spiral pi coil

[waltr]

As you are starting to realize is that coil design involves many trade offs.

Example of your Original Spool coil verse the one with added spacing.
The spacing does reduce the C between the wires but it also enlarges the coil which decrease the L.
To get the same L you need to increase the number of turns which then also increases the C.

Modeling a coil for physical properties can not be done in Spice since spice uses electrical models.
What is needed is a EM simulator that take a 3D physical model then does analysis. These EM simulators are available but the software packages are VERY expensive (several 10k's of dollars) and not simple to use.
The best we can do is build coils and measure results.

[bbsailor]

As you are starting to realize is that coil design involves many trade offs.

Example of your Original Spool coil verse the one with added spacing.
The spacing does reduce the C between the wires but it also enlarges the coil which decrease the L.
To get the same L you need to increase the number of turns which then also increases the C.

Modeling a coil for physical properties can not be done in Spice since spice uses electrical models.
What is needed is a EM simulator that take a 3D physical model then does analysis. These EM simulators are available but the software packages are VERY expensive (several 10k's of dollars) and not simple to use.
The best we can do is build coils and measure results.

I would add one more part to your last sentence. The best we can do is build coils and measure results and understand the consequences on things that this relates to.

Try using this mental model. If there was no coil capacitance, coax capacitance or circuit capacitance ,the flyback pulse would drop and not oscillate because there is no capacitance energy that needs damping. The value of the damping resistor is related to the coil energy that needs to reach near zero so the RX circuit can be turned on sooner. Typically, the energy in gold, being fully stimulated, falls to zero energy within the metal in 5 time constants. A 2uS target TC would be very hard to detect at 10uS (time delay) but at 8us could be detected. Theory tells us that to fully stimulate a target, the TX decay pulse time constant needs to be 5 times faster than the target time constant. The TX pulse TC for a 2us target would need to be 2 divided by 5 or .4uS. The TX decay TC is calculated by dividing the coil inductance by the effective damping resistor (Rd) value. A 300uH coil with a 750 ohm Rd is 0.4uS.

As you make the coil faster you now need to ensure that the coil wire itself or anything near the coil, like a solder joint, is not being detected as a target.

As you can see, coil design involves many tradeoffs, but understand how they interact and fit together helps understand what you need to consider to detect that low TC target.

Joseph J. Rogowski

[green]

Is there a specific way to test a shielded coil should I be hooking up the shield?

The shield should be connected to one of the coil leads. Circuit common if coil is connected a circuit, if exciting resonance with another operating PI(test coil floating)connect shield to one of the test coil leads. I sometimes forget and connect the shield for a floating coil to circuit common. Less effect, not correct.

[Guzzi]

Hi all

I found this a long time ago, maybe to late now.

http://chemelec.com/Projects/Metal-1...g/Coil-Jig.htm

[chemelec]

Hi all

I found this a long time ago, maybe to late now.

http://chemelec.com/Projects/Metal-1...g/Coil-Jig.htm

I Still have this Jig.

[dbanner]

Aha! I was trying to remember where I saw this jig a long time ago. Thanks for reminding me. Always wanted to make such a gizmo.

[godigit1]

Is there a specific way to test a shielded coil should I be hooking up the shield?

The shield should be connected to one of the coil leads. Circuit common if coil is connected a circuit, if exciting resonance with another operating PI(test coil floating)connect shield to one of the test coil leads. I sometimes forget and connect the shield for a floating coil to circuit common. Less effect, not correct.

Thanks,
I was pretty dissapointed with that coil Im glad it was me. Ill retest it see what it actually is.
Ive been pulsing with a stock MPP with a dual field coil on it.
Thats why I mentioned notice the voltage on the 8 coil with too much spacing it has almost no transformation or induction voltage from the MPP.
I think thats a sign of a good antenna

[godigit1]

I would add one more part to your last sentence. The best we can do is build coils and measure results and understand the consequences on things that this relates to.

Try using this mental model. If there was no coil capacitance, coax capacitance or circuit capacitance ,the flyback pulse would drop and not oscillate because there is no capacitance energy that needs damping. The value of the damping resistor is related to the coil energy that needs to reach near zero so the RX circuit can be turned on sooner. Typically, the energy in gold, being fully stimulated, falls to zero energy within the metal in 5 time constants. A 2uS target TC would be very hard to detect at 10uS (time delay) but at 8us could be detected. Theory tells us that to fully stimulate a target, the TX decay pulse time constant needs to be 5 times faster than the target time constant. The TX pulse TC for a 2us target would need to be 2 divided by 5 or .4uS. The TX decay TC is calculated by dividing the coil inductance by the effective damping resistor (Rd) value. A 300uH coil with a 750 ohm Rd is 0.4uS.

As you make the coil faster you now need to ensure that the coil wire itself or anything near the coil, like a solder joint, is not being detected as a target.

As you can see, coil design involves many tradeoffs, but understand how they interact and fit together helps understand what you need to consider to detect that low TC target.

Joseph J. Rogowski

Thank you both for the input,
thats one of the reasons I been saying its hard to beat a pro coil they have all the nice toys.

As far as understanding all the dynamics Im far from that yet, Im sure I will have many lame questions allong the way so please all bear with me.

Ive been trying to decide on what data would be most useful for my spacing tests.
Im thinking now I have to do 6 coils instead of three.

Three with the same amount of wire, a tight one, and one wire spacing, then two wire spacing. this will give me both inductance and capacitance readings.

Then three coils built the same way but with the inductance adjusted to match each other.

Thinking that the inductance matched coils are a better representation of actual performance changes. The effects of adjusting the capacitance and SRF.

If anyone has any input on that I would appreciate it I would like to provide the proper data.
Thanks Again.

[godigit1]

I Still have this Jig.

I was just walking through HD last night trying to find something that I could use as a center spacer. Ended up at the for sale signes, I didnt take a mic but looks like the tin ones are a little thinner than the plastic.
I didnt commit beacuse I was still thinking about what to sandwhich it between.
this answers that question.

Well you dont make anything skimpy thats for sure thats a very nice Jig.
I may have to do something similar scaled down a bit sure would make adding spacing easier.
Thanks again and thank you Guzzi for posting this as well.

[green]

Have you tried your suggested coils with a coil calculator first? http://www.deepfriedneon.com/tesla_f_calcspiral.html Won't give capacitance readings but capacitance should be low with a flat spiral coil. What wire are you planning on using? Coil diameter?

[green]

I Still have this Jig.

Did you try different spacing? I yes, did the added spacing make it better?

[godigit1]

Have you tried your suggested coils with a coil calculator first? http://www.deepfriedneon.com/tesla_f_calcspiral.html Won't give capacitance readings but capacitance should be low with a flat spiral coil. What wire are you planning on using? Coil diameter?

Hi Green,

Ive been using the calculator form Q here on the forum. Most of the time its within a wrap or two.
For the first tests im going to use 30 Awg single strand wire. I figure the extra spacing is kinda like using a wire with better insulation but good old cheap magnet wire.

Coils are 6.5 inch inner diameter.
For now I will be forming on paper with double sided tape and transfering the coil to foam.

I just finished the first one
Wire length is 54 feet with leads
Calculated inductance for a 6 .5 inch I.D coil with 28 wraps came out to 410uh.
The actual coil is measuring 390 uh 5.9 ohms so pretty close.
Hoping to hit a target point of 300uh after adding one then 2 lines of spacing to the coils.

This is the coil on paper.


This is foamed. I got a little out of hand.


Here it is trimmed out and with a coat of super corona dope over the windings. Its still curing.


Beacuse of the over use of foam and the coil only had the rigidity of a peice of paper the coil itself is not perfectly flat. It sits pretty flat but the surface has rolled a bit. the outer diameter is about 2 wire thicknesses lower than the I.d. it has maybe a 5 to 8 degree nice even tilt.
I need a better transformer for a foam cutting table I made. this way I can wind directly onto a cured formed slice of foam.

[green]

The flat coil calculator i suggested http://www.deepfriedneon.com/tesla_f_calcspiral.html calculates 298uH for AWG30, 28turns, 6.5inch ID. Maybe Q's calculator is closer?

I purchased a 20 x 30 x 3/16 inch foam board. Elmer's foam board #950805 at Walmart. Thinking of making a larger coil Rx_(2)round figure8, Tx surrounds Rx. Trying to determine coil size, wire size and number of turns for Rx and Tx. Rx on one side of board, Tx the other side.

[godigit1]

The flat coil calculator i suggested http://www.deepfriedneon.com/tesla_f_calcspiral.html calculates 298uH for AWG30, 28turns, 6.5inch ID. Maybe Q's calculator is closer?

Thinking my inductance meter could be off a bit by maybe 30uh on the high side. however the calculator did come out higher. Wondering if we are inputing the same perimiters.
OK I just redid my input I must have done something a little different last time
3.25 inner radius
.2546 mm wire
28 turns came out to 392uh
Just did the deep fried calc and same as you 298uh
bang Q nails it for this coil right on.

[green]

The flat coil calculator i suggested http://www.deepfriedneon.com/tesla_f_calcspiral.html calculates 298uH for AWG30, 28turns, 6.5inch ID. Maybe Q's calculator is closer?

I purchased a 20 x 30 x 3/16 inch foam board. Elmer's foam board #950805 at Walmart. Thinking of making a larger coil Rx_(2)round figure8, Tx surrounds Rx. Trying to determine coil size, wire size and number of turns for Rx and Tx. Rx on one side of board, Tx the other side.

Theory tells us that to fully stimulate a target, the TX decay pulse time constant needs to be 5 times faster than the target time constant. The TX pulse TC for a 2us target would need to be 2 divided by 5 or .4uS. The TX decay TC is calculated by dividing the coil inductance by the effective damping resistor (Rd) value. bbsailor reply#92


Thinking about a larger coil. Coil I'm using now is Rx_(2)8inch round figure8, Tx surronds Rx. SRF(Tx and Rx) close to 1MHz. Should be good for targets above 1.6us TC. Thinking larger coil not good for faster TC targets, maybe .318 MHz SRF(1us TC)good for targets with TC greater than 5us might be a place to start. Is there a disadvantage to making the coil with the highest inductance other than weight?