DIY Mad Litz Wire for ULTRA FAST PI Coils

[Sean_Goddard]

OK I have wound a coil for the SM PI using the following method and it is F A S T...

Calculate the overall length of wire you need to wind the coil you want to make and cut TWO LENGTHS of the finest enamelled (magnet wire) you can find, just TWO strands of it. Secure them at one end and then using a HAND DRILL, twist CLOCKWISE until you have around 4 turns per inch.

Take two more strands and twist ANTCLOCKWISE until you have the same 4 turns per inch.

Make up FOUR sets of each, clockwise and anticlockwise.

Now take ONE SET of ANTICW and one of CW and wind THEM together, CLOCKWISE.

Repeat the above but this time make the twist ANTICW.

Repeat the above process using OPPOSITELY wound pairs until you have TWO strands of oppositely wound wires bundles. Wind these together in whichever way you choose and use to make your coil. You will end up with a stupidly fast mono-coil with a razor sharp rise time.

You can starts with any number of strands you want but you are aimng for the FINAL CSAe (cross secional area EQUIVALENT) or 16AWG, thus you will need to work out how may "pairs" you need to twist up to get to the final COPPER CSA (not bundle as that includes the enamel).

Points to note:
You MUST keep a good tension on the wire at all times (until finished) or it will come unwound.

For your length, you can calculate the number of turns on the drill, simply take the length in inches X 4.

This stuff is a ***** to solder.

This stuff is virtually non-inductive (hence the very fast rise time).

When winding (the final coil with the finished bundle) you have to keep a fair amount of tension on the wire or it will bunch up and look like a birds nest.

You need a LARGE outdoor space to make it and usually two people make the whole process a LOT easier.

Your turns per inch MUST match on the oppositely wound strands in order to get the best effect.

Starts here > http://www.ebay.co.uk/itm/42AWG-0-06...3D112332885949
for wire.

ENJOY!

[ionut_mtb]

If i understand you are using only twisted finest enamell for making the coil? But if measure lenght of wire for making the coil, by twisting it the wire will become shorter, and uH lower. The enamell will not add capacitance in the windings? You have a picure of your coil wire finished? Thanks

[Jose]

Hi Sean Goddard
Thank you for sharing this interesting topic.
Unfortunately some concepts I can not understand, partly by the translation and by my ignorance of the subject.
Would appreciate if someone does a graphic scheme, or publish an image to understand better.
Thanks again, best regards
Jose

[bbsailor]

To everyone tinkering with coils,

Here is a link that will give you more information about Litz wire. http://litzwire.com/nepdfs/Litz_Design_PDFs.pdf
The most important thing to consider is the actual strand size as the strand sizes are frequency dependent. An AWG 30 strand would be good to use from 1KHz to 10KHz. As the frequency goes up the strand size gets smaller.

I hope this helps.

Joseph J. Rogowski

[homefire]

A Square wave in made up of a Infinite number of Harmonics of the Base Wave. How would that effect Damping ? Thin could be good on
damping times?

[6666]

Hi Sean Goddard
Thank you for sharing this interesting topic.
Unfortunately some concepts I can not understand, partly by the translation and by my ignorance of the subject.
Would appreciate if someone does a graphic scheme, or publish an image to understand better.
Thanks again, best regards
Jose

Basicly he is saying instead of twisting a big bunch of parallel wires in the one direction ,
Groups of wires are twisted together in opposite directions

start by twisting two pairs of wires in opposite directions, < >

then twist those two pairs together (2+2) = 4 in one direction
then twist another two pairs together (2+2) = 4 in opposite direction

So then you have 4 + 4 =8, etc.

[Skippy]

I wonder how much of this observed 'speed' change is due to the wires no longer running absolutely parallel, and how much is due to the wires being further apart, and there being less coupling and interaction between strands?

Interesting experiment idea: in the first step, twist 2 copper strands and one strand of monofilament nylon together. Then proceed as before.

[Jose]

Thanks for the link, bbsailor.
Thanks for the clarification, 6666.
Jose

[bbsailor]

The twist of the wires is called "wire lay". If you look into the end of the wire and see the slant of the twist that matches the slope of the center of letter Z, it is called a "Z-lay". If it matches the slop of the letter S,(wound in the opposite direction) it is a "S-lay".

The purpose of winding the parallel strands in this way is to try to minimize the inductive component in the wire. Straight wire has about a 0.4 micro-Henry inductive component per foot. So, winding strands and wire bundles in opposite ways is one technique to try to minimize parasitic inductive component of the wire. Then, increasing the spacing between winds is a way to try to minimize the parasitic capacitance coupling between coil turns.

All electronics components follow electronic theory as pure representatives of their component type and value. However, in the real world, real electronic components have parasitic values of other components that change at various power levels, frequency and speed.

Here is a common example that applies to Pulse Induction Metal detectors. The value of the damping resistor is an attempt to quench the energy in the flyback pulse as fast as possible to quickly change the coil from the transmit mode to the receive mode. Why? After the TX pulse turns off the TX energy, that quick change induces energy (eddy currents) into nearby metal objects. While the PI circuit is in the delay time, the energy in the target is starting to fall. As soon as the RX circuit is activated, the decaying eddy currents in the target are now capable of being detected. In small targets such as gold, the currents decay so fast that there is little left when the RX mode is activated. That is why low delay is desirable for these low Time Constant type targets.

When you have parasitic capacitance in a coil, the damping resistor does not immediately suppress the flyback pulse because the parasitic capacitance of the coil, coax cable and MOSFET collectively cause the flyback pulse energy to oscillate and take more time to be damped to very near zero when the RX circuit can be activated.

How you get the delay time lower can be done in a few ways. Lower TX circuit capacitance or lower the power in the TX pulse to have less energy to damp. This is why gold detectors operate at a higher frequency with shorter TX pulse widths and typically have higher values of damping resistors. If the delay time is able to be very low but there is still eddy currents decaying in the coil wire itself, then the physics of the coil wire is preventing any further delay reduction. Using Litz wire is a way to have many parallel strands carry enough current to stimulate a target but have thin enough strands that eddy currents in the wire itself does not restrict the very early delay transition to the RX mode.

The winding techniques of using S-lay and Z-lay bundled wire is another technique to attempt to minimize the parasitic inductive components of the wire itself in an attempt to better stimulate a target and then quickly detect that target.

It would be interesting to see actual use of this wiring technique in some gotech1 forum member experiments with shared results.

This is just my best analysis as to the value of using this type of wire. I hope Sean jumps in to comment on my analysis share his results.

Joseph J. Rogowski

[Sean_Goddard]

Well I think BBSailor explained it better than I did. TY also to 6666 ;-).

I have tried the monofilament idea (was mine in the first place) to wind a spiral coil using copper wire then 100lbs monofilament along side it. Better still, use the Rx coil and wind the two side by side for fantastic coupling. Even BETTER is to overwind (more turns in the Rx than the Tx) the Rx for a transformer type action.

Better than nylon, use FLOUROCARBON, it has a much better dielectric coefficient than monofilament. Try the FlouroCarbon with the above multi core wound in a spiral or basket weave and see how that works.

Can someone post some oscilloscope shots on here when they do comparisons please? I'd do the work but I am too short of time atm and can only suggest things to try. Sorry.

One last thought, why not put the Tx drive FET and Rx Preamp IN the search head? No coax capacitance to discharge then and you could use a current loop system to drive the signal to the control box.

ENJOY!

[green]

Well I think BBSailor explained it better than I did. TY also to 6666 ;-).

I have tried the monofilament idea (was mine in the first place) to wind a spiral coil using copper wire then 100lbs monofilament along side it. Better still, use the Rx coil and wind the two side by side for fantastic coupling. Even BETTER is to overwind (more turns in the Rx than the Tx) the Rx for a transformer type action.

Better than nylon, use FLOUROCARBON, it has a much better dielectric coefficient than monofilament. Try the FlouroCarbon with the above multi core wound in a spiral or basket weave and see how that works.

Can someone post some oscilloscope shots on here when they do comparisons please? I'd do the work but I am too short of time atm and can only suggest things to try. Sorry.

One last thought, why not put the Tx drive FET and Rx Preamp IN the search head? No coax capacitance to discharge then and you could use a current loop system to drive the signal to the control box.

ENJOY!

http://www.geotech1.com/forums/showt...770#post213770

Some coil comparisons I did awhile back with awg28, awg24 and some litz wire. Not sure it's the best test to compare coils. Amplitude was close for all the coils. The larger diameter wire was a little lower amplitude, lower inductance. Adjust turns to get same inductance should get amplitude closer. The coil comes out of saturation sooner with the smaller strands. litz, then awg28, then awg24. Need to define a good test procedure to see the differences when winding the coils.

[6666]

The coil comes out of saturation sooner with the smaller strands. litz, then awg28, then awg24.

Thats a good graph, but I'm not sure I understand the results properly, when I saw the graph a while back, I was looking at the SRF number and 24 awg PVC wire had the highest SRF, meaning to me, that it had the lowest capacitance (maybe not ?) but it comes out of saturation last of the three, I'm confused .

[6666]

One last thought, why not put the Tx drive FET and Rx Preamp IN the search head

MikeBG published a circuit to do that, been looking for it, will keep trying to find it.

[green]

Thats a good graph, but I'm not sure I understand the results properly, when I saw the graph a while back, I was looking at the SRF number and 24 awg PVC wire had the highest SRF, meaning to me, that it had the lowest capacitance (maybe not ?) but it comes out of saturation last of the three, I'm confused .

I think it's because the larger wire is acting as a target longer. http://www.geotech1.com/forums/showt...725#post213725 Litz_2 chart includes a chart of wire TC for awg12, awg19 and awg24 that I recorded. Maybe not the reason but it's my best guess. The awg24 had the highest SRF, mayby because of the lowest inductance.

The SRF of the coils are probably higher than charted. They were measured in circuit with the damping resistor removed. I now excite the test coil with another PI and measure the resonance with the operating PI which gives a higher SRF

[6666]

I think it's because the larger wire is acting as a target longer

That makes sense, thanks.