Looking for a solution to improve slow coil electronically to let it decay fast and keep R-dump high

[Waikiki_Sweep]

From memory it was about 4-5mm. The shield material is aluminum braided wire which I removed from some TV cable called RG6 quad. The wire showed next to no response when waved over the coil.

Good for you! 4-5mm spacer will add a centimeter to my coil thickness I will brake my arms to move such coil in water.

[green]

Interesting subject. I think I remember reading the coil (circuit) resonance should be at least 5 times faster than the target resonance. Target resonance equals .159/target time constant. Target TC = 1usec, target resonance = 159khz. Circuit resonance needs to be greater than 5*159000 =800khz. 2usec TC =400khz. I'm thinking target TC determines minimum coil (circuit) resonance. A critically damped coil decays to near zero volts in 3/circuit resonance L=300uh, C=500p, resonance=410khz, decay time=7.3usec (I/T=E/L) I think the fastest way to get to zero current is to disconnect the damping and input resistor and let the fet clamp voltage decay the current. The problem is when the current reaches zero the capacitance is at clamp voltage. If we could short the coil at exactly the right time to decay the capacitance it would speed the decay up. Takes a lot of added circuit and I'm not sure its possible except maybe on the bench.

[green]

Was wondering if shield material effected resonance. Graphite shield is higher resistance than aluminum foil. Wondered if adding resistance from shield to ground might help. I have a single layer coil wound on a wood hoop. 9.3 inch diameter, .25 inch thick, .5 inch wide. (20 turns of awg 28 magnet wire, L=280 uh, R=3.4 ohms, SRF 2.24Mhz). I don't have any graphite so I wrapped the coil form with aluminum foil, no spacer between wire and foil on one side, .25 inch wood between wire and foil on the other side. foil not grounded SRF 1.68 Mhz, foil grounded SRF 1 Mhz, the resonance damped out in a couple cycles with a 1k resistor foil to ground. Changing resistor value does change things, if it helps I don't know. Its not spiral wrapped but it is single layer with foil next to wire on one side. Just learning about coils, but wondering why your coil is so slow.

[Waikiki_Sweep]

Was wondering if shield material effected resonance. Graphite shield is higher resistance than aluminum foil. Wondered if adding resistance from shield to ground might help. I have a single layer coil wound on a wood hoop. 9.3 inch diameter, .25 inch thick, .5 inch wide. (20 turns of awg 28 magnet wire, L=280 uh, R=3.4 ohms, SRF 2.24Mhz). I don't have any graphite so I wrapped the coil form with aluminum foil, no spacer between wire and foil on one side, .25 inch wood between wire and foil on the other side. foil not grounded SRF 1.68 Mhz, foil grounded SRF 1 Mhz, the resonance damped out in a couple cycles with a 1k resistor foil to ground. Changing resistor value does change things, if it helps I don't know. Its not spiral wrapped but it is single layer with foil next to wire on one side. Just learning about coils, but wondering why your coil is so slow.

Thank you for your input. My biggest problem is to make a coil with best hydrodynamic properties to let it move in water without drag.
As I see no MD company move a finger to help detectoricsts searching in water to stop apply unbelievable physical forces to move detectors in water.

I have made several coils with good results but best for hydrodynamic is a flat spiral which is 4-5 mm. thick including shield and fiberglass protection on bottom layer.

But such a thin coil has no spacer between shield and windings. As result shield forms capacitors with begin and end of spiral windings converting coil to LC circuit and making it slow.
Sure graphite is best for the shield. Also all coil surface have to be shielded or salt water will find a place where to form capacitor to ground and detector will start chattering.
Resistor between shield and ground may help a little but because start and end of windings are close to the shield there will be a capacitor between them anyway even if I disconnect a shield.

So now I maneuvering between coil construction and schematic solutions to continue to keep coil as thin as I can but make it decay faster to catch smaller gold items in salt water.

[green]

I see where you want to have 3 to 4 mm thick. What outside dimensions are you trying for?

[Waikiki_Sweep]

I see where you want to have 3 to 4 mm thick. What outside dimensions are you trying for?

Ooo... It is pretty huge. 17" x 9". I like to cover some area too.

[Waikiki_Sweep]

Finally I have calibrated my oscilloscope and made some accurate measurement for my new flat spiral coil 18 turns AWG 24 stranded 19 strands Teflon 17" x 9" x 5 mm. thick 300uH with no spacer between shield and windings, graphite shield and Belkin coaxial cable. (You can see it at the back on the photos of my coils. One with gap in graphite shiels visible through fiberglass)

Resonant frequency without damping is 435 kHz and with usual SurfPI gain=1000 preamp saturation lasts only 15 uS. (With 220 Ohm damping resistor which is to low for sure and have to be replaced with higher value)
Looks not so bad. Much better than I have expected!
Probably because of use Teflon insulated wires internal capacitance is not too big.


And I am sure later I can even make it faster with reducing preamp gain and underdamping.

12-15 uS delay looks OK for salt water to prevent chatters.

Will go hunting with it soon.

[6666]

WS whats the 00 coil for ?

[Waikiki_Sweep]

WS whats the 00 coil for ?

17" x 13" homemade SEF for waterproofed Fisher CZ-6 Quscksilver is a killer.

[6666]

17" x 13" homemade SEF for waterproofed Fisher CZ-6 Quscksilver is a killer.

Thanks, cheers

[punagold]

Waikiki Sweep,

Can you elaborate on your graphite shield? I was thinking about using a carbon twill to stiffen up a large coil and I was thinking that it might also suffice as a good shield material. I also looked into adding graphite into epoxy, but the problem there is if it doesn't work, the whole project would have to be tossed out. How did you make your graphite shield? Thanks

BTW, I really admire your coils. I know what you mean about drag in the water. I've had 3 shoulder surgeries. Don't like that drag.

[sinclairuser]

Waikiki Sweep,

Can you elaborate on your graphite shield? I was thinking about using a carbon twill to stiffen up a large coil and I was thinking that it might also suffice as a good shield material. I also looked into adding graphite into epoxy, but the problem there is if it doesn't work, the whole project would have to be tossed out. How did you make your graphite shield? Thanks

BTW, I really admire your coils. I know what you mean about drag in the water. I've had 3 shoulder surgeries. Don't like that drag.

waikiki sweep has not posted in almost a year, i think he still visits thunting, but he does not come here anymore, he was quite badly insulted and left, just another good contributor lost due to stupidity.

[6666]

waikiki sweep has not posted in almost a year, i think he still visits thunting, but he does not come here anymore, he was quite badly insulted and left, just another good contributor lost due to stupidity.

Yes he was a good contributor

[punagold]

That's too bad. He seems to know what he is doing. Loved those coils he made. I have come across some crazy insulting comments digging through the archives. Seeing as how this forum has members all across the globe, I suppose it's not hard to find those with a chip on their shoulder, but it's too bad, cause we all are trying to achieve the same goal. Even worse for those of us who are clueless looking for answers.

[sinclairuser]

the best advice here is stick to the topics at hand, dont take comments or even insults to heart, quite a few misunderstandings here are caused by discussing topics other than detectors.
when you add translators like google and bing etc, it gets worse, there used to be a few troublemakers who used to come here just stir up crap, thankfully they have gone.
so now its a better place, we need to attract a few good guys back and it will be perfect.