DIY Mad Litz Wire for ULTRA FAST PI Coils

[bbsailor]

To stay within the scope of the title of this thread, please keep in mind that the ability of a current that rises to its maximum and is quickly cut off, creates a discharge time constant. The more vertical this discharge current is depends on the value of the damping resistor (Rd). Higher Rd values means less capacitance to suppress as well as potentially faster times to turn on the RX portion of the circuit (delay time). With extremely small targets that are desired to be detected, it becomes extremely important to fully stimulate the target with the fastest discharge current to fully stimulate most eddy currents in the target. Here is where Litz wire comes into play.

Look at this web link: http://litzwire.com/litz_types.htm. Look at type 2 as it contains bundles of strands wound in opposite directions. This type will minimize the straight wire inductance, typically 0.4 Micro Henries per foot, and create a better opportunity for the current discharge to be more vertical, with less of a rounded edge on an oscilloscope pattern display. Quicker discharge Time Constants (TC) stimulate smaller targets better. You need to choose Litz wire with very small strands in the bundles to easily represent the harmonics many time higher than the PPS rate of your pulse induction metal detector. The rule of thumb is to make the coil discharge TC 5 times smaller than the TC of the target you are seeking.

Here is where total coil circuit capacitance, damping resistor value and coil wire eddy current retention minimization come into play to optimize your coil for what you are seeking.

Joseph J. Rogowski

[6666]

I think this may reduce the effectiveness of the coil. It will reduce it's depth. At least this was my experience with enamel copper wire, not sure about mult strand.

I was talking about speaker wire, not solid enamel copper wire,

[rickb]

@green
The size of the wires being used in your tests are so large, that I think that the other characteristics of litz wire that result in high srf and fast eddy current decay are being masked. Try using #20-24 litz wire made up of 300-400 strands of #36-40 wire. I suspect that you will see a difference.

As an aside, for those wondering how on earth can you solder the stuff, think "caustic paste paint remover". Dip the end in the paint remover, wait a few minutes and gently wipe with a cotton swab on a hard surface until the enamel is gone. If not, dip again, rinse and repeat. Some types of enamel are tougher than others, but eventually they all fail. Dip in rosin flux before soldering. Some litz wire may use enamel that melts at soldering temps.

[dbanner]

Good experiment Mr Green, This is just for interest, but there is a crude trick you can do with non tinned multi strand copper wire to increase the resistance between the strands, WM6 once posted, that he put the ends of a full length of wire (PI coil) across a car battery (basicly a short) to heat the wire enough to make it change colour , this draws a lot of current and you need to watch what you are doing, the wire heats up FAST, all you want is enough heat to discolour the wire, not fuse the strands together or make it brittle , now, if this crude trick has the same effect on eddy currents as useing tinned wire strands is hard to know, .

I think this may reduce the effectiveness of the coil. It will reduce it's depth. At least this was my experience with enamel copper wire, not sure about mult strand.

[6666]

Good experiment Mr Green, This is just for interest, but there is a crude trick you can do with non tinned multi strand copper wire to increase the resistance between the strands, WM6 once posted, that he put the ends of a full length of wire (PI coil) across a car battery (basicly a short) to heat the wire enough to make it change colour , this draws a lot of current and you need to watch what you are doing, the wire heats up FAST, all you want is enough heat to discolour the wire, not fuse the strands together or make it brittle , now, if this crude trick has the same effect on eddy currents as useing tinned wire strands is hard to know, .

[green]

The silver plating reduces the resistance between adjacent strands of wire thus making the entire wire cross section appear electrically as one solid wire. Unplated stranded copper wire will oxidize over time worsening the contact between strands allowing it to take on some of the characteristics of litz wire.

I read a masters or phd thesis on this subject that came to the conclusion that for non-critical high volume applications, unplated stranded copper wire was sufficiently like litz wire to work where low cost was paramount. Not something I would use in a pi coil.

Thanks for the reply. Looks like my test this morning didn't show the problem. Maybe not enough strands? Another test. Charted some different size solid wire, 3inch long pieces. Best guess on size of stranded wire(plain copper, strands not coated), 19 strands of I think awg27 wire. Stranded wire charted about the same TC as awg22 at start then TC increased. Need to get some pieces of silver coated, tin coated and not coated stranded wire to compare.

[rickb]

I don't see how the platting would make much difference.

The silver plating reduces the resistance between adjacent strands of wire thus making the entire wire cross section appear electrically as one solid wire. Unplated stranded copper wire will oxidize over time worsening the contact between strands allowing it to take on some of the characteristics of litz wire.

I read a masters or phd thesis on this subject that came to the conclusion that for non-critical high volume applications, unplated stranded copper wire was sufficiently like litz wire to work where low cost was paramount. Not something I would use in a pi coil.

[green]

You need to add one more thing to your thinking. When attempting to use a low delay between 5 and 10 microseconds you need to make sure that eddy currents are not being retained in the coil wire itself. A thin silver plated AWG 30 single strand will work well if you can use a coil of about 5.5 ohms. However, multiple thin strands that are silver plated may act more like a thicker solid strand relative to eddy current retention due to the highly conductive silver plating on the strands. Tin plated strands have less conductivity between
strands and will tend to hold less eddy currents. Small gold nuggets need very fast delays to have enough signal left within them to be detected after the delay time. If your coil wire retains any eddy currents, it may also look like a small gold target.

I hope this helps.

Joseph J. Rogowski

However, multiple thin strands that are silver plated may act more like a thicker solid strand relative to eddy current retention due to the highly conductive silver plating on the strands. Tin plated strands have less conductivity between
strands and will tend to hold less eddy currents.

I see the word may, did a test this morning with some materials I have. The not coated awg16 looks like it is plated(maybe tin). Adding more wires didn't change the decay TC. TC the same wires coated or not coated. Test maybe not the same as silver or tin platted stranded wire but I don't see how the platting would make much difference.

[bbsailor]

Hi,
I'm new to detectors but have radio and electronics experience.
I always thought that litz wire was used to reduce resistance due to skin effect at high frequencies as well as reduce self capacitance of the coil.
I have some multistrand silver coated wire that I thought would be perfect but apparently not?
I thought the ideal coil would be low resistance, low capacitance and driven to just below saturation.
years ago radio coils used to reduce capacitance (I hope I describe the following in a sensible manner)
You wind the coil on a round card with an odd number of slots, winding the wire in and out of the slots so that there is a gap between adjacent windings.
would this strategy work with a detector coil ?

You need to add one more thing to your thinking. When attempting to use a low delay between 5 and 10 microseconds you need to make sure that eddy currents are not being retained in the coil wire itself. A thin silver plated AWG 30 single strand will work well if you can use a coil of about 5.5 ohms. However, multiple thin strands that are silver plated may act more like a thicker solid strand relative to eddy current retention due to the highly conductive silver plating on the strands. Tin plated strands have less conductivity between
strands and will tend to hold less eddy currents. Small gold nuggets need very fast delays to have enough signal left within them to be detected after the delay time. If your coil wire retains any eddy currents, it may also look like a small gold target.

I hope this helps.

Joseph J. Rogowski

[green]

Tried a flat spiral to compare with flat basket(spider coil)I've been using awhile back. 200mm diameter, awg28 magnet wire, spider coil wrapped around toothpicks, flat spiral wrapped on sticky tape. Not much difference, data measured with no added leads, both coils have calculated capacitance less than 1 foot of coax. Coil calculator calculates 400uH for a bundle coil instead of 330uH I got. Would be interested in SRF and inductance measurements of different coils to compare winding methods.

[andybiker]

That's exactly what I was thinking of.
I was trying to think what they were called.
IBGold's spider coil is really a work of art though.
I look forward to a little experimentation when I get my kit.

[bernte_one]

Like this part built 18" Mono Spider coil for PI.

Regards Ian.

can you tell something about the Performance of this coil?

[qute1]

Nice work IBGold i too have a crystal radio background and will be experimenting with this when the correct litz gets here i only have high strand on hand and for MONO pi coils you need lower 15 strand or so as not such high freqs involved and better not to saturate as both out going and incoming signals. I have high hope for PI coils made to this standard. Let us know your results and if there is any improvement in depth or quality of the return signal.

[IBGold]

Like this part built 18" Mono Spider coil for PI.

Regards Ian.

[6666]

Yes that will work to make a spiral coil.