AWG wires for soldering.


Frequency use vs. AWG size of the individual wire strands used within the Litz bundle.
All Values in KHz; a fast coil should be in the 800-3200 kHz range.
1 To 10.0 - 30 AWG
10 To 50.0 - 33 AWG
50 To 100.0 - 36 AWG
100 To 200.0 - 38 AWG
200 To 400.0 - 40 AWG
400 To 800.0 - 42 AWG
800 To 1600 - 44 AWG ------ Fast coil
1600 To 3200 - 46 AWG ----- Fast coil
3200 To 5000 - 48 AWG ----- Hard to find


There are several listings on eBay for Litz wire. The larger (mm) diameter listed in advertisements is normally the overall wire plus serving/insulation measurement. Litz wire made up of 330 strands of #46 AWG (330/46) or (250/46) would be approximately equivalent to an overall 22 AWG solid wire size. Litz 175/46 is approximately equivalent to 24 AWG. These sizes would be good for a fast coil.

Smaller wire will reduce coil capacitance a small amount and be a little faster. I haven't seen any literature on comparisons of coils wound with different size Litz wire. With this modified basket weave there probably is not much difference since there is no parallel wires as wound in commercially built Litz metal detector coils.

Smaller overall wire size will increase the resistance of the coil which results in increasing the time constant of the coil (current charge time) for each transmit pulse. This in turn will limit the pulse repetition frequency (transmit pulses per second). There is a lot of leeway and trade-offs but its good to build in some margins for experimenting with different charge currents, pulsewidths and pulses per second rates.

[KRinAZ]

Litz improves coils by means of eliminating eddy currents in the cross section of a wire. E.g. you can detect a mains cable that is made of thick bulk wire, but you can't detect a cable of a same thickness made of braided wires. Same goes with a coil, and the copper decays quite slowly. Tau of copper is ~140us. This signal presents itself as a constant offset on top of all other signals, but because it also produces noise, it is a good idea to reduce it as much as possible. Hence Litz.
The other method is using braided wires, as they too reduce cross-section eddy currents.
OK thanks much Davor - do you know if the 7/36 unserved Litz would be a good choice for the basket weave coil and if 8K feet for about $55 is a good price?

.

[Davor]

Litz improves coils by means of eliminating eddy currents in the cross section of a wire. E.g. you can detect a mains cable that is made of thick bulk wire, but you can't detect a cable of a same thickness made of braided wires. Same goes with a coil, and the copper decays quite slowly. Tau of copper is ~140us. This signal presents itself as a constant offset on top of all other signals, but because it also produces noise, it is a good idea to reduce it as much as possible. Hence Litz.
The other method is using braided wires, as they too reduce cross-section eddy currents.

[KRinAZ]

I can get some Litz wire - either unserved 7/36 (28AWG equiv) or served 12/36 (25AWG equiv), about 8000 feet - I know nothing about Litz wire - never heard of it until joining this forum - would either of these work for my basket weave coil, is one better than the other for this coil (I'm thinking the 7/36 would be better as it's smaller equiv gauge), and served/unserved (what is that!?) does it matter which one for the coil build? Would about $55 be a good price? To me it seems like a lot of wire for the $ but I've of course never shopped Litz wire. It's not chinese I'm told...

[KRinAZ]

I was a bit lost at first too until I figured out the key is after 1 complete loop you go forward one slot and start looping on the other side.

Thx for that, I'll watch for that step

.

[KRinAZ]

I used awg 24 Teflon coated wire. I just hope the hot melt doesn't slow it down too much!

I just fired up my Hammer Head yesterday so will be able to test it soon.

To wrap mine I'd need to cut the Lexan. In working with mine I found it can crack if you
are not careful. I might just put it in a full case like the Minelab one above.
Yesterday I also picked up some new sharp 35 tooth thin metal jigsaw blades, will use those to cut the lexan into the ring shape (with a nice slow feed rate), then use the dremel to cut the slots (also nice slow feed rate) and then will clean up and soften the edges of the cuts with 320 grit sandpaper strips.
I should have my magnetics separator (for prospecting) project finished today, if I have time left I will cut the Lexan, otherwise should have it cut and pic'd tomorrow.

Best wishes on the Hammerhead!

.

[KRinAZ]

Hi KRinAZ, Attached is a photo of my coil. It is in an old 8” coil housing that a friend gave me. The coil form was box knifed out of foam filled art board which was lying around. It wasn't strong enough for the wire used so the windings have some BO-BOs from broken or bent sections. Even with the mistakes the principal goal was maintained by having good crossover of the wires (no parallel wires that would cause more coil capacitance). It is wound with 18 gauge Litz wire which was also available in one of my boxes of stuff.

Qiaozhi has a coil calculator at the top of the Coil Forum. Because of the zig zag pattern this coil will require some additional wire and turns to achieve the required inductance. It is easier to remove extra turns than to add if the inductance is wrong.

Air is the best dielectric for a fast coil. Sections of foam filer (tubes) used in the Chance PI Postings is a good way to stabilize the windings from bumps against rocks and bushes. Avoid using very much solid material such as epoxy or goop directly within or against the windings.

The coil works well on the bench. It will be tested outside after through experimenting and devising some kind of Ground Balance circuit. Click on the photo to expand or shrink.
Looks good Chet, boy I'd be happy acquiring some of those shells! Thx for those tips - I will heed them. So...I just emailed Don at DBCoilShells with the coil specs (will be 8"OD, appx 1" wide, appx 1/4" thick) to see if he has/can build shells...

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[Silver Dollar]

It's still vague to me how to wire it but I'm guessing it will be clearer when I actually sit down and lace it up.

I was a bit lost at first too until I figured out the key is after 1 complete loop you go forward one slot and start looping on the other side.

[Silver Dollar]

I used awg 24 Teflon coated wire. I just hope the hot melt doesn't slow it down too much!

I just fired up my Hammer Head yesterday so will be able to test it soon.

To wrap mine I'd need to cut the Lexan. In working with mine I found it can crack if you
are not careful. I might just put it in a full case like the Minelab one above.