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I'm wanting to build a faster (shorter TC) SS3D coil for my modded Minipulse Plus & need to get wire. I know the wire I used in the past was not optimum: 1) the wire size was too small (26AWG) resulting in a ohms/MFT that was too high (my coils ended up having around 3.1 ohms resistance). 2) the PTFE insulation is probably too thick (0.06" total wire & PTFE thickness) resulting in wires that are spaced too far apart (good for reducing coil capacitance though). I believe the wire I used was triple thickness PTFE. So I want to purchase new wire & I'm looking for thoughts on: 1) the optimum wire size in AWG and stranding configuration 2) the minimum PTFE thickness without compromising (increasing) the coil capacitance. Phoenix Wire and Cable has a nice ordering chart that includes PTFE thicknesses (1, 2, &3 layer/thicknesss) as well as their ohms/MFT - as an example of what info is needed to order (let me know if you want the url to their chart). Many Thanks!
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You can build fast flat spiral coil out of cheap TV coaxial cable (with removed outer insulation and shielding before) and inner (ptfe or something like this) insulation left, then you winding coil tight turn to turn, to get about 330uH:
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Wondering what coil resistance has to do with coil speed. Thinking coil resonance ( inductance and capacitance) controls speed.Originally posted by KRinAZ View PostComment
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Probably more impedance (C-reactance) than resonance.Comment
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Hi Kyle
As stated above the coil speed is dependent on the inductance and total circuit capacitances. A smaller coil wound with the same #26 AWG wire with a lower inductance might be a better choice.
The circuit time constant (TC) for charging the 360uH coil is determined by the MOSFET resistance = approximately 0.7 ohms, the MOSFET series resistor = 3.3 ohms and the coil resistance of 3.1 ohms; TC = L/R = 360 uH / 7.1 ohms = 50.7 usec. If you wind a new coil and reduce the coil resistance to 0.5 ohms the TC will = 360 uh / 4.5 ohms = 80 usec.
The time constant for coil discharge is mostly determined by the circuit resonant frequency as determined by the coil inductance and total circuit capacitance. The capacitance is a combination of coil capacitance, MOSFET/series diode capacitance, and coil cable capacitance. The discharge TC = L/R; R is the damping resistor value in parallel with the 1000 ohm input resistor. The internal 3.1 ohm coil resistance has insignificant effect on the discharge time constant. One TC is approximately 63% down the discharge slope. It takes close to 5 TCs to reach the bottom of the damped discharge curve.
Some approximate examples are;
1. With a circuit capacitance of approximately 280 pf and a 360 uH coil;
TC = 360 uH / 333 ohms (500 damping across 1000 input) = 1.08; 5 TCs = 5.4 usec
2. With a circuit capacitance of approximately 70 pf and a 360 uH coil;
TC = 360 uH / 500 ohms (1000 damping across 1000 input) = 0.72; 5 TCs = 3.6 usec
3. With a circuit capacitance of approximately 60 pf and a 300 uH coil;
TC = 300 uH / 500 ohms (1000 damping across 1000 input) = 0.60; 5 TCs = 3.0 usec
4. With a circuit capacitance of approximately 60 pf and a 250 uH coil;
TC = 250 uH / 500 ohms (1000 damping across 1000 input) = 0.50; 5 TCs = 2.5 usec
Using the values listed in example 4 versus example 1 reduces the decay time in half. There will be some signal strength lost from having fewer turns in the coil. This loss could be compensated for by reducing the coil diameter.
Teflon #26 AWG wire is a good choice for a new coil. It can carry an average current of 7 amperes which is much higher than required. And it has low capacitance characteristics which is very important. Since you are trying to detect very small gold nuggets it might be worthwhile to reduce the new coil size to 4-6 inches similar to Gold Master or Gold Bug detector sizes. This would increase the intensity of the magnetic field and reduce the amount of ground mineralization in view around tiny nuggets.
Happy hunting,
ChetComment
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To avoid confusion on the name of this coil I believe the term 3DSS (3 Dimensional winding pattern, Self Shielding) as used in the CHANCE PI COIL thread should be used. In the past this unique coil was referred to as a basketweave or spider coil, and I was guilty of the spider coil term too. However the 3 Dimensional winding pattern is unique and as a result there is a benefit in self shielding that no basket or spider coil geometry can provide.
Regards,
DanComment
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Can you post some photos of a ss3d coil? I want to build one but i dont have idea how.Originally posted by baum7154 View PostComment
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----------------------------------------------------------------------------------------Originally posted by jladre View Post
Construction details and photos of the 3DSS coils are in the CHANCE PI Coil thread of this forum. An 8" round coil is detailed beginning with post #30 in that thread. Another version called the 'Racetrack' is posted in the last 5 pages of the thread. Many pictures are provided of both coils.
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The details of the Racetrack Coil posts are here in the Chance PI Coil thread:
3DSS (3D wind pattern - Self Shielding) Racetrack Coil Build Since this build has been going on for quite a while my posts on its construction are spread across several pages in this thread. Below are the post numbers in this thread that pertain to this build.
POST NUMBERS: 126, 134, 136, 138, 139, 142, 169, 173, 177, 179, 215, 216, 228, 229, 232, 236, 254, 255, 256, 257, 258, 262, 263.
Hopefully this will help to find construction details quickly.
Dan
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Thanks dan!Comment
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Ok thx all & especially Chet for that excellent analysis of the factors that influence the speed of the coil. That will be very useful in lowering the overall TC my MPP is capable of detecting. So I should have been more clear - I'm focusing specifically on how to make the coil itself as fast as possible - separate from the electronics & the coil's interaction with the electronics (separately I will also work on the electronics aspect). I'm thinking that ultimately for the coil's magnetic field to collapse and recover as fast as possible the electrons must flow out of the coil field as fast as possible, & the coil's impedance is going to slow down the flow. This is why I'm focused on lowering the coil impedance. I am under the impression that Minelab & Coiltek coils are sub ohm for the same inductance and I am trying to achieve this. This is why I'm looking at finding the optimum wire for the balance between impedance and capacitance for a given inductance. Many thanks! - KyleComment
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