Question on MPP coil

[frenchy]

is there any info on how to wind a 3DSS coil ?

[geoscash1]

Here are 2 threads that have all the info you need.

http://www.geotech1.com/forums/showt...Chance-PI-coil
http://www.geotech1.com/forums/showt...size-for-coils






some pics of my last 3dss @ 7-3/4" round with 250/46 litz

[baum7154]

Hi Geoscash,

Nice work on your 3DSS coils. Did the Litz show a significant improvement over the teflon wire?

[geoscash1]

Hi Geoscash,

Nice work on your 3DSS coils. Did the Litz show a significant improvement over the teflon wire?

Hi Dan,

yes, in my testing and coil building I find the litz to be more sensitive to small gold than the Teflon silver plated. Also I have built them using the different size litz and find that the 175/46 and the 250/46 are the best sizes to use. Both work well but in retrospect I would use 175/46 as it seemed to have the best sensitivity.

All my tests were done with modified MPP's and I have them setup specifically for small gold. With the same size coil in litz and one in Teflon the litz won by over 1 inch in air tests with small gold test nuggets of 1 gram and 8 grain.

I have all my builds setup so I can interchange coils so these also have been tested with Svens Mirage with very good improvement too.

I bought 3000 ft of the 600v 24awg silver plated wire and built numerous coils but my personal opinion is the litz is just a bit better and worth the extra investment.

[bbsailor]

Everyone building coils,

Silver plated stranded Teflon insulated wire starts to look like a solid wire because the silver plating has very good conductivity between strands and will sustain eddy currents much better than tinned strands. This is because the tin plating on the strands produces a higher resistance between strands and tends to minimize the sustaining eddy currents in the wire core area itself.

When the delay gets lower (typically below about 7uS) the eddy currents sustained in the coil wire becomes more critical. One way to visualize this is to look at the size of the small gold nuggets or jewelry links that you want to detect and compare their size to the size of your coil wire. When they are close to the same size then Litz will allow lower delays because each strand is independently isolated from each other and all strands carry the current in parallel.

Another trick is to move the coil wire connection to the coax cable about 9 inches to 1 foot up the shaft (and outside the outer coil circumference) to not have the large solder mass of the coil and coax connection to be detected as a target at very low delays.

If using a ferrite core to make a probe, only use soft ferrites as they will not sustain the TX pulse energy into the RX time as other harder ferrites will. Also, ground and shield the ferrite probe to minimize noise.

i hope this helps?

Joseph J. Rogowski

[geoscash1]

Everyone building coils,

Silver plated stranded Teflon insulated wire starts to look like a solid wire because the silver plating has very good conductivity between strands and will sustain eddy currents much better than tinned strands. This is because the tin plating on the strands produces a higher resistance between strands and tends to minimize the sustaining eddy currents in the wire core area itself.

When the delay gets lower (typically below about 7uS) the eddy currents sustained in the coil wire becomes more critical. One way to visualize this is to look at the size of the small gold nuggets or jewelry links that you want to detect and compare their size to the size of your coil wire. When they are close to the same size then Litz will allow lower delays because each strand is independently isolated from each other and all strands carry the current in parallel.

Another trick is to move the coil wire connection to the coax cable about 9 inches to 1 foot up the shaft (and outside the outer coil circumference) to not have the large solder mass of the coil and coax connection to be detected as a target at very low delays.

If using a ferrite core to make a probe, only use soft ferrites as they will not sustain the TX pulse energy into the RX time as other harder ferrites will. Also, ground and shield the ferrite probe to minimize noise.

i hope this helps?

Joseph J. Rogowski

Thank you Joseph for chiming in, good info as usual and it is much appreciated.

All of my coils I keep 36/40 inches of tag line or feed line directly from the coil as the leads to my coil plug on my rear panels, twisted at a rate of 3 twists per inch so I have no breaks or solder points any where near the coils or coil housing. Seems to work very well.

I believe Dan (baum7154) came up with the twisting of the leads, not sure but it was something I had been doing for awhile. It keeps the sensitivity at max or unsaturated keeping all solder points away from the coil itself.

[green]

Hi Dan,

yes, in my testing and coil building I find the litz to be more sensitive to small gold than the Teflon silver plated. Also I have built them using the different size litz and find that the 175/46 and the 250/46 are the best sizes to use. Both work well but in retrospect I would use 175/46 as it seemed to have the best sensitivity.

All my tests were done with modified MPP's and I have them setup specifically for small gold. With the same size coil in litz and one in Teflon the litz won by over 1 inch in air tests with small gold test nuggets of 1 gram and 8 grain.

I have all my builds setup so I can interchange coils so these also have been tested with Svens Mirage with very good improvement too.

I bought 3000 ft of the 600v 24awg silver plated wire and built numerous coils but my personal opinion is the litz is just a bit better and worth the extra investment.

Hi geoscash,
Have you looked at amplifier out with a scope with the different coils? I did some tests awhile back in another thread.http://www.geotech1.com/forums/showt...725#post213725
The amplifier did come out of saturation sooner with the litz coil. After the coils came out of saturation the coils looked close to the same with 175/46 litz, 28awg solid enamel coated and 24awg solid pvc coated. Wondering why you are seeing a difference. Maybe a no target scope trace would show something, maybe not.

[baum7154]

Geoscash1, you are correct that I did develop the continuation of the coil wire as the feed in order to minimize capacitance, max the coil speed, and eliminate solder joints in the coil. This was after much disappointment with all kinds of coax and other cable. There are some other refinements I am still working on to enhance 3DSS coil speed. One concern I have with Litz is that I have not found it in 600 volt PTFE insulation. I have found that the thicker PTFE increases wire to wire separation and hence coil speed. Would be glad to try Litz in this format when I find it.

Does the detection advantage of Litz diminish with testing targets in soil? The method I use to replace air testing was initially suggested by 6666.

1. Place the coil on a 3/4" wood spacer on 'clean ground'

2. Place a plastic ruler vertical in the coil.

3. Note the measurement of the top of the coil.

4. Present the target to the coil on the end of a plastic or dry wood yardstick.

5. For consistency and repeatability, use two spacers (Small cardboard boxes) of desired sizes representing detection distances and bridge them with the target yardstick as the target is moved over the coil.

6. Note the detection distance on the vertical ruler and subtract the STEP 3 measurement from it.

Photos of this setup in my basement are posted in the Chance PI Coil thread.

Regards,

Dan

[geoscash1]

Hi Green,

You know, I did use my scope to base my thinking off of, I don't remember if I took any screen shots or pics of the scope at the time, I'll have to look thru my files.

The big thing that was very noticeable was the recovery of the litz wire coils versus the Teflon coated wire or ptfe plated wire coils. Now one thing I didn't mention was the mmp boards were modified, using 30 and 34us TX pulse, freq/pps set to 3000 along with sample delay set at min 7us, both sample widths set to 15us, secondary sample set to 147us.

With 2 boards set exactly the same, the 3 coils used were 7-3/4 round, one with 24awg 600v ptfe, one with 175/46 litz and the last with 250/46.

During initial tests (at least from my limited knowledge stand point) both litz wound coils, at output of opamp would return to 0 within 3 to 4us whereas the ptfe coil was around 10us before return to 0. The part that was very noticeable was the very quick recovery from a target signal to 0, it was snappy, very crisp and clean. Again, I don't have the knowledge or know the technical terminology to use for some of the things I mentioned so it may sound odd to educated ears.

I tried adjusting the TX pulse up, reduced the freq/pps and basically after much messing around found a sweetspot for the litz wire coils with my mpp's the TX pulse set to 34us, the sample widths set to 15us, minimum sample delay set to 7us was best and fastest with the cleanest and most crisp response and air test depths. With those same settings and using the ptfe coil, I lost sensitivity, air depth loss was extreme for some test objects and especially my small gold.

So, I hope that sort of answered your question, I'll try to dig up my scope shots if I took some, which normally I do for my own records so just have to find them, when I do I'll post what have.

Again, jus my uneducated opinion, my litz coils smoked my ptfe coils while using the same exact settings on 2 exactly the same built mpp boards and the scope showed clearly which of the 2 coil types was best (or to me seemed to be the best) for those particular boards. I did try them with my mirage builds as well, 2 different mirage boards, different settings and both clearly excelled with the litz wire as well.

I wish I knew more about how to explain things and had a better understanding of some of this stuff, I sure love building these things and do enjoy learning it hands on. With guys like you and George or Carl and Eric it is a blast to know that at 51 yrs old I am being educated by some of the best in the business and sincerely appreciate everything this forum has offered me as a member.

[baum7154]

Hi Geoscash,
are your Litz and PTFE coils about the same inductance and resistance? Also are each of the coils optimally damped or did you use the same damping resistance value for all three coils? It is critical that each coil be optimally damped. A higher coil resistance will also limit coil current and require a longer TX pulse to achieve the same coil energy. It would be interesting to know the Self Resonant Frequency of each of the coils.

Dan

[geoscash1]

Hi Geoscash,
are your Litz and PTFE coils about the same inductance and resistance? Also are each of the coils optimally damped or did you use the same damping resistance value for all three coils? It is critical that each coil be optimally damped. A higher coil resistance will also limit coil current and require a longer TX pulse to achieve the same coil energy. It would be interesting to know the Self Resonant Frequency of each of the coils.

Dan

Hi Dan,

I'll answer those in the order asked.

1 - Are litz and ptfe coils inductance about the same - Yes, all within 10uh of each other, 304uh to 312uh. Resistance 2.1 to 2.6 ohm.

2 - Are the coils critically damped or using the same damping - No, each coil was critically damped using my adjustable damping tool so each coil used a different damping resistance.

As far as self resonance, I have no idea how to check or test for that or if I even have the proper tools to do so..

[green]

Hi Green,

You know, I did use my scope to base my thinking off of, I don't remember if I took any screen shots or pics of the scope at the time, I'll have to look thru my files.

The big thing that was very noticeable was the recovery of the litz wire coils versus the Teflon coated wire or ptfe plated wire coils. Now one thing I didn't mention was the mmp boards were modified, using 30 and 34us TX pulse, freq/pps set to 3000 along with sample delay set at min 7us, both sample widths set to 15us, secondary sample set to 147us.

With 2 boards set exactly the same, the 3 coils used were 7-3/4 round, one with 24awg 600v ptfe, one with 175/46 litz and the last with 250/46.

During initial tests (at least from my limited knowledge stand point) both litz wound coils, at output of opamp would return to 0 within 3 to 4us whereas the ptfe coil was around 10us before return to 0. The part that was very noticeable was the very quick recovery from a target signal to 0, it was snappy, very crisp and clean. Again, I don't have the knowledge or know the technical terminology to use for some of the things I mentioned so it may sound odd to educated ears.

I tried adjusting the TX pulse up, reduced the freq/pps and basically after much messing around found a sweetspot for the litz wire coils with my mpp's the TX pulse set to 34us, the sample widths set to 15us, minimum sample delay set to 7us was best and fastest with the cleanest and most crisp response and air test depths. With those same settings and using the ptfe coil, I lost sensitivity, air depth loss was extreme for some test objects and especially my small gold.

So, I hope that sort of answered your question, I'll try to dig up my scope shots if I took some, which normally I do for my own records so just have to find them, when I do I'll post what have.

Again, jus my uneducated opinion, my litz coils smoked my ptfe coils while using the same exact settings on 2 exactly the same built mpp boards and the scope showed clearly which of the 2 coil types was best (or to me seemed to be the best) for those particular boards. I did try them with my mirage builds as well, 2 different mirage boards, different settings and both clearly excelled with the litz wire as well.

I wish I knew more about how to explain things and had a better understanding of some of this stuff, I sure love building these things and do enjoy learning it hands on. With guys like you and George or Carl and Eric it is a blast to know that at 51 yrs old I am being educated by some of the best in the business and sincerely appreciate everything this forum has offered me as a member.

Thanks for the info. Looks good what you're doing. Including the scope recordings before and after subtracting the no target recording from the target recording. Don't remember if they were before or after shielding. They weren't critically damped. Don't look very clean, overshoot and oscillation. Scope was external triggered(zero time) when signal to fet driver commanded of. The main thing I was looking for, does the wire act as a target? The amplifier comes out of saturation at about 2.5usec with the litz. All three were out at about 5usec. I wasn't trying to sample before 5 or 6usec so I assumed litz wasn't necessary. I didn't try 24awgTeflon. Yours and my test aren't the same. I'm just trying to understand what's different to make your litz test a lot better. Maybe someone could suggest what I'm missing.

[geoscash1]

Thanks for the info. Looks good what you're doing. Including the scope recordings before and after subtracting the no target recording from the target recording. Don't remember if they were before or after shielding. They weren't critically damped. Don't look very clean, overshoot and oscillation. Scope was external triggered(zero time) when signal to fet driver commanded of. The main thing I was looking for, does the wire act as a target? The amplifier comes out of saturation at about 2.5usec with the litz. All three were out at about 5usec. I wasn't trying to sample before 5 or 6usec so I assumed litz wasn't necessary. I didn't try 24awgTeflon. Yours and my test aren't the same. I'm just trying to understand what's different to make your litz test a lot better. Maybe someone could suggest what I'm missing.

Hi Green,

Well, I wish I was more knowledgeable on the subject. Maybe the only difference is I was using completed PI boards that I spent many many hours tweaking and tuning for best performance after modifying and using closely matched parts along with all 1% resistors. The coils were wound in the same manner as well, using the same force so to speak so the wire laid in tightly wound as close as I could, along with close uH and resistance. Maybe all those things put together relate to the good performance & bench test results along with the fact it was well after 1 in the morning so electrical interference was low???

As soon as the weather is better and God willing I am healthy enough to get out this coming spring I can get some field testing done too. Since my cancer has come back faster than anyone expected I will be starting chemo and radiation this coming week so not sure on how well I will be, hoping good...

I did look thru my files and found a couple scope shots along with a 15 second clip of the target reaction for the 250/46 litz coil using my 1 gram test nugget. Other scope shots were not labeled even though they were the mmp coil shot results, I wont post those just because I'm not sure which is which or what is what.

[baum7154]

Everyone building coils,

Silver plated stranded Teflon insulated wire starts to look like a solid wire because the silver plating has very good conductivity between strands and will sustain eddy currents much better than tinned strands. This is because the tin plating on the strands produces a higher resistance between strands and tends to minimize the sustaining eddy currents in the wire core area itself.

When the delay gets lower (typically below about 7uS) the eddy currents sustained in the coil wire becomes more critical. One way to visualize this is to look at the size of the small gold nuggets or jewelry links that you want to detect and compare their size to the size of your coil wire. When they are close to the same size then Litz will allow lower delays because each strand is independently isolated from each other and all strands carry the current in parallel.

Another trick is to move the coil wire connection to the coax cable about 9 inches to 1 foot up the shaft (and outside the outer coil circumference) to not have the large solder mass of the coil and coax connection to be detected as a target at very low delays.

If using a ferrite core to make a probe, only use soft ferrites as they will not sustain the TX pulse energy into the RX time as other harder ferrites will. Also, ground and shield the ferrite probe to minimize noise.

i hope this helps?

Joseph J. Rogowski

_________________________________________

Hi Joe and Geoscash,

In researching available silver plated stranded PTFE insulated wire I found that there are at least two levels of silver plating. The standard seems to be a very thin sliver plate on each wire but there is also a much thicker version called 'cladding'. It is enough to bind the strands together so that they won't easily separate in the wire bundle. This is not a good wire to use because of the eddy current problem.

Just for fun I took a 51 1/2" scrap of my 24awg silver plated PTFE 600 volt insulated coil wire and presented it to my CHANCE PI detector and it was not seen at a 7us sample delay...even coiled up and laying on the search coil. I then presented a .25 X .25"" square of the sidewall of an aluminum soda can to the search coil with and without the wire scrap test target laying on the coil and in both cases its was readily detected at a few inches. I should mention that my coil was resting on a 'metallically clean' spot of my basement floor so this is not an air test. The detector was allowed to warm up for about 6 minutes and then zeroed to the floor. Thinking that a bigger gauge of silver plated wire would be easier to detect I took a piece of 22AWG silver plated wire 54" long and it too went undetected.

In desperation I took 73 feet of 18 gauge silver plated wire and it was detected about 20". I think wire gauge, length, and coil wind are significant factors in eddy current strength and smaller gauges create much less of it.

I found a 40 foot length of the 24 awg wire I use for my coils and it was wound in a 4.5" dia. loose bunch and it was detected at about 6". I am going to wind it in a larger loop to see the effect.

Rewound the 40' length of 24 awg coil wire into an 8" diameter bunch wound coil and it is detected at 7".

Took my 7.75" 335uh round 3DSS coil with feed (24 awg PTFE) and it is detected at about 9". (approximately 80' of wire)

It would be interesting to see the detection distance of the 7.75" dia. Litz 3DSS coil as a target and at what sample delay it is detected.

Thanks,
Dan

[bbsailor]

_________________________________________

Hi Joe and Geoscash,

In researching available silver plated stranded PTFE insulated wire I found that there are at least two levels of silver plating. The standard seems to be a very thin sliver plate on each wire but there is also a much thicker version called 'cladding'. It is enough to bind the strands together so that they won't easily separate in the wire bundle. This is not a good wire to use because of the eddy current problem.

Just for fun I took a 51 1/2" scrap of my 24awg silver plated PTFE 600 volt insulated coil wire and presented it to my CHANCE PI detector and it was not seen at a 7us sample delay...even coiled up and laying on the search coil. I then presented a .25 X .25"" square of the sidewall of an aluminum soda can to the search coil with and without the wire scrap test target laying on the coil and in both cases its was readily detected at a few inches. I should mention that my coil was resting on a 'metallically clean' spot of my basement floor so this is not an air test. The detector was allowed to warm up for about 6 minutes and then zeroed to the floor. Thinking that a bigger gauge of silver plated wire would be easier to detect I took a piece of 22AWG silver plated wire 54" long and it too went undetected.

In desperation I took 73 feet of 18 gauge silver plated wire and it was detected about 20". I think wire gauge, length, and coil wind are significant factors in eddy current strength and smaller gauges create much less of it.

I found a 40 foot length of the 24 awg wire I use for my coils and it was wound in a 4.5" dia. loose bunch and it was detected at about 6". I am going to wind it in a larger loop to see the effect.

Rewound the 40' length of 24 awg coil wire into an 8" diameter bunch wound coil and it is detected at 7".

Took my 7.75" 335uh round 3DSS coil with feed (24 awg PTFE) and it is detected at about 9". (approximately 80' of wire)

It would be interesting to see the detection distance of the 7.75" dia. Litz 3DSS coil as a target and at what sample delay it is detected.

Thanks,
Dan

Dan,

Good to see you probing the effects of delay on target size. Here is a subtle point to consider when using raw wire coils to see if they are detected. Put a resistor of one half the damping resistor (Rd) value across the wire ends and place that resistor outside your detection area to see if the wire is detected when loaded by a typical damping resistor value and the input resistor (typically 1K ohm) which in parallel with Rd due to the clamping diodes.

Small targets have low time constants and much of their signal may drop during the delay time. If a 2uS target is detected 10 uS (delay time) later then it will fall to the noise level in 5 TCs and not be detected. But now, if you can detect that same target at 5uS then you may be able to detect the tail end of that target's decay if the coil wire decay is faster than that targets decay and not masking it. That is why if you drop the delay on some widely adjustable PI delay circuits you may reach a point where the detection suddenly locks up. This could be below the lowest level your circuit can work at and it may be caused by several things: (1) the coil wire itself being detected, (2) the preamp time coming out of saturation, (3) detecting the coax wire or shield material, (4) poor damping, (5) target too small for PI circuit design, response and coil size used.

Remember one key thing about small targets. They need a discharge coil TC that is 5 times faster than the target TC to fully stimulate that target for maximum potential detection. A 2uS target needs a coil discharge TC of 0.4uS. A 300uH coil needs a damping value of 750 ohms to meet this criteria and that means that low a capacitance coil and TX circuit is required to achieve this.

Keep up the good work and make a PI machine that will go down to about 5uS or 6uS delay and use this to test various target samples and wire samples to determine their actual time constant. Once you know the actual TC of your desired target you can do some reverse engineering to optimize your machine parameters (TX pulse width, RX window width, delay time, opamp gain and SNR) along with the optimum coil size for that particular target.

Joseph J. Rogowski