Big foot style for a PI

[Chet]

After some more thought the two receiver coils would need to be spread to the outside of the transmit coil to achieve a null balance.

Sorry about that,
Chet

[Sean_Goddard]

If you want to make an insane fast spider coil wind TWO coils together side by side. If you use Raychem type 44 wire that is about 0.91mm in 24 awg and it will take 600V. now your second wire is FLOUROCARBON fishing line OR THE SAME DIAMETER (90lbs)!! Wind them side by side wire/line/wire/line, like this oxoxoxoxoxo (x = line, o = wire). You get a very fast coil with much lower capacitance OR you can substitute the line for wire but use the second winding ar the Rx coil. I have done this and sampled at 5us.

ENJOY.

[WM6]

One more great solution from you Sean, thanks. Never heard about conductive fishing line.

This part of instruction in not really clear for me, can you explain some more?:

".....but use the second winding ar the Rx coil."

[WM6]

Attached is an idea similar to your coil and a “OOO” coil found in a previous forum. Open for discussion.

An old forgotten Bounty Hunter Outlaw solution:

[baum7154]

[QUOTE=Sean_Goddard;204169]If you want to make an insane fast spider coil wind TWO coils together side by side. If you use Raychem type 44 wire that is about 0.91mm in 24 awg and it will take 600V. now your second wire is FLOUROCARBON fishing line OR THE SAME DIAMETER (90lbs)!! Wind them side by side wire/line/wire/line, like this oxoxoxoxoxo (x = line, o = wire). You get a very fast coil with much lower capacitance OR you can substitute the line for wire but use the second winding ar the Rx coil. I have done this and sampled at 5us.
----------------------------------------

Hi Sean,

What Spider Coil winding pattern are you using?

Regards,

Dan

[6666]

I was going to ask the same question
from the desription oxoxoxoxoxo (x = line, o = wire) it sounds a bit like this one.

Interesting idea winding the tx and rx together .

[baum7154]

The form you attached looks much like the original Chance PI monocoil form design. Actually quite low capacitance, unshielded, but desperately needing a shield, and there lies a problem.

Dan

[6666]

The form you attached looks much like the original Chance PI monocoil form design

Yes

[Sean_Goddard]

Hi All,

Thanks WM6, I do my best.

Yes 6666 that's the one, flat wound coil. I use my CNC mill to make them with 17 divisions outer diameter 300mm total turns = 26. I got a resonant frequency of 14MHz (OMG) with the experimental version not sure what the other one I made is....Yet .

It's not conductive, but being fluorocarbon it has the same capacitive (or very similar) coefficient at PTFE very low in other words.

If you wind the Tx and RX coils side by side on the spider coil, I have found that there is negligible interwinding capacitance change between using fishing line and using wire, so why not use wire and make a two coil PI machine? After all, with a separate Rx coil you can damp with a lower wattage resistor, you wont overload the front end amp as much and even better you can sample at stupid speeds thus get very good sensitivity to small and thin section gold (BONUS).

I have finished one of these coils so far and have found it works very well (great depth AND sensitivity) but it does have a problem when used in monocoil mode; HUGE flyback voltage which in turn kills FETs for fun so you have to take precautions to either damp or actively snub the flyback and limit it to a sensible voltage. I tried a tranzorb but found that the avalanche noise just caused more problems that it solved to I ditched that and went back to a passive snubber network. The ONLY problem with that solution is that you need to tune the time constants of the snubber to the pulse rate or you get a steadily decaying depth due to the DC bias "buildup" on the FET which causes, and the only word I can think of is, "polarisation" by putting a reverse bias voltage on the FET until the FET effectively doesn't turn on properly.

It appears this PI design is a little more involved than I thought, but when you crack it the results are well worth it.

I will try to post pictures and a "how to" when I can. I don't get on here much anymore as I'm too busy with projects and work.

[6666]

Thanks for the feedback Sean about coil and snubber.

[baum7154]

I was going to ask the same question
from the desription oxoxoxoxoxo (x = line, o = wire) it sounds a bit like this one.

Interesting idea winding the tx and rx together .

---------------------------------------------------

So is this coil wound as a SPIRAL WOUND coil with all windings parallel to each other like the original CHANCE PI coil? I understand the windings will cross each other in the slots as with the CHANCE coil, or do the windings crossover each other in the transition from slot to slot as in a spider coil. Is there a picture of this coil posted?

Thanks,

Dan

[6666]

I think they are just wound in parallel just like figure 8 speaker wire.

[baum7154]

I think they are just wound in parallel just like figure 8 speaker wire.

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Based on your description I would class this coil as a 'Basket Wound Spiral Coil' as I would the original Chance PI Coil.

Regards,

Dan