Please tell us how your wonderful deep penetrating Surf PI 1.2 coil is made.

[Firebird]

Hi Kenny444,
I'm at that stage with my Surf PI as well, I built a 11inch/28cm coil 0.5mm enamelled winding wire 20 turns or 25cm 21 turns, haven't tested yet as waiting for new meter to arrive, got the info of this vid: https://www.youtube.com/watch?v=EWwW...ZnA1g&index=30

[kenny444]

Gentlemen. May I again thank you for the information and guidance. I will be reading and learning during the evening and just to add a happy note the LCR meter I have been waiting for has eventually arrived, so that has had me modifying my present coils. I will be looking at Litz wire and will have to determine the length required, but I do not know what size the cable is yet so it is not possible for me to use the coil calculator, but I will move on and look at the post's you have left. Appreciated.

[kenny444]

Hello KT315 If I am to substitute Litz for copper does the following sound correct please? have I the full information, is there a problem soldering or joining Litz wire. thank you.

Recommended Litz

"16 turns of .14 Litz at 174mm Radius" Coil Calcs to 371uH

So length required 2pi * R = C, C * 16 = 17.5mts of Litz

instead of

"18 turns of .5mm en/copper at 174mm Radius" Coil Calcs to 377uH"

Thank you

[kt315]

http://www.geotech1.com/forums/showt...ighlight=field
http://www.geotech1.com/forums/showt...ighlight=field
http://www.geotech1.com/forums/showt...ighlight=field

[Silver Dollar]

I think the dual field was proven to not be much better than a simple mono (possible
the damping wasn't optimized).

For a 14" housing I'd use 24 awg Teflon coated wire in a loose bundle. My initial testing
says this configuration can be adaquitly fast.

[kenny444]

Thank you for that information is that multi strand wire?, I was planning to wind and test a dual field coil trial over the holiday period, and will get there as I can. As for the 14" housing it is not here yet, but may I ask please with regard to the Surf pi 1.2 which uses a 390 ohm damping resistor, would a lower value of R make the coil faster? or should the coil be tuned specifically? Further to this I now have my LCR meter so I can check the uH inductance. This raises another question, which is "what is the overall inductance range of the SPI 1.2"? and "is there an optimum inductance"? Thank you to all who contribute and I hope that all that read this thread appreciate the input of the more knowledgeable members. Very grateful here.

[Qiaozhi]

... may I ask please with regard to the Surf pi 1.2 which uses a 390 ohm damping resistor, would a lower value of R make the coil faster? or should the coil be tuned specifically?

It is important to adjust the damping resistor for critical damping, or very slightly underdamped. The best way to do this is with an oscilloscope. If you underdamp, the signal will continue ringing and delay the point at which you can sample. If you overdamp (by using a smaller resistor), you will lose sensitivity as more of the energy stored in the coil will be dissipated in the damping resistor.

[kenny444]

Hello again, What considerations should be given to the use of 30 Awg instead of 24 Awg? Thank you again.

[baum7154]

30 AWG will result in even less capacitance but also more resistance and therefore less transmit power.

regards,

Dan

[kenny444]

Hello again, I am reading "Making a Fast Pulse Induction Mono Coil" and I wondered if someone can tell me please what the best/optimum coil delay setting might be for the Surf PI 1.2 Thank you again.

[kenny444]

Thank you.

[kt315]

Kenny,


I am feeling to tell you about ITMD copy you can get from Carl or George http://www.geotech1.com/forums/conte...metal-detector
right moment. may be it ll be best to begin reading from this book, like a start in your metaldetecting electronics understanding.
anotherway you ll only run around like a much of people here who wish to get lady-on-deck on-line all-time. if anybody can find whites double field
broken coil I am ready to open it, measure L&R and tell all details to Geotechers.

Anatloy

[baum7154]

Hello again, I am reading "Making a Fast Pulse Induction Mono Coil" and I wondered if someone can tell me please what the best/optimum coil delay setting might be for the Surf PI 1.2 Thank you again.

--------
The optimum delay setting depends on what kinds of targets you seek. Very small gold i.e. 5 grains or less requires delays in the range of 8 us.

Regards,

Dan

[kenny444]

Thank you for the guide information. I will have to look in my Christmas stocking to see if there is enough funding. Appreciated.

[kt315]

--------
The optimum delay setting depends on what kinds of targets you seek. Very small gold i.e. 5 grains or less requires delays in the range of 8 us.

Regards,

Dan

Hi All,
Below is a repost of an article I did a while back on my Tech forum.
"There are many factors that influence the design of a PI coil. However, let's decide on two starting parameters i.e. 100uS pulse width and 2kHz pulse rate. One very important factor is what the earliest sampling time is required to be, as this does have a major effect on the maximum useable coil inductance. The greater the inductance and the higher the current, the longer it takes to reduce the field to zero and the later the sample delay has to be. If you are designing a detector for beach and shallow water hunting a good minimum sample delay to aim for is 15uS. With the right components in the transmitter circuit you can then use a coil inductance of 300uH. I have found it best to wind the coil with 10/0.1 PVC insulated stranded wire (10 strand, 0.1mm strand diameter) as this minimises eddy currents generated in the wire cross section. For a 10in coil the winding resistance is about 4 ohms and with an additional 4.7 ohm 4W resistor in series (in the drive circuit) gives a coil time constant (L/R) of about 30uS. For a 100uS TX pulse width (3.3 times the coil TC) the current will have reached over 95% of its maximum value at switch off, which is a satisfactory situation.
Another important factor to decide at the outset is the type of battery and the required battery life. At 2kHz the current consumption with the above values will be about 250mA at 12V which really requires the use of 10 C cells as a minimum. You can of course design for less current and smaller batteries by using a shorter TX pulse, a higher inductance coil with higher series resistance but there will be some performance trade off. Some PI detectors, particularly earlier designs, do not use a series resistor in the coil circuit to limit the current. Is this case the pulse current is often limited by the coil inductance i.e. the current is switched off well before it has reached its final value which would be determined by the coil resistance. Alternatively, the pulse repetition rate can be lowered until the current is of an acceptable value. There are disadvantages to both of these techniques:- Inductance limiting results in the magnetic field still changing rapidly in a positive direction at the point of switch off. This can cause some signal loss due to switch on eddy currents cancelling those generated at switch off. In pulse frequency limiting, the response time and noise averaging of the detector may not be as good.
The shunt, or damping, resistor, will be determined by the resonant frequency of the coil plus stray and cable capacitance. Not being able to display formulae yet (hope to soon as scanner arrives on Monday) you could look up in a radio or electronic text book the formula for a critical damping resistor for an LC with a given resonant frequency. Or you could just try it experimentally. Start with 1k ohms and look at the receiver output on a scope. The receiver (again it depends what front end I.C. you use) should recover cleanly with no ringing before the point of sampling. Add more resistors in parallel until this is achieved. You should end up with maybe 500 ohms across the coil as described above. Use a good quality 0.5W metal film as the peak emf at switch off can reach several hundred volts and I have found that smaller wattage carbon film resistors can break down and go noisy. I have tried a cermet preset in series with a fixed resistor to give an easily variable adjustment but again the high voltage can cause burning at the wiper contact and eventual failure.
Eric.
-------

Re: depths on various metals


Posted by: Eric Foster (---.ipt.aol.com)
Date: May 26, 2000 01:26PM


Hi Jeff,
It is the characteristics of both the object and the detector that gives this strange result. Dealing with the object first. One of the fundamental laws of electromagnetics is that any change in equilibrium is resisted. If you change a magnetic field, as happens at the end of the TX pulse, then eddy currents will flow to try and maintain the field at its original value. In the absence of a metal object, the coil will produce a very high voltage in an attempt to jump between the ends of the coil and maintain the field. Hence the back emf. If there is a metal object, then a small portion of the field is coupled to the object and on field removal, currents are set up in the object to try and maintain this local field. For a metal with a relatively high resistance (nickel) the emf necessary to set up the current to maintain the field is higher than for a lower resistance metal (quarter). This results in a higher signal initially for the nickel, but which decays quicker due to the resistive losses. For the quarter, the initial signal amplitude is lower but due to the lower resistance, carries on for longer. It’s the same principle that if you have a 1K resistor, you will need 1V to get 1mA current, whereas for 100 ohm resistor, you only need 0.1V. I have tried to illustrate this in the diagram below. The two curves represent the decaying signals from a nickel and a quarter but it would apply to any objects of different conductivities. The hypothetical nickel and quarter are at the same distance from the coil and, for the reasons above, the nickel starts off at a higher amplitude. If your sampling window is at A (say 15uS) then the nickel signal is greater than the quarter, which translates into a bit more range. If you move the sample window to B (say 75uS), then the reverse is the case as the nickel signal is decaying faster. Move the sampling window to C (say 150uS) then you will only detect the quarter, although not at the same range that you would at B or A. On the Deepstar, the sampling window is moved by means of the REJECT control and that is how you can reject pull tabs and bits of foil. You will also lose thin section rings and nickels but at least it gives the user some adjustment for different conditions and objects. Most users will keep the control set at the shortest pulse delay to get maximum sensitivity and range for all objects and this will result in nickels and the majority of rings being detected at greater ranges than copper, or silver coins.
The second factor is the width of the transmitter pulse. I mentioned in an earlier post that the TX pulse should be longer than the object time constant, otherwise maximum signal will not be obtained. Most beach hunters want to find gold rings primarily and coins or silver objects are of secondary interest. Having a longer TX pulse than necessary is wasteful of transmitter current and the 100uS of the Deepstar is about right at the pulse frequency used to give good range on most rings and acceptable battery life. For the more conductive coins and particularly silver ones, the TX pulse is a bit short, hence a further small reduction in range on top of that due to the object characteristics.
Eric.