DIY Mad Litz Wire for ULTRA FAST PI Coils

[Melbeta]

This brings up a question in my mind. Would it be better to use a solid wire? If not, what kind of wire would you suggest?
Melbeta

[bbsailor]

Thank you. So that correspond to AWG28, not the very commonly used awg24 i can see in most of the surf pi posts?Or awg 28 applies only for the litz?

Look up on the web “wire skin effect”. This will explain how as the frequency gets higher the less penetration to the center of the wire.

In in relation to PI metal detectors is the minimum delay you need for detecting very small targets. At very low delays the coil wire itself can hold eddy currents. If your delay is set to a lower value than the coil retained eddy currents, the coil wire will respond like a target.

The Litz wire stranding related to frequency is a way to get the required coil wire gauge size to carry the required current but also select a stranding size to use the full potential of each strand at the operational frequency range but to also minimize the wire bundle from holding larger amounts of eddy currents if the wire were a solid larger diameter type of wire or the strands were too large for the frequency range.

The key test is to test wire samples an inch or two long and see if they respond like a target at your lowest delay. Litz wire is a wire type that uses each strand in parallel to carry the required current but also minimize any retained eddy currents in the individual small strands.

Joseph J. Rogowski

[nonkapo]

Thank you. So that correspond to AWG28, not the very commonly used awg24 i can see in most of the surf pi posts?Or awg 28 applies only for the litz?

[Qiaozhi]

Thanks for the interesting topic guys and Joseph in particular;-). I start thinking of SWR meter at the coax(kidding. My question is what is the typical frequency Surf PI 1.2 is working on,so i can choose the proper litz wire for the coil. The amount of information i get here and in the net is only confusing me.The bunch of standards(AWG,SWG etc) is only making the stuff harder. So i see somewhere surf pi operates on 15khz but for sure it can be adjusted in some border as well. Than where is the golden middle or typical frequency range it will work and is still the AWG30 maximum/minimum size of wire advice applicable for litz too? Thanks.

The pulse rate of the Surf-PI is nowhere near 15kHz. In fact, the Surf-PI uses the same oscillator to generate the audio frequency.
If you look at the 555 timer circuit, you will see that R1=100k, R2=3k3, and C1=22nF. These values give a pulse rate of 622pps.

[nonkapo]

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

Thanks for the interesting topic guys and Joseph in particular;-). I start thinking of SWR meter at the coax(kidding. My question is what is the typical frequency Surf PI 1.2 is working on,so i can choose the proper litz wire for the coil. The amount of information i get here and in the net is only confusing me.The bunch of standards(AWG,SWG etc) is only making the stuff harder. So i see somewhere surf pi operates on 15khz but for sure it can be adjusted in some border as well. Than where is the golden middle or typical frequency range it will work and is still the AWG30 maximum/minimum size of wire advice applicable for litz too? Thanks.

[Sean_Goddard]

Try using TWIN-AX ;-)

[phred01]

below 10uS i have frustrated Coax Detected

Using a coaxial cable doesn't become a silver bullet in all cases. To get the benefit of coaxial shielding etc a basic requirement must be implimented to take advantage of shielding & signal delivery. In radio work the driving & receiving circuits must present the impedance that of the coaxial cable used otherwise there will ring/standing waves that will interfere with signals < 10us. Assuming u have PI that delivers a current into the coil so some of the energy ends up on the coaxial cable flowing up & down the length coaxial cable (ringing) only after it dies down that u will c the return signal from the target. So the solution is put ur amp @ the coil's o/p & c if there is an improvement. If this helps then do impedance matching @ both ends of the cable

[bbsailor]

Why should Tx power(4 above) have an effect on the damping resistor. I was thinking the resistor damped a LC resonance and power would have little effect.

This is just an observation based on seeing the lower peak fly back voltage when the coil DCR and series resistor DCR cause a lower fly back voltage below 100V and short TX pulse widths between 30 to 60 uS at about 3000 to 10000 PPS allows for higher value damping resistors. The CS6PI is a low power example of this that has the potential to be made faster than its initial design.

L/C is a easily measurable variable to speed up a coil but power to be damped also plays a role maybe to a lesser degree. You can easily test this by putting a resistor in series with the coil to reduce the fly back voltage and see how changing just this variable affects the damping resistor value.

Joseph J. Rogowski

[green]

As delays drop to 10 uS and below coil construction and attachment to the coax cable requires some creative designs to prevent the coax cable and solder joint to the coil from being detected. Some forum members extend the coil wire as a twisted pair up to the control box to eliminate this joint. If you can use this twisted pair design all depends on the noise in the area where you are detecting.

Here are the things that collectively allow early sampling.

1 Fast preamp response after coming out of saturation. Using a dual stage, low gain and higher preamp voltage power source will tend to speed this up.
2. Minimize all capacitance in the TX circuit including coil turn to turn capacitance; coil to shield capacitance; coax capacitance, coil driver MOSFET COSS.
3. A higher damping resistor value indicates less capacitance to damp and thus a faster coil.
4. Power of the TX pulse. Higher power TX pulses need lower damping resistor values.
5. The soil where you are detecting. Wet salt sand will respond at about 15 uS delay.

There is no single right answer. It all depends of what targets you are seeking, the electrical noise environment, the soil environment and size of the coil relative to the target size.

Read various coil forum posts to see how other forum members have attempted to speed up their coils.

Joseph J. Rogowski

Why should Tx power(4 above) have an effect on the damping resistor. I was thinking the resistor damped a LC resonance and power would have little effect.

[bbsailor]

below 10uS i have frustrated Coax Detected

As delays drop to 10 uS and below coil construction and attachment to the coax cable requires some creative designs to prevent the coax cable and solder joint to the coil from being detected. Some forum members extend the coil wire as a twisted pair up to the control box to eliminate this joint. If you can use this twisted pair design all depends on the noise in the area where you are detecting.

Here are the things that collectively allow early sampling.

1 Fast preamp response after coming out of saturation. Using a dual stage, low gain and higher preamp voltage power source will tend to speed this up.
2. Minimize all capacitance in the TX circuit including coil turn to turn capacitance; coil to shield capacitance; coax capacitance, coil driver MOSFET COSS.
3. A higher damping resistor value indicates less capacitance to damp and thus a faster coil.
4. Power of the TX pulse. Higher power TX pulses need lower damping resistor values.
5. The soil where you are detecting. Wet salt sand will respond at about 15 uS delay.

There is no single right answer. It all depends of what targets you are seeking, the electrical noise environment, the soil environment and size of the coil relative to the target size.

Read various coil forum posts to see how other forum members have attempted to speed up their coils.

Joseph J. Rogowski

[Wahyu]

When operating pulse induction metal detectors at low delays, below 10 micro seconds, any lingering eddy currents in the coil wire itself could be detected. Typically, thinner strand wire below about AWG30, will not hold many eddy currents. Using thin tinned strands puts a slight resistance between the strands and minimizes eddy currents in the tinned stranded wire bundle. Silver coated stranded wire does not offer these benefits as silver is a good conductor. Litz wire is good but you need to make sure that each strand of Litz wire is making a good coil connection to have the maximum current flowing.

Joseph J. Rogowski

below 10uS i have frustrated Coax Detected

[phred01]

Yes, that's true but it is very important that each stand is enameled (ie insulated from the other strands) otherwise the advantages of litz wire is negated. Good litz wire has a excellent feature that the cloth & enamel will melt and tins easily

[bbsailor]

Basically litz wire is use in making coils in radio applications. In practice current does not flow evenly thru the cross section of a wire but tends to use the surface of the wire....Known as skin effect. By bundling thinner wires the surface area is considerably higher for the same wire diameter. This translates to lower resistance to higher frequencies. Suggestion is to use proper litz wire for better predictable performance. BTW commercial litz wire uses very fine strands and is bound with a cloth type material sheath whilst still maintaining high flexibility

When operating pulse induction metal detectors at low delays, below 10 micro seconds, any lingering eddy currents in the coil wire itself could be detected. Typically, thinner strand wire below about AWG30, will not hold many eddy currents. Using thin tinned strands puts a slight resistance between the strands and minimizes eddy currents in the tinned stranded wire bundle. Silver coated stranded wire does not offer these benefits as silver is a good conductor. Litz wire is good but you need to make sure that each strand of Litz wire is making a good coil connection to have the maximum current flowing.

Joseph J. Rogowski

[phred01]

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

Basically litz wire is use in making coils in radio applications. In practice current does not flow evenly thru the cross section of a wire but tends to use the surface of the wire....Known as skin effect. By bundling thinner wires the surface area is considerably higher for the same wire diameter. This translates to lower resistance to higher frequencies. Suggestion is to use proper litz wire for better predictable performance. BTW commercial litz wire uses very fine strands and is bound with a cloth type material sheath whilst still maintaining high flexibility

[alex61]

Hi all I haven't posted here before but have been following for a while Id like to mention while making my own litz wire I also found it tended to twist into a heap .so on one of my experimental coils for a minelab I whipped a very thin spread of engine gasket silicon then applied the twist and let it dry .what I found is the wire was easier to use like a lenth of rubber .the coil seams to work ok .I don't no however because I don't have a scope if there is any negative effects .