Litz wire size for coils

[baum7154]

Thanks baum7154,
Then I would be safe making coils for both detectors ( MPP for gold & SM pro for beach ) with this 24 awg like you used ?

~~~~~~~~~~~~~~~~~~

Yes, in the range of 300uh to 340uh, lower is faster, but lower is also less sensitive.

Dan

[baum7154]

Maybe I should tell you I just started trying to learn electronics / detectors last year as if it is not abvious. I have only tried a few coils I have made with the telephone wire and it seems that the inductance was under what the coil calculator said ( instead of 300uh my meter set at 2mH would read .250 to 290 depending on the coil ). Most where round bunch coils. I figured it was the wire I used as I know it not the best to use I just had it laying around.
What meter do I need to measure inductance ?

Thanks again for your help

Darin

~~~~~~~~~~~~~~~~~~~

I use an LC200A purchased on Ebay for about $35, made in China of course. It seems to be pretty good in accuracy when compared to some old Dale Electronics (Now Vishay) coils I have.

Dan

[Darin]

That was it I did not know there was insulation on each wire you cant see.
I will check out that meter and try to pick one up when I can.
I will work on this some more later when I get home.

Thank you so much Dan

[rickb]

Thick paste type epoxy paint remover will remove the enamel if soldering heat won't melt it. Be careful as it is very caustic.

Rick

[6666]

Or you can make a solder pot from small tin can and a hot stove.

[Darin]

I Got a chance to finish the 8" 3DSS mono coil with the 330/46 LITZ wire. Cleaned the insulation off much better readings know thanks for your help.
Here is what I got know

37 turns with 42" twisted leads
Resistance 1.7 ohm
Inductance .329 mH

Hooked it to my MPP it does not perform as good as the 7" 3DSS round made from telephone wire at .294 mH ( not 2.94 as posted above sorry )
I only get about 6" on a US nickel on the 8" coil and the 7" gets about 8.5" and that's not inline with others I have seen posted.
My MPP is still as per the build document the dampening resistor has not been changed so I'm not sure how that effects it.
O well such is the learning curve, but I have not give up on it yet.

[baum7154]

I Got a chance to finish the 8" 3DSS mono coil with the 330/46 LITZ wire. Cleaned the insulation off much better readings know thanks for your help.
Here is what I got know

37 turns with 42" twisted leads
Resistance 1.7 ohm
Inductance .329 mH

Hooked it to my MPP it does not perform as good as the 7" 3DSS round made from telephone wire at .294 mH ( not 2.94 as posted above sorry )
I only get about 6" on a US nickel on the 8" coil and the 7" gets about 8.5" and that's not inline with others I have seen posted.
My MPP is still as per the build document the dampening resistor has not been changed so I'm not sure how that effects it.
O well such is the learning curve, but I have not give up on it yet.

----------------------------------------------

Darin, I would definitely do the Critical Damping resistance change for this coil. Also I believe this coil should run at a shorter sample delay than the solid copper telephone wire does. This is why Mine Labs uses the Litz wire. You might be pleasantly surprised! I would expect that the 8" coil would operate at 8us and possibly lower. By the way your 42" feed line adds about 1pf of capacitance per inch of length so it is best to keep it as short as practical. Capacitance is the enemy of coil speed and that is why the 3DSS coil is designed to keep it low with all that air space and winding separation in its design.

Dan

[Darin]

----------------------------------------------

Darin, I would definitely do the Critical Damping resistance change for this coil. Also I believe this coil should run at a shorter sample delay than the solid copper telephone wire does. This is why Mine Labs uses the Litz wire. You might be pleasantly surprised! I would expect that the 8" coil would operate at 8us and possibly lower. By the way your 42" feed line adds about 1pf of capacitance per inch of length so it is best to keep it as short as practical. Capacitance is the enemy of coil speed and that is why the 3DSS coil is designed to keep it low with all that air space and winding separation in its design.

Dan

Thanks Dan, I have to pick up some resistors to make the damping tool to adjust it then hopefully it will be a good coil. I left the leads long as I do not have a shaft or box to mount everything yet and did not want to come up short. I am thinking of using a straight shaft and mounting the control box underneath at the back. Once I get all the parts I will cut the leads. Would like to find some connectors like Garrett uses that are waterproof for the coil and headphone connections. I'll keep you posted how it turns out.

Thanks again Dan

Darin

[green]

The two factors that restrict target detection are: 1. coil circuit capacitance which affects the damping resistor value and, 2. the residual eddy current decay time in the coil wire itself which is affected by the delay setting.

The coil resonance is related to 1 above, however when detecting very small targets like gold nuggets, you need to quickly switch the RX circuit on to catch the quickly falling eddy currents in a small gold nugget or the links of a fine link gold chain. Using Litz wire prevents eddy currents from being generated in the coil wire. The issue is: at what delay are you sampling? The lower the delay, the more care must be used to allow the fastest RX turn on time (lowest delay).

Another issue is what is the peak current per pulse? Lower current pulses can be damped quicker.

Typical delays of 10 uS to 15 uS can be used on beach PI machines looking mostly for coins and large jewelry like rings at higher delays. However, when you get near and below 10 uS delay, the ability to reduce the delay and not lock up the RX circuit can be a challenge. You will be detecting the residual signal in the coil caused by less than optimal coil damping and the residual eddy currents decaying in the coil wire. Remember, we are only dealing with a few microseconds here.

The PI machine needs to have the ability to operate at low delays to see if the chosen search coil can operate at that PI machines lowest delay. One trick is to use a variety of small target sizes and materials and then reduce the delay to see if the PI machine locks up. If it does then you have found the limit of your PI machine and that coil! Eric Foster reported making a PI machine and coil that went down to 5 uS. I believe he used Litz wire on this 5 uS coil. All of the above issues are interrelated.

I hope this helps?

Joseph J. Rogowski

Wanting to see if I can see the residual eddy current decay of coils wound with different wire. Where I'm at and what I'm thinking. I have two coils, spider wound with solid wire, one AWG24 and one AWG28. Don't have Litz wire but I have AWG34 magnet wire. Think of twisting 4 strands of #34, same resistance as AWG 28 and winding a coil. 23 turns, same as other coils. Including some data on the two coils. Coils not shielded, about 12 inches Teflon coated twisted pair for lead wire. Thinking it makes sense to leave the coil resonance as high as possible to allow early sampling for the comparison. Need to connect coils to a coil driver and pre amp and compare decays, scope and maybe LT spice simulation of the same circuit. The scope resonance pictures were taken with the coil connected to the coil driver with no damping resistor and the pre amp disconnected with a x10 probe across the coil. The resonance is higher with the scope probe disconnected laying close to the mosfet. Suggestions appreciated.

[godigit1]

Hi Green,
I'm really trying to understand how all the coil nuances come together to make that one great coil but so far I've not included resonance testing.
I would like to start!
I have found methods like on the chemelic site.
I was wondering if you would share your set up so I was comparing apples with apples?

The next thing is am I missing something? Really if we have a magnet wire coil clean at 3 us is a litz wire coil at the same speed going to be any better?
The litz wire coil of the the same uh is going to have much less resistance. Say 320 uh 26 awg magnet wire 2.5 ohms resistance. depending on the litz your resistance is under 1 ohm.

For circuits like the gpx these low resistances are a must but form what I read and maybe this is where my lack of understanding comes in the SM Pro, HH Mini pulse. Barracuda all want 1.5 to 3 ohm for the original circuit design..

I have a couple professionally made coiltek litz coils for my Sd 2200 v2 and Tdi Pro. The only major difference I can measure between the Tdi coils and Minlab is resistance.
All of my Tdi coils with higher resistance out perform the minelab coils on my minipulse. Its not even a debate as far as my machine goes.

So Am I missing something are are we over designing for the intended circuit by using such low resistance wire is it overkill?

By the way very nice coils you make and I always look forward to your experiments.

Oh yea I recently twisted a 100' pair of 30 awg magnet wire and was going in the same direction as you with the 3 twists. I did one coil this way in the past but my wire was not long enough only had 50 feet. Resistance was cut by over a third . The wires in parralel only 140uh and in line over 400 uh so it makes a huge difference.

Best
digit

[green]

Hi Green,
I'm really trying to understand how all the coil nuances come together to make that one great coil but so far I've not included resonance testing.
I would like to start!
I have found methods like on the chemelic site.
I was wondering if you would share your set up so I was comparing apples with apples?

The next thing is am I missing something? Really if we have a magnet wire coil clean at 3 us is a litz wire coil at the same speed going to be any better?
The litz wire coil of the the same uh is going to have much less resistance. Say 320 uh 26 awg magnet wire 2.5 ohms resistance. depending on the litz your resistance is under 1 ohm.

For circuits like the gpx these low resistances are a must but form what I read and maybe this is where my lack of understanding comes in the SM Pro, HH Mini pulse. Barracuda all want 1.5 to 3 ohm for the original circuit design..

I have a couple professionally made coiltek litz coils for my Sd 2200 v2 and Tdi Pro. The only major difference I can measure between the Tdi coils and Minlab is resistance.
All of my Tdi coils with higher resistance out perform the minelab coils on my minipulse. Its not even a debate as far as my machine goes.

So Am I missing something are are we over designing for the intended circuit by using such low resistance wire is it overkill?

By the way very nice coils you make and I always look forward to your experiments.

Oh yea I recently twisted a 100' pair of 30 awg magnet wire and was going in the same direction as you with the 3 twists. I did one coil this way in the past but my wire was not long enough only had 50 feet. Resistance was cut by over a third . The wires in parralel only 140uh and in line over 400 uh so it makes a huge difference.

Best
digit

http://www.geotech1.com/forums/showt...465#post212465

Teleno gives a formula for calculating Rd in the link above. When I started learning about metal detectors I made a circuit to excite the coil to measure coil resonance and then added a .01u capacitor across the coil and measured resonance to calculate inductance. I found the coil resonance was close to the same measured with the test circuit or the coil driver after I added a MUR460 diode in series between the mosfet and coil. I measure circuit resonance with the coil driver after removing Rd and input resistor to the pre amp, includes lead wire and circuit capacitance to calculate Rd. Rd=pi x inductance x circuit resonance. Teleno's formula gives the same answer but you assume external capacitance. Gives me a calculated Rd to compare against adjusted Rd.

I've been assuming coil wire eddy current wasn't a problem with AWG28 solid. Trying to find out.

I think coil resistance just effects circuit efficiency with a PI detector, if the current profile is the same with a 5 ohm coil as a .5 ohm coil the target signal will be the same. Maybe not? The 5 ohm coil needs a different PS and a bigger battery.

Hope that answered a couple of your questions, if more I would be happy to give more opinions.

[Chet]

Using multiple strands of #28 or #30 AWG will only add more coil capacitance and eddy currents. Refer to the attached address for a Litz chart for the following discussion. Very small #46 or #48 strand sizes are required to have a high self-resonant frequency response / fast coil. The number of strands will determine the coil resistance.


Example; the Litz equivalent of #24 AWG solid wire is 165 strands of #46 AWG wire.
The resistance of 60 feet is; 28.7 ohms / 1000 X 60 = 1.722 ohms.


Increasing the number of strands increases the ability of the coil to carry more current. But more strands will increase the coil capacitance and eddy currents. More strands than needed to carry the current will reduce the self-resonant frequency of the coil.


A coil with higher resistance will require a longer transmit pulse width or a higher power supply voltage to reach the desired coil current. If building a coil for a commercial detector the coil characteristics may need to closely match the manufacture’s coil.


Have a good day,
Chet


http://www.newenglandwire.com/produc...d-cables/round

[godigit1]

Mr Goddards reply on the next post suited my response well!

But being serious thank you The .01u cap seems to be what I'm seeing others use as well and being new I wanted to be on the same page.

I keep reading posts that say resistance is not that important. I think you right when you say circuit efficiency and would ad tolerance as well. Example being the Ml machines vs TDI which is a much more tolerant design.

I'm anxious to try litz but I cant just get it and I wanted to really learn how far I could different coil designs, I wanted to keep my wires consistent between coils.
The twisted .30 magnet wire will be my last experiment with that small wire unless things turn out good. I'm thinking flat wound fractal pancaking and stepping forward every ten wraps with teflon spacing between layers.

This was my first attempt but it was too tight and tedious, I didn't like the way it was stacking up. I'm working on a different wind now that I think will have better fill.



Thanks again for your reply, Its time to back track on some coils and see where the good and bad ones spec out .

[godigit1]

Hi Chet,
Your reply came as I was posting, Guess that throws a hammer in the works. Thanks for saving me the time on the twisted pair. I still may try a coil for giggles but it wont be a complicated wind.
And thanks for the perspective on resistance, you have a way of putting things between the lines, I don't know what you do but I bet you would make a good teacher.

Little do you all know your my un willing professors.

[green]

Was thinking Litz wire was used to increase coil Q, not necessarily coil resonance. Example: the AWG28 magnet wire coil, 273uH, 1.92MHz without probe, 1.515MHz with x10 probe. Calculates 25pf without probe and 40pf with probe. The probe is labeled, x10 10meg 13.5pf close to the 15pf difference. The calculated capacitance includes 12 inches of Teflon coated twisted pair + driver capacitance. How much lower would the capacitance be using Litz wire the same resistance as AWG28 solid. The magnet wire has a resonance Q with probe of about 50, what would the Litz wire have?