Dual coil for mpp

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Yes, I knew I was confusing myself. Thanks for clarifying, So the whites inner coil(if taken on it's own parameters) is underdamped. But when taken as part of the whole dual coil setup, including the other damping resistor, it is just good enough to get rid of any residual ringing of the small coil.
That would make sense. If it were true?? More evidence that in setting up the dual coil arrangement, you damp the inner coil LAST.

Again we see more evidence that it is the "in circuit" ringing that must be dealt with by damping "tool", on a practical "cut and dry" basis.

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If Whites used standard value Rd(inner coil) for all of there dual field coils, I anticipate some machines performing better than others. The coil construction quality control would have to be kept very high to avoid this.

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Some more preliminary conclusions:
By disconnecting coil completely from the circuit and measuring it separately by various methods to obtain SRF measurements, and using the HIGHEST obtained mearurement, you arrive at the coil "Self Resonant Frequency" (SRF). However, this result cannot be used to calculate a critical damping resistor value (Rd). Because It does noy take into consideration the specific operating mode of TX circuit, and unpredictable behavior of transistor parasitic capacitance, effect of clamping diodes etc, etc.

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For the dual field coil, I cannot be sure of how to setup such a coil without actually trying to build one and put it to the test.
For example, how do you know which of the two Rd's to tweak? How do you know which of the two coils is ringing, or maybe both? How do you calculate a theoretical Rd for just the inner coil based on the in-circuit resonance?
Overall, how do you know when you have the sweet spot nailed down on a dual field coil?? Or maybe it is best to just copy TDI front end, and reverse engineer one of there dual field coils.

[green]

Used the small and large coil to make a dual field coil. inductance and SRF measurements a little different than picture. Maybe measurement difference or I super glued the wire to the backing after measuring the first time. Wires were loosening from tape.
coil__________uH________SRF MHz
small________115________6.67____calculated Rd=2410, small and large coils measured one at a time, not near each other
large________115________4.5
series________271_______4.16_____connected in series, outer small to inner large
series________271_______1.25 MHz circuit resonance with added graphite shield and 30 inches twisted pair lead. connected to Tx circuit, MUR460 diode to inner small, outer large connected to +supply. external Rd not connected. 2410R across small coil to damp small coil.

adjusted external R for critical damping looking at coil decay on scope, Rd=1390R, calculates 1064R, didn't see any ringing, scope trace decay looked good. Didn't try internal damping both coils, looked good with just the small coil damped.

[dbanner]

Great, so you damped the inner coil first using calculated Rd 2410 based on the 6.67mHz SRF.
Are you saying that you connected each coil individually to the Tx to measure the in circuit resonance, is that how you got the 6.67mHz and 4.5mHz respectively?
Or by other method?
OK, so no ringing, but how can you be sure the inner coil isn't woefully underdamped (or overdamped) and the whole setup is compensated by external R.

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Can you bring the your dual field coils anywhere near critical damping by just using external R (no inner coil Rd), then put in damping tool and adjust inner Rd to get rid of remaining oscillations? Then measure the Rd for inner coil and see where it is in relation to 2410.

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I'd sure like to do some testing of my own, but my left hand is damaged from brawl at local watering hole a couple of weeks ago. I am a one handed man for the time being.

[green]

Great, so you damped the inner coil first using calculated Rd 2410 based on the 6.67mHz SRF.
Are you saying that you connected each coil individually to the Tx to measure the in circuit resonance, is that how you got the 6.67mHz and 4.5mHz respectively?
Or by other method?
OK, so no ringing, but how can you be sure the inner coil isn't woefully underdamped (or overdamped) and the whole setup is compensated by external R.

from reply #105

6.7MHz____PI coil exciting small coil, loop pickup
3,87MHz___small coil connected to Tx with diode, loop pickup
2.82MHz___small coil connected to Tx with diode, x10 probe across coil
550kHz____small coil connected to Tx no diode, x10 probe across coil
1.3MHz____PI coil exciting small coil, x1 probe across small coil
3MHz_____ PI coil exciting small coil, x10 probe across small coil
The small and large coils were measured one at a time, PI coil excited, loop pickup on test coil spaced from PI coil. Connecting test coil to Tx lowers resonance. I just tried to follow the patent instructions except for the formula. Not sure the inner coil is critical damped, used formula Rd=pi*L*frequency. Think I could use the loop method to see if critical damped if PI circuit was faster

[dbanner]

OK. According to the patent, the small coil is isolated from the pulse source by the inductance of the larger coil and will ring at the frequency determined by its own parameters.
I think this statement is somewhat misleading.
Although it is isolated from the pulse source, its ringing will still be influenced by the totality of the circuit.
The inner coil is never really "critically damped" in the conventional sense as it applies to a standard mono loop. Why? My theory is that the inner coil is actually part of one single coil split in two parts. So it is the two coils taken as a WHOLE that is damped by the dominant external Rd. Any residual ringing could then be attributed to the small coil and damped out by inner Rd.

[dbanner]

To verify all of this, and to actually understand a dual coil behavior in circuit, you need to connect two damping tools and have a play with adjusting both of them, to see how the coils behave with preferred targets.
Another thing you can do is to try increasing the value of your 2410 Rd and see when you start to pick up overall ringing on the scope.
I'd bet you could go as high as 4K or even higher before you see anything.

[godigit1]

To verify all of this, and to actually understand a dual coil behavior in circuit, you need to connect two damping tools and have a play with adjusting both of them, to see how the coils behave with preferred targets.
Another thing you can do is to try increasing the value of your 2410 Rd and see when you start to pick up overall ringing on the scope.
I'd bet you could go as high as 4K or even higher before you see anything.

I can see that none of my actual building damping and testing on a working PI and reporting the results have been worth anything.
Did the dual damping set up earlier in the thread no go THE INNER COIL IS ISOLATED.
Why Green and I are getting different actual damping im not totally sure but I will say we are not testing on the same PI.
maybe the front end impediance of his tester is so high he barly need any damping. but i tell you fact out on a mpp that inner coil damped at 2000 + ohms you will hear ringing under the bridge.
the other thing you will see is the fet starting to get very hot very fast. as that little coil will hit 600 + volts like nothing unless you turn down your on time.
Im no big expert but on a surf pi HH MPP Baracude all react the same way. Test circuts at 1pps may be exciting resonance but not paracitic field effects you would see under full voltage. Thats my conjecture.

When testing SRF with a Square wave on the signal gennerator Per the methods i studied, I set the peak to peak at 10 volts 20 volts the coil is seeing.
I forget the exact # but green said he was seeing in the millivolt level.
It makes sense that we would get different paracetic effects.

I see no valid testing comparisons between green and I happening unless we are testing with apples and apples.

[godigit1]

Hi green,
No disrespect but it sounds like you still have your coil, I see no reason to really send it at this point ive allready validated and matched the same SRF ranges as you have on coils that are constructed as close as you could to mine. Except my inductance is higher. and Im not using SRF to measure it.
We are not playing with apples and apples Ive tried to remedy this.
the other conclusion Ive come to is the SRF measurement doesnt matter 6.9, 3.333 as long as i establish a baseline and repeat my test method consistantly, I can track the effects of the coil fet shielding it just may not be exctly the same #s as you. its still a guide to fine tuning your capacitance.
I no longer feel the need to get on the same page as everyone about everything and starting to realise on this forum that may be impossible .




Ive shown my respect and appreciation for your contributions for the forum and to my learning I hope I havent offended you too much but im moving on throwing in the towel driving away on three wheels.
I have 4 pi's to build 5 coils to test a wire twisting machine to finish and a vacumm former ive been puttin off untel I felt like I could test a coil properly.
Ive spnt 4 + months pounding over coil design articles and coils upgraded my equipment. Time for a break go dirt fishing.
Peace out.

[dbanner]

I am simply trying to posit an argument to account for whites 4.7K Rd on their inner coil. If the toal inductance is around 300uH, you can safely assume that their inner coil is below 200uH.
If you do the math, to end up with a Rd of 4.7K, the self capacitance at 200uH would have to be somewhere less than 3pF, an unrealistic value. At 150uH, less than 2pF. At 100uh, even lower. These are not realistic values for the inner coil self capacitance.

The conclusion is that whites isn't damping the inner coil with standard formula. That is why I was intrigued by the formula that they have in the patent, even so, the 4.7K seem high.

Also, if somebody can state what is the value of TDI external Rd, I think this would help put more light on subject.

[green]

Hi green,
No disrespect but it sounds like you still have your coil, I see no reason to really send it at this point ive allready validated and matched the same SRF ranges as you have on coils that are constructed as close as you could to mine. Except my inductance is higher. and Im not using SRF to measure it.
We are not playing with apples and apples Ive tried to remedy this.
the other conclusion Ive come to is the SRF measurement doesnt matter 6.9, 3.333 as long as i establish a baseline and repeat my test method consistantly, I can track the effects of the coil fet shielding it just may not be exctly the same #s as you. its still a guide to fine tuning your capacitance.
I no longer feel the need to get on the same page as everyone about everything and starting to realise on this forum that may be impossible .




Ive shown my respect and appreciation for your contributions for the forum and to my learning I hope I havent offended you too much but im moving on throwing in the towel driving away on three wheels.
I have 4 pi's to build 5 coils to test a wire twisting machine to finish and a vacumm former ive been puttin off untel I felt like I could test a coil properly.
Ive spnt 4 + months pounding over coil design articles and coils upgraded my equipment. Time for a break go dirt fishing.
Peace out.

Hi godigit
I apologize for not sending the coils and SRF tester yet. It's important that I do. I'm interested if we measure the same inductance and SRF with the same coils. And if using the same procedure with different equipment we can get the same numbers. We are getting different answers for our Rd because I'm just looking at the coil, amplifier not connected. Think you are looking at amplifier out. I like looking at amplifier out and coil decay when I'm adjusting Rd. The amplifier isn't connected when measuring circuit resonance so I think it shouldn't be when comparing calculated Rd against adjusted Rd. SRF measurements are important for each stage of coil building to learn something but mean little for final Rd. Circuit resonance should be used when calculating Rd for a mono coil. Not sure but I think the small coil SRF should be used for calculated Rd for a dual field coil. The amplifier needs to be connected to make a detector so what you are doing is right for final Rd but not when comparing calculated vs adjusted. Rin effects Rd, don't know the formula for combined effect. The adjusted Rd is what's important, would be nice if calculated matched adjusted. If they don't, knowing why would be interesting. I need to connect the amplifier to see how it effects what I have been doing.

My thoughts, maybe correct maybe not. Just trying to learn.