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More questions about coils

A

Anonymous

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Hi all,
Has anyone tried a PCB coil ? I was going to try doing a postscript coil and printing it onto a toner transfer sheet (using the centre cutout for the main circuit board). With regards to screening has anyone tried using the self adhesive aluminium tape used for windows alarms ?
Nick
 
Hi Nic,
I see you are from Aussie too?.. what sort of Detector are you developing the coil for?
I messed with the concept of "winding" a coil using an etched PCB, but after doing some early experimenting with winding thin copper wire into a flat disc to see how the pulses were propogated,
(wound kinda like in a snail shell format)...It didn't seem to work too well for the SD series in the tests I did. <IMG SRC="/forums/images/frown.gif" BORDER=0 ALT=":(">
Good luck <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=":)">
 
It's for a homemade PI detector (suprise suprise). Apparently some commercial coils are made this way, although I have no idea how minelab and coiltek make their coils. In the "shadow" PI design on the geotech page the article states that a PC coil should have about 1/2 the capacitance of a wire bundle coil.
Nick
 
ahhh.. well perhaps you'll have a lot more success with a home made PI. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=":)">
I have only worked on Minelab SD series coils and detectors.
 
Hi Nick,
Building a PI and doing what you are doing now is a great way to learn about PI's and how they work.
Sorry, I can't help you on the pcb coil, but it is my understanding that Tesoro does or did make their PI coils that way.
The first PCB coil I ever saw was on an AH Pro detector, some 25+ years ago. They etched the coil windings on the top and etched a pattern of very fine lines on the bottom side for shielding. Just a thought if you should try to make a coil this way.
Good luck with your PI project. Which PI are you building?
Reg
 
My own design roughly based on Mark Stuart's designs- still very experimental at this stage !
 
Hi Nick.
If you do a search on this site for "SYMPATICO" you should find a post by Carlos. He posts a site with a number of DIY projects including a PI design using wound and PCB coils.
WWW3.BC.SYMPATICO.CA/CHEMELEC gets me there, its very interesting.
Regards John S.
 
As Reg said, Tesoro does make printed spiral coils for their SandShark as well as the old Pirahna. I just obtained the 10.5" and 5x10 elliptical for mine. I plan to compare them to std wire coils on my own PI project.
Advantages of printed coils are lower capacitance and very repeatable manufacturability. Disadvantages are higher metal resistance (try to use heavy copper clad) and poorer inductive qualities. Spiral inductors are not as good because the magnetic flux from outer windings begins to cut the inner windings. The best coil to make is a cylindrical coil.
I also have a printed elliptical spiral on a toner transfer but never got around to etching it. I would also suggest making it double-sided, you'll get more bang for your buck.
Reg, I didn't know the A.H. used printed coils. I have and old "Pro" and noticed that the coil was really thin. Makes sense since it's an off-resonance design and uses a mono coil. Allen Hametta has a new probe out, it's pretty slick.
- Carl
 
Hi Carl,
I'd be interested to know what the inductance and resistance is for the above Tesoro coils.
Eric.
 
I tried to measure the inductance of the stock (8") coil using an HP 4261 LCR meter. I could never get a good reading. Strangely, I had the same problem with the White's PI coil and old Garrett BFO coils. Seems that all my mono coils had problems, but every single IB coil would measure fine. I did jot down some numbers, but they're at home so I'll post them later.
- Carl
 
Hi Carl,
Yep, AH used pcb coils back in those days. At least they did on the coil I used to have. I lost it a long time ago, and I can only vaguely remember what the pcb looked like. If I remember correctly, it had more than a single winding on the top. I wasn't sure what he was doing at the time. It has been too many years ago for me to remember which model the coil fit.
Reg
 
The stock coil read 358uH and 11.0 ohms. I have not got round to measuring the other 2 Tesoro coils, the LCR meter belongs to another group at work so I will have to borrow it again.
The SMPI coil measured 230uH at 1.37 ohms. The big honkin' Garrett BFO coils (12x24 and 24x24) each measured about 120uH at 3.2 ohms.
- Carl
 
Carl, a quirey, what Freq is your RCL meter operating at during these tests?
 
I think the 4261 has a 100Hz setting and a 1kHz setting, or something like that. I was having problems with the mono coils so I tried both frequencies. As best I can remember, the higher frequency would not work at all, and the lower gave suspect measurements.
Anyone ever use one of those little BK Precision handheld LCR meters? Jameco has a branded version for $140.
- Carl
 
Hi Carl,
Under different conditions all the little cheap meters can choke as well. But not usually with things the size and construction of metal detector coils. Most of the little meters I have seen have been 1KHz, that is the good ones. The really bad ones oscillate with the coil and measure that. Usually too much series resistance on a many many turn coil will upset them, or too much parallel capacitance, or both.
Make sure the search coils don't have a cap in them as most resonant both the xmit and rec coil, but think you know this. Usually the cap is back with the rest of the electronics, but some got cute and put some cap in coils.
Anyway, when all else fails, parallel the coil of interest with a know capacitance (0.1uf ?), excite this arrangement with a sign wave generator, with a 10,000 ohm resistor in series with the generator. Hook up a scope probe across the coil and cap, hooking the ground of the scope to the ground of generator, the probe tip at the junction of the coil cap and resistor.
Sweep the generator in frequency till the you find the highest peak in voltage on the scope. Then you know the cap value and the resonant freq, and solve the equation ((1/(freq*2*pi))^2)/cap = L.
Hope I got that right.
Carl you probably know all this, but might be helpful for others without a RLC meter, but have scope and generator.
This is the most definitive method I have found, never quite sure with them new fangled digital read out stuff. Might be right might be wrong. You pays you money and takes you chances.
Which is the way cold fussion got started, with the digital readout on a neutron detector. Stupid scientist thought that when it read 151 neutrons that there were really 151 neutrons.
JC
 
Hi Carl and everyone,
By the way RLC meters really choke when measuring Q or 1/dissapation factor. Watch this one. Top of line equipment usually does ok, especially if it is made to measure this.
Once you get done measuring L with the above setup. Then remove the parallel cap and sweep again. This peak will be at a higher frequency. Solve the same equation putting L in the place of C. This will now yield C which will be the self capacitance of the coil. In case anybody is interested in this value for any reason.
JC
 
Hi Carl,
I have a BK model 878 hand held LCR meter and a Wavetek.
On the coils I have made, the two LCR meters read very close to the same.
Reg
 
I have one as well and use it to look at detector coils as well as the small inductors I'm using in my power supply circuits. I have been very happy with its performance. I'm using the B&K, not the Elenco.
Thanks,
Charles
 
I did try the manual method to start with, but most IB coils apparently do have internal caps. Or a whole lot of self capacitance. So I was having to measure the peak with no external cap, then measure it with an external cap, to calculate the value of the internal cap, in order to calculate the inductance.
Then I found the LCR meter at work, so I said to hell with that and just jabbed 'em with the meter!
- Carl
 
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