Dual coil for mpp

[green]

I think you are on to something here.
The two coils would be strongly coupled when one is completely inside the other.
But he does have them coupled with this directive "K1 L1 L2 .4".


I tried with the three damping Rs at MegaOhm values and the coil does ring at 966kHz.
I then find that there is no ringing with R2 = 1200 Ohm and Rd at 1500Ohm with R1 = 1100MegOhm (not there). This seems to me to be close to Critical damping.

So no need to have three damping Rs, Just one across L2 and one across both.

Make damping resistors 1100meg and C1=10nf, coil inductance calculates 300uH with .4 coupling and 330uH with .5 coupling.

Tried your two resistor damping. Not as clean with Y axis log scale. No difference in signal amplitude, don't see disadvantage of using three resistors. Don't know if could tell the difference between two or three resistor damping with MPP.

[green]

Have you tried a simulation using the schematic from the whites patent?
They use an Rd for inner coil and then also another Rd for overall damping of both coils. Your simulation should support the conclusions of the whites patent, otherwise something is wrong with the simulation.
My instincts tell me that three resistors, the way you have shown, is not going to be any better, because it does not isolate the damping of the inner coil with respect to the pulse drive, but it now introduces a path via a third resistor feeding back to the outer coil. This might be ideal for a "centre-tapped bipolar dual field coil", but maybe no good for mono coil, which is what the dual field coil is. It is essentially a mono coil split into two parts.

https://patents.google.com/patent/US7994789 #1 under claims(12) method for calculating the two damping resistors. I'm confused trying to calculate damping resistors.

Does coil have two resistors mounted in the coil? If yes both methods have the same number of resistors. Whites, one across 1 coil and one across both coils. My suggestion, one across each coil.

Rd in my circuit is for final damping, not mounted in coil.

[waltr]

The resistor across the inner coil needs to be in the coil housing.
The other, across both coils, could be in the coil housing or in the coil cable connector or on the PCB.

If you plan on changing coils then best to have a largish R on the PCB then add a resistor in the cable connector to match damping to that coil.
This way each coil built can have its own finial damping R value and it is simple to change coils.

I did this on the three coils, 10" mono, 10" DD & 8" concentric for my HH2 detector. The PCB has a 2k R and another R inside the coil's cable connector.
I can then simply change to the coil I want for the location I am detecting.

[green]

The resistor across the inner coil needs to be in the coil housing.
The other, across both coils, could be in the coil housing or in the coil cable connector or on the PCB.

If you plan on changing coils then best to have a largish R on the PCB then add a resistor in the cable connector to match damping to that coil.
This way each coil built can have its own finial damping R value and it is simple to change coils.

I did this on the three coils, 10" mono, 10" DD & 8" concentric for my HH2 detector. The PCB has a 2k R and another R inside the coil's cable connector.
I can then simply change to the coil I want for the location I am detecting.

Are you suggesting the Whites dual field coil could have only one resistor in the coil? I think both methods have 2 resistors in the coil and an external Rd to adjust for added circuit capacitance.

Have you tried calculating the damping resistors with the method described in the patent?

[godigit1]

So many things can be left to interpretation.
In figure 6 it shows the final wiring of the coil to a pulse source. Only two resistors.
It also says damping of the inner coil is determined by it own electrical perimiters. Simply calc the resistor and be done.
I dont see how the calculation takes into account mutual coupling.
I think my method pre criticle damping is the most accurate you can adjust later but the coil is perfectly damped before coupling its at least a head start.

So here I go again. I have a small coil that had no direction. Its now the inner coil for a 7.5 dual field.
Inner and outer coil inductance both at 135uh came out to 309uh finished inductance, inner coil 25 wraps outer 15 wraps. even this contradicts the patent as it says the total induction is equal to the summation of the two coils yet there is added capacitance from the coupling that is proven to me now.
Blew my mpp test board but its back up now, hopeing tonight Ill have time play with the different procedures discussed in this thread.

Im thinking looking at the Fet out and op amp if one procedure should be better than the other we should see it in the speed of the coil.
Is this correct, Or where should I look to see actual results other than just the op amp?
I think you could have the inner coil off and compinsate it with the outer coil or visa versa.

[Carl-NC]

Are you suggesting the Whites dual field coil could have only one resistor in the coil? I think both methods have 2 resistors in the coil and an external Rd to adjust for added circuit capacitance.

White's DF coil has one resistor in the housing for the inner coil. The "overall" resistor is in the box.

[godigit1]

I was looking at the spice circut again.
Final damping value is pretty damn close to what the dual field damps to on the MPP.

Both resistors R1 and rd are in parralel value for them calcs to 634 ohms final damping.
657 ohms is where the 12 inch dual field is damping on the Mpp. the sim is pretty close there.
What happens if you damp with 634 ohm single resistor on spice? I havent tried to learn how to do that yet? whole different tanget right now.
Thanks

[green]

White's DF coil has one resistor in the housing for the inner coil. The "overall" resistor is in the box.

Thanks, If I read the patent correctly, Rd=pi*L*C. I use Rd=pi*L*SRF. Am I doing it wrong. The formula in the patent uses coil capacitance to calculate "overall" Rd. A PI with a series diode would require a different "overall" resistor than a PI without the series diode so I assumed the "overall" resistor was in the coil.

[green]

So many things can be left to interpretation.
In figure 6 it shows the final wiring of the coil to a pulse source. Only two resistors.
It also says damping of the inner coil is determined by it own electrical perimiters. Simply calc the resistor and be done.
I dont see how the calculation takes into account mutual coupling.
I think my method pre criticle damping is the most accurate you can adjust later but the coil is perfectly damped before coupling its at least a head start.

So here I go again. I have a small coil that had no direction. Its now the inner coil for a 7.5 dual field.
Inner and outer coil inductance both at 135uh came out to 309uh finished inductance, inner coil 25 wraps outer 15 wraps. even this contradicts the patent as it says the total induction is equal to the summation of the two coils yet there is added capacitance from the coupling that is proven to me now.
Blew my mpp test board but its back up now, hopeing tonight Ill have time play with the different procedures discussed in this thread.

Im thinking looking at the Fet out and op amp if one procedure should be better than the other we should see it in the speed of the coil.
Is this correct, Or where should I look to see actual results other than just the op amp?
I think you could have the inner coil off and compinsate it with the outer coil or visa versa.

I would be interested in what the SRF of each coil is if you haven't put the dual field coil together. If you have each coils inductance, SRF and resistance. And completed dual field coil inductance and SRF we could compare simulation with your PI.

[godigit1]

I would be interested in what the SRF of each coil is if you haven't put the dual field coil together. If you have each coils inductance, SRF and resistance. And completed dual field coil inductance and SRF we could compare simulation with your PI.

Perfect I was actually hoping you might say that.
Got srf of the smaller coil last night. It was difficult using a pulsing pi with this coil.
With the probe on ten times I coud not get resonation. with the probe on 1 times I get 1.316 MHZ
But then I looked at my measurements and I was pumping 488 volts into my scope on 1 times. Very large transformation ratio from the dual field wich is running 388 volts.
So there was a100 volt gain from the 3.75 inch coil due to tranformation from the live coil.
Not sure how to protect my scope and still not kill SRF on a coil this small with such low capacitance.
I will get you all the specs so far all my testing has just been twisting wires.

[green]

Perfect I was actually hoping you might say that.
Got srf of the smaller coil last night. It was difficult using a pulsing pi with this coil.
With the probe on ten times I coud not get resonation. with the probe on 1 times I get 1.316 MHZ
But then I looked at my measurements and I was pumping 488 volts into my scope on 1 times. Very large transformation ratio from the dual field wich is running 388 volts.
So there was a100 volt gain from the 3.75 inch coil due to tranformation from the live coil.
Not sure how to protect my scope and still not kill SRF on a coil this small with such low capacitance.
I will get you all the specs so far all my testing has just been twisting wires.

Try making a loop(connect scope ground lead to scope probe)lay probe on test coil(test coil leads open) and put test coil near operating PI coil. CH gain maybe 5mV/division, x1.

Scope probe not connected to test coil or any circuit just laying on test coil. Put test coil on a piece of cardboard if coil isn't stiff.

Test coil just close enough to get a signal with 5mV/ division.

[godigit1]

Try making a loop(connect scope ground lead to scope probe)lay probe on test coil(test coil leads open) and put test coil near operating PI coil. CH gain maybe 5mV/division, x1.

Scope probe not connected to test coil or any circuit just laying on test coil.

Test coil just close enough to get a signal with 5mV/ division.

Thanks I wasnt sure if it would work with the coil not connected to anything. I will try that next time.
I went ahead and got the #s on the coils.
Im a bit suprised by the capacitance of the single coils and the end result.

the coils are 30 awg tested with no leads.

Inner coil----------- outer coil---------------combined
25 wraps135uh-----15 wraps135uh--------309uh
2.9 ohms------------3.3 ohms---------------5.6 ohms
1.316 Mhz-----------1.496 Mhz-------------1.639 Mhz
108Pf----------------83.8pf------------------30.516pf

scope shot is of both coils together.

[waltr]

I was looking at the spice circut again.
Final damping value is pretty damn close to what the dual field damps to on the MPP.

Both resistors R1 and rd are in parralel value for them calcs to 634 ohms final damping.
657 ohms is where the 12 inch dual field is damping on the Mpp. the sim is pretty close there.
What happens if you damp with 634 ohm single resistor on spice? I havent tried to learn how to do that yet? whole different tanget right now.
Thanks

Just tried a single 634 Ohm Rd in LtSpice. It damps but decay takes more than twice as long. Therefore it is Over Damped.

[green]

Thanks I wasnt sure if it would work with the coil not connected to anything. I will try that next time.
I went ahead and got the #s on the coils.
Im a bit suprised by the capacitance of the single coils and the end result.

the coils are 30 awg tested with no leads.

Inner coil----------- outer coil---------------combined
25 wraps135uh-----15 wraps135uh--------309uh
2.9 ohms------------3.3 ohms---------------5.6 ohms
1.316 Mhz-----------1.496 Mhz-------------1.639 Mhz
108Pf----------------83.8pf------------------30.516pf

scope shot is of both coils together.
[ATTACH]47610[/ATTACH][ATTACH]47611[/ATTACH]

I would expect SRF to be a lot higher using the loop method with a spiral wrap coil.

[godigit1]

it was still set up so a quick try and I dont get SRF.
Im seeing the Mpp Tx charge fly back pulse cycle of the dual field.
The loop worked when I tried it on a connected coil with the damping and imput resistor removed.
But I did get the same #s both ways.
When I returned home I had forgotten I snipped one side of the input resistor I did not see it when I re-setup so i installed the damping resistor.
Wonder why im not seeing a coil, took me about the time of the pop to figure it out.
The 555 blew itself partially out of the sockett. easy fix nothing else happened.