Damping dilemma

[Ism]

So I rewound my 18" Litz coil, wrapped it and shielded it with Scotch 24 according to best practices outlined by BBSailor (If I remember right).

Here's how my coil tests out....
356.5uH
4.55 ohms
445.3 kHz SRF
128 pF (shield to coil capacitance)

According to an online calculator, using my inductance/SRF/resistance, my parasitic capacitance is 359 pF.

Using the formula by BBSailor (Rd = 2 X the square root of L over C), I get a critical damping at almost 2k ohms (left image).

However, by adjusting Rd for best looking waveform, I get a damping resistance just under 1.2k ohms (right image).

Can you seasoned builders tell me if I should go with the formula and ignore the bounce I'm seeing (left) or go with the (right) better looking waveform?

Images are created with an induced pulse from a PI detector using my factory TDI coil. An adjustable damp resistor is used to vary the waveform.

[Teleno]

So I rewound my 18" Litz coil, wrapped it and shielded it with Scotch 24 according to best practices outlined by BBSailor (If I remember right).

Here's how my coil tests out....
356.5uH
4.55 ohms
445.3 kHz SRF
128 pF (shield to coil capacitance)

According to an online calculator, using my inductance/SRF/resistance, my parasitic capacitance is 359 pF.

Using the formula by BBSailor (Rd = 2 X the square root of L over C), I get a critical damping at almost 2k ohms (left image).

However, by adjusting Rd for best looking waveform, I get a damping resistance just under 1.2k ohms (right image).

Can you seasoned builders tell me if I should go with the formula and ignore the bounce I'm seeing (left) or go with the (right) better looking waveform?

Images are created with an induced pulse from a PI detector using my factory TDI coil. An adjustable damp resistor is used to vary the waveform.

When the coil is in the circuit the capacitance of the MOSFET adds to that of the coil. More capacitance, lower damping resistance.

[Carl-NC]

Your images did not come through.

RD = 0.5*sqrt(L/C) (0.5x, not 2x) so I get 499Ω. But slightly underdamped usually gives better performance.

[Ism]

Thanks Carl, I pulled the damping resistor formula I used from an old post by BBSailor on Find forum. I copied and pasted it into my post as not to get it wrong.

I did see your formula several days ago while perusing the posts here but couldn't find it again when I actually needed it.
Thank you for the correction. I should be alright from here.

I'm not sure about the images, they are showing up on my end both my post and Teleno reply post.

[Ism]

Teleno, are you saying that when I hook the coil up to a front end that the capacitance will change thus affecting my damping?
Right now I've been inducing a pulse into the unconnected coil by laying a known good coil (TDI 12" coil) on top of the coil under construction.

Carl, here's the coil under construction. I use a simple PI detector I built some years ago that I wired to accept the TDI coil connector.
I've built a couple coils simply using the waveform to adjust damping. The coils worked well, but I don't want "good enough" on this coil.
I want the very best coil I can possibly produce at home.

Here's the weird part I find with my coil capacitance. I plug the numbers into different formulas and get different results.
One formula asked for only 2 inputs (wire length and coil diameter), it spit out a capacitance of 230pF. Coil Capacitance Calculator | Design & Analysis | Tesla Scientific

Another (RCL circuit calculator) gave me the 358.8pF for coil capacitance. RLC Circuit Calculator (omnicalculator.com)
However, when I use the capacitance between the shield and coil (128pF) I get 833 ohms for my damping.
That number gets me a waveform very much in line with the results I expected on the scope.

The only waveform change I see on the scope when I go below 1000 Ohms is the initial pulse peak voltage dropping.
Is seeing the voltage drop the result of using an induced pulse vs directly driving the coil and viewing the waveform?

[Teleno]

Teleno, are you saying that when I hook the coil up to a front end that the capacitance will change thus affecting my damping?
Right now I've been inducing a pulse into the unconnected coil by laying a known good coil (TDI 12" coil) on top of the coil under construction.

Correct. You add the MOSFET Coss capacitance and the cable capacitance. The optimal damping resistor has to be found empirically with the coil attached to the front end.

I use a binary ladder of resistors in series: R, 2R, 4R, 8R, 16R, 32R ... with R being 12.5. The ladder is on the PCB. Each resistor end is connected to a PCB terminal. Using bridge plugs you can short individual resistors until you find the good value.

Start by shorting the largest one, if you get a damped response remove the bridge, if not then leave the bridge there, pick a new bridge and restart the procedure from the next largest value.

[bbsailor]

Damping resistor value requires you to have a good mental model about how the pulses interact with the target as well as the coil seen capacitance. Here is how to form a good mental model.

1. The TX current rises to a flat level and then turns off.
2. When the current quickly drops it tends to oscillate due to all the capacitive energy that the coil sees.
3. Before the RX stage can be turned on, all the oscillations need to be damped by the damping resistor.
4. The damping resistor value determines the TX discharge slope which stimulates the targets. Higher values make this slope more vertical and better able to stimulate smaller TC targets. Higher damping resistor values allow earlier sampling or less delay before the RX stage can be turned on. Remember that you need to sample early to pick up the decaying eddy currents in small targets before they die out. The main issue to sampling early is to reduce as much coil seen capacitance as possible by a design that pushes the variables listed below all in the same direction.
4.1 Coil wire turn to turn capacitance is based on the wire spacing due to the physical lay out of the coil. Dry air has a dielectric of 1 which is ideal. Find a coil wire with the lowest dielectric constant such as Teflon. Insulation thickness also holds the coil wires farther apart but lessens the inductance compared to wires that are closer together.
4.2 The coil shield adds distributed capacitance to the total coil seen capacitance but not the full amount. If you measure the coil self resonance without the shield and calculate the coil capacitance and then do it again with the shield you will find out that only about one quarter of the coil to shield capacitance is seen by the coil.The coil to shield capacitance is affected by the spacing between the coil wire bundle due to both the distance and dielectric constant of the spacer materiel. It is also affected by the area of the shield. That is why when doing my research, I found Scotch 24 which is a wire mesh that has less area than a flat solid shield like foil. This contributes to lowering the coil seen capacitance. The best use of Scotch 24 is to use a single layer. Also, as you cover the coil wire bundle, try to not overlap conductive mesh around the cross section of the coil bundle by carefully cutting it wide enough to not overlap or putting a piece of tape under the overlapping portion to prevent a conductive loop around the coil viewed as a cross section. Male sure you leave a circular shield gap where the coil wires come out of the coil.
4.3 The MOSFET that drives the coil also contributes coil seen capacitance so choosing a low capacitance MOSFET or putting a capacitor in series with it will again reduce the coil seen capacitance.
4.4 The coax that connects the coil to the electronics circuit also adds capacitance.

When tuning your damping resistor you need to consider the damping resistor value relative to the heat that it may collect while operating in your detecting environment for several hours. As resistors warm up their value may change to being higher depending on the type of damping resistor you choose to use. This may require you to slightly over damp to keep your RX circuit stable in a warm environment.

When planning to make a coil, look up the skin effect of your chosen wire. Thinner wires have a higher frequency skin effect where the current frequency fully goes into the core of the wire.

Once you understand how all these variables interact you can begin to understand how you want to push as many that are practical to your situation in the right direction to get the best performance on your desired targets.

I hope this helps?

Joseph J. Rogowski

[Ism]

It's been years since I built a coil or even perused Geotech Forums so I'm sorry for what is probably seems like such a noob question about damping.
In the past, info was readily available but it appears on the surface that many of the older posts are gone. As I do word searches, I don't get a whole lot on the topic.

Delano, I thought that may be what you were referring to. Thank you for taking the time to share your method.
I need to factor the extra capacitance into my formula. But for now, I'm just trying to understand the purpose of such a low damp resistance when it doesn't appear to improve upon the decay curve as seen in my images.
As far as on board damping, I'm using this coil on my TDI so I need the damping in the coil housing.

BBSailor, Your insight and detailed explanation is much appreciated. I have used Teflon stranded wire in the past. but this time I used Litz wire for my coil.
Overkill for a 18" coil? That's fine, I just hope it doesn't factor negatively on my build.

Left to right 1.2K Ohm / 1k Ohm / 860 Ohm / 680 Ohm / 560 Ohm.




As you can see, the decay curve doesn't tighten up at all, only the peak voltage drops. That's where I'm baffled, I thought I would see on the scope an improvement at critical damp.
Would an in-circuit test better show me the 500 Ohm damping that I'm not seeing with the induced signal?

At any rate, I really appreciate the help on this. Of course I will test before I epoxy everything, I just want to get it right the first time.

[Carl-NC]

You need to drive the coil with a real PI transmitter. Yes, passively coupling the coil to a driven coil will not work because you are first limited by the damping of the driven coil. After that, the 18" coil looks like a target and as you raise the tau (lower R) it drops the peak and alters the log-linear slope (difficult to see in a linear-linear oscope plot).

[Op04]

It's been years since I built a coil or even perused Geotech Forums so I'm sorry for what is probably seems like such a noob question about damping.
In the past, info was readily available but it appears on the surface that many of the older posts are gone. As I do word searches, I don't get a whole lot on the topic.

Delano, I thought that may be what you were referring to. Thank you for taking the time to share your method.
I need to factor the extra capacitance into my formula. But for now, I'm just trying to understand the purpose of such a low damp resistance when it doesn't appear to improve upon the decay curve as seen in my images.
As far as on board damping, I'm using this coil on my TDI so I need the damping in the coil housing.

BBSailor, Your insight and detailed explanation is much appreciated. I have used Teflon stranded wire in the past. but this time I used Litz wire for my coil.
Overkill for a 18" coil? That's fine, I just hope it doesn't factor negatively on my build.

Left to right 1.2K Ohm / 1k Ohm / 860 Ohm / 680 Ohm / 560 Ohm.




As you can see, the decay curve doesn't tighten up at all, only the peak voltage drops. That's where I'm baffled, I thought I would see on the scope an improvement at critical damp.
Would an in-circuit test better show me the 500 Ohm damping that I'm not seeing with the induced signal?

At any rate, I really appreciate the help on this. Of course I will test before I epoxy everything, I just want to get it right the first time.

By experience, I will try 580R and lower, sorry can't explain too many variable.

[Ism]

Yes Carl, I've been doing it backwards.....Thank you!

Last night I was pondering why I was having such difficulty when I've built coils before.
Then I remembered that my coil under construction was the driven coil with an adjustable 1/2-1 watt resistor pack like the one BBSailor shows in his "Fast Coils.pdf".
Not the backward way I have it set up.

Thank you all for your patience and help getting me back on track.

Randy

[Ism]

UPDATE ....I'ma so happy!

I properly tested my coil. Adjusted the variable damping resistor to give me the best waveform on the scope.
I tested my resistor pack and it read 422 Ohms. I added the additional coax capacitance (104pF), to the formula which resulted in a solution of 441 ohms.

Now that's what I like to see, the formula and bench test results practically matching. I presume the difference of ~21 ohms is from the added capacitance of the driver circuit.

Carl, you said that slightly under damping is best. Would you say that a damping resistor of 470 Ohms would be a good choice?

[Carl-NC]

I don't know, I go by what it looks like. But 470Ω sounds about right.

[Ism]

Honestly guys, I have been trying to find info on the forum before being a nuisance here, but what I do find doesn't go into enough detail enough for this amateur.

Update: I've been doing a bit of reading and discovered my coil resistance was pretty high in comparison to other builds.

I found that my LCR meter was on 1KHz when I measured the coil resistance. I dropped it to 120Hz and I got 1.9 Ohms which falls in line with the Litz wire chart for 175/46 (resistance per 1000ft.)
I punched in the new number for my damping calculation, but it only changed the Q value of the formula, so no worries there.

The results of air testing were unimpressive. It performed the same or just a little better than the 12" TDI factory coil. That may be why I abandoned the effort some 4-5 years ago.
In addition, the damping resistors (2-1k in parallel for testing) get quite hot to the touch. They aren't marked but they are either 1W or 2W resistors equaling 2-4Watts dissipation.
I bought them years ago specifically for use as damping resistors, so I know they are at least 1W. I would think that 2-4 Watts should be more than adequate to dissipate the heat. Am I wrong?

Now the Litz wire is equivalent to 23 AWG. That should be fine for a coil build based on what I read on the forum, but maybe too small for an 18" coil?
Would you guys be comfortable with 175/46 or would you go larger like 330/46 which is 1.1mm (closer to 18 AWG)?
I didn't have enough 330/46 on hand to make a coil, that's why it wasn't used in the first place.

Thanks for any insight.

[Op04]

If I understand correctly when use low R damping value, target receiving higher current stimulan in short period while high damp value will get higher flyback for longer stimulan periode but what target receives low current which is less deep. High damp value for shorter TC before start to ringing / vice versa. Wish I can explain better. To me, it's better with lower Q for coil.