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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...
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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.Comment
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Hi KRinAZ, nice job. How did you build your template so accurate?Comment
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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?Comment
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Hi KRinAZ, using the printer---neat trick. The rest of the steps are very clear.
Thank you,Comment
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Well I will test my basket coil tonight. It has AWG 24 teflon wire and a low resistance.
The biggest benefit (if it works) is self shielding. My New Litz came in today. I might build
a big foot DD with it though you have me thinking a basket might be nice.
I got 110' of 176 / 46 for $22 on ebay. This guy has it and ships fast!
http://www.ebay.com/itm/160567800290...%3AMEBIDX%3AIT
Last time I got 100' of 60/46 and was able to make 2 coils out of it. The resistance was on the higher side so I went
with 176 strands this time...Comment
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OK!, Behold!, my very first coil wind is a basket weave and it works! No errors on the lacing (thanks completely to all the pointers I got from you all), but I did make an error on the form that wasn't apparent until the end of the lacing - I somehow cut the inner wire slots more shallow than the intended .375" and there wasn't enough room for the 6th layer, so it's got 5 layers for 25 windings. I guess it gives me an opportunity to see how a low inductance coil behaves. I will be much more careful about measuring and squaring cuts when I build the next form. Pics of course:
So the wire turned out to be stranded instead of solid core - was a surprise to me & hope it works out better than solid core would have, it has a very thick insulation jacket, overall diameter is about 1.9mm.
It's specs:
180uH
2.1 ohms
My capacitance meter wasn't able to give a reading so don't know the static capacitance.
Out of curiosity I plugged the coil's parameters into Qiaozhi's Coil Calculator and they came out pretty close - it said should be 193uH - I'm guessing the difference is the basket weave instead of a standard tight wound coil - and the difference seems close to the difference between a standard and a D shaped coil (from what I read in the Coil threads).
Hooked it up and it does work. The flyback voltage is about 450 volts (my scope w/10x probe only goes to 400V so I'm guesstimating by how much I had to turn the position knob to find the peak of the pulse) - is that OK? The manual says it should be about 350 volts, that's a lot higher, could the 100V higher flyback be related to the low inductance?
Along those lines, what are the implications of lower inductance coils - my understanding is that the range for the MPP is 300uH to 500uH and a coil closer to 300uH makes for a faster coil, but why is 300uH the minimum?, what happens when the inductance is lower? I guess I need to learn more about how coil parameters interact with the electronics...Comment
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That basket coil is fast! It damps at 800 ohms.
Higher flyback voltage indicates more current flowing.
Lower inductance will allow more current.
The problem is low inductance = less receive signal so the
idea is to balance transmit power and receive signal.
It seems 300 uh has been found to be a good compromise
though sometimes RX is set to 450 uh in a DD to gain a little.
I thought I had enough space too but ended up adding an
extra row of holes to fit all my wire. I think I'm going to try
a basket with the 176/46 Litz maybe 12" using a plastic
tray I found. Can you post some dimensions ? I'm wondering
how wide is the material? Maybe litz will allow more wire to
fit in the slots?Comment
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OK that makes sense Silver Dollar, so this coil at it's low inductance isn't of much use since I don't want to run higher than needed current & I'll re-use the wire for something else. I think I'll add a resistor to lower the current and use it to finish the MPP for now while I'm making a new coil.
Nice job on the coil! Pics! It sounds like from what you said that the lower the damping resistor the faster the coil? Good to know...
The dimensions on the form on this coil:
8" outer diameter of lexan ring
5 7/8" inner diameter of lexan ring
Wire OD 1.9mm I believe, will verify later today
37 slots cut inside & outside, 3/8" deep
Note - for this particular (very thick OD) wire and to make the 7 layers/35 turns I plan to use next time - the form will need 1/2" deep slots
I will get my mic out today and get an accurate wire OD measurement, & curious what the OD is of your LitzComment
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Well according to the description on ebay the 176/46 Litz is 0.90mm.
A higher value resistor indicates a faster coil. Regular coils use say 300 - 500 ohms.
Fast coils are 800 - 1200 ohms.
So the ring is approximately 2" wide? I'll have to scale it up to use 12"...
I'm a bit worried about flopping of the form and attaching the stick. I have
found filling the coil with epoxy does slow it down so I'll try filling the inside
with foam and fiberglassing the outside. I thought of using some spokes but
will have to figure how to glass around them..
I use a series resistor on all my coils to control the current. Somewhere around
6 - 10 ohms will keep you in the ball park. I may change that later when I try to go
real deep but for now it works.
A detector is designed to have a pulse width range to work with it's coil inductance
without using too much power. So you can use a low inductance coil and just make
some changes to control the current. I have a center tapped coil that I will use
as 80 uh for the transmit for fast high current and as 300 uh for the RX to see a
bigger signalComment
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Here's the start of the Apollo Pi Basket Litz 90;
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Hmmm, interesting on the center tap coil, & thanks for all the info again - gaining knowledge from all you seasoned coil builders...I'll use a resistor to bring the flyback down to 350V until I get another coil built.
OK this wire mic's to .060 - just under 1/16" - a whopping 1.524mm. When I use this wire up I'll get "normal" thinner wire, but I will say this stuff is super durable...& maybe with the super thick teflon the capacitance will be very very low...
One thing I'm thinking with these basket weaves - is that it can maybe just be stuffed into a tight fitting clam shell - this one I made would be quite durable - both the lexan and the wire are quite tough - might be a simple way to enclose the coil...
The ring is 1 1/16 inch wide - which is counted on both the left and the right sides to make the difference between inner and outer dia's.
And...wait...I've got an idea! - how cool would it be to have a 8x10 oval/ellipse version of this basket weave? I've found I can use Quaozhi's coil calculator on the basket weave and it works out that the basket weave is 95% of the calc'd inductance.
This evening I will be working on making a PDF of a complete cut ready 8x10 oval/ellipse template that can be printed and scaled to different sizes...once I get that done I will make a round version that can also be scaled (ring width would also scale) - should work on your 12"Comment
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