Instruction monocoil for minelab GP series

[bigrex]

Hello,

I was pointed in the direction of this forum by a Mr. Bill on a treasure forum. Anyway, I was just wondering if anyone had any fairly easy to follow instruction or directions pertaining to building a monocoil for a gp series minelab detector. I have an 8"mono, 11"DD, and 18" mono which I have purchased but thought it could be an interesting project to try to build a monocoil. I was looking at building a somewhat larger one maybe around a 25" round or 32" x 18", or 34" x 16" elliptical. I did run across some information on building a monocoil at:

http://www.thunting.com/geotech/page...Hv1p0_150c.pdf

Page 9 gives some instruction about producing a monocoil, yet I was wondering if there is anything out there that specifically focuses on monocoils which could be downloaded or purchased. I already ran a few searches on this forum just in case and looked at the projects section as well. I know for larger coils there is a some sort of problem that can be aided by a lower AWG or higher wire diameter, although the weight would suffer somewhat. Anyway, any comments or help would be greatly appreciated.

RCB

[Largesarge]

The first thing I would say is to measure the inductance of the 2 mono's you have, that should give you an idea of the requirements of the electronics. Then using a coil calculator such as the ones at www.chemelec.com to get an idea of the wire gauge, turns and such. Good luck.


From the desk of a total amateur.

[bigrex]

Thank-you for the reply Largesarge,

I noticed the chemelec website in the past and sort of forgot about it. Forgive my ignorance but how would you go about measuring the inductance? I found a site that gives a method for doing so without using an inductance meter, but I'm not sure how I would go about connecting the unknown L Or coil to the setup shown. The webpage I'm referring to is:

http://www.oz.net/~coilgun/theory/measureinductance.htm

[bigrex]

Also, I managed to find an inductance meter on eBay for about $30, but again am not too sure how to go about hooking up a meter to the detecting coil. Thanks for reviewing this message. Do I need to open it up and attach it to each end of the wire coming off the loop?

RCB

[Fred]

Yes, mesuring inductance is easy with an inductance meter, just connect it to the coil from its the wires, but be carefull to be far from anything that can change its value.

Fred.

[Charles (Upstate NY)]

Yes, mesuring inductance is easy with an inductance meter, just connect it to the coil from its the wires, but be carefull to be far from anything that can change its value.

Fred.

I second that, at least 3 feet from any metal objects. I would recommend the Extek LCR meter as entry level for coil building. There are cheaper meters out there but you need one that measures Q. My LCR meter of choice is the BK Precision 879. I have both and they get the job done nicely.

[Charles (Upstate NY)]

Hello,

I was pointed in the direction of this forum by a Mr. Bill on a treasure forum. Anyway, I was just wondering if anyone had any fairly easy to follow instruction or directions pertaining to building a monocoil for a gp series minelab detector. I have an 8"mono, 11"DD, and 18" mono which I have purchased but thought it could be an interesting project to try to build a monocoil. I was looking at building a somewhat larger one maybe around a 25" round or 32" x 18", or 34" x 16" elliptical. I did run across some information on building a monocoil at:

http://www.thunting.com/geotech/page...Hv1p0_150c.pdf

Page 9 gives some instruction about producing a monocoil, yet I was wondering if there is anything out there that specifically focuses on monocoils which could be downloaded or purchased. I already ran a few searches on this forum just in case and looked at the projects section as well. I know for larger coils there is a some sort of problem that can be aided by a lower AWG or higher wire diameter, although the weight would suffer somewhat. Anyway, any comments or help would be greatly appreciated.

RCB

A monster size coil now thats my kind of coil! I'll offer a suggestion on construction to keep the weight down. Pick up a 4x8 foot sheet of 8-10lb rigid urethane foam, 1 inch thick. Cut a template the shape of the coil you want to build in 1/2 plywood which you can use to route out the foam coil. You can cut a channel in the foam for the windings with a dremel or panel router, lay the windings in, pot the channel with epoxy. Then coat the entire outside of the foam with epoxy, and maybe a little fiberglass in key areas for strength.

I once built a 21 inch coil using an ABS shell and potted with unfilled epoxy. She weighed a lower rod flexing 4 pounds. So keeping the weight to a minimum should be a key design goal on a coil this size.

[Largesarge]

Rex, I've found that Gary's (Chemelec) calaculators are pretty near spot on.

If for instance for needed a coil of 350uhy, using the most recent calc., a 25" coil is 18 turns of 20.5 gauge wire.

I've found a couple of coils including a rectangular (took outside dimension and used as circumference to calculate as a round coil) and then checking with my Tenma meter (e-bay special ) and they were within a couple micros of what calaculator said.

I think you could use them with confidence that the coil will be what you want.

[Reg]

Hi RCB,

First, I have not tried to build a coil for a ML but have heard stories about the difficulty involved to get them to work correctly. I have built a bunch of coils, both mono and DD for the Hammerhead, my GQ clone and my Goldscan PI's and they all worked just fine. However, none of them will work on the ML as far as I know.

Fortunately, it is very easy to make a coil work on the Hammerhead and Eric Foster's machines (my GQ clone and my Goldscan).

Unfortunately, I do not have access to a ML, so I can't try to make one. I would have to look at the coil signal to get an idea of what I needed to do if it didn't work. Since no machine is available, I won't attempt it at this time.

Now, what you will most likely have to do is to match both the inductance and the resistance of the Factory ML coils to work correctly. At least, that is a good place to start. Then, the shielding of the coil can have an effect, so you may want to try different methods there also. As for wire, it will require a fairly large wire, maybe an 18 awg or so. Just wind the coil to an inductance of 300 ohms and a resistance of about .5 ohms or so to start.

You could use a Litz wire also, but it is more expensive and difficult to find in the larger sizes. Also, someone told me that some of the third party coils are made with stranded wire, rather than Litz, so I would start with that and see how it goes.

There aren't too many people in the US that have tried to make a coil for the ML, but quite a few people in Australia have. As such, you might ask the question of what is involved on one of their websites.

Good luck and keep us posted as to how it works out.

Reg

[Unregistered]

Hello,

I guess I don't have to worry about balancing the coil with an oscilloscope since it isn't a DD coil? Should I also measure the capcitance? I guess if the inductance is the right level the capacitance would be as well? I just want to make sure I have a basic handle on things before I begin to start purchasing parts, equipment, and disassembling a factory coil. Thanks for all the replys and good advice, it's greatly appreciated. I'll try to also see if I can find any Aussie forums for a little extra minelab info. Does anyone know of any Aussie tech/electronics forums? I know plenty of nugget detecting forums, but most of them would probably be hostile toward creating your own coil and going around the minelab folks.

Regards,

Rex B.

[bigrex]

Additional Note

On that last post I failed to login, so I'm the "unregistered" user. I also wanted to mention that I too have heard that companies such as coiltek don't use litz wire. The slight advantage of the litz wire in theory seems to not make a difference detectable by the ear as far as what I've heard from some people who were doing some of their own tests.

[bigrex]

Additional note 2

Ok, here's another ignorant question (I guess most questions stem from ignorance since that's the nature of the beast?) Anyway, now I'm just wondering how ohms are related to inductance, I already have something which will measure ohms. I thought inductance is measured in Henries and the symbol for inductance is L?

[Reg]

Hi Rex,

Inductance and resistance are not completely dependent upon each other, so you can have a coil with an inductance of 300 uh and that coil my have a resistance of anything from .5 ohm or less to maybe 10 ohms or so. The resistance will depend or vary because of the size of wire used.

So, the inductance is primarily determined by the number of turns and the diameter of the coil, and the resistance is determined by the total length of wire and the wire size.

If you want a low resistance coil, then you will use large wire.

Now, you asked about the capacitance of a coil. Normally, this is determined by the type and thickness of the insulation as much as anything. Use a thicker insulation or better insulation and the capacitance goes down. Care must be taken because the inductance may go down also simply because the windings are thicker and longer. So, all factors do interact.

Now, you don't have to dismantle a coil to get an idea of what is happening inside. You just need good test equipment that is accurate. On the plus side, much of what you need to know is already been figured out or can be measured, which are the inductance and the resistance.

Just measure a ML coil that you have and you will now know what you need to try to match. My guess is the inductance will be 300 uh and the resistance will be about 0.5 ohms. So, given that information and the size of coil you want to build, you can figure out the approximate size of wire needed.

Lets say you want to build a 11" mono coil. Ok, you need to figure out the number of turns of wire needed. In this case, the coil will probably need about 19 turns. This may vary by a turn or so, depending upon the insulation used.

Ok, lets assume you need 19 turns at 11" diam. This results in a total wire length of pi times the diameter times the number of turns or 3.14159 x 11 x 19 or 656.59 inches, which is 54.71 feet. For simplicity, we will round this off to 55 feet.

Now, you can go to a chart for wire and find size where 55 feet is about .5 ohm. Now, by going to a wire chart you can determine the approximate wire size. In this case, it appears to be somewhere between a 19 awg and a 20 awg is what would probably work, depending upon the wire itself.

Ok, now to get an idea of a factory made coil and its self resonance frequency, you will need a scope. Just take the connector apart on the coil you are testing or get a matching connector and connect the two and then connect the scope to the coil lead pins. With this done, turn on your ML with a coil attached and move the coil you are testing near the operating one and notice the ringing frequency. This will give you something to shoot for in terms of what your coil you are building should do to work.

Anyway, this should get you started. As for contacting someone in Australia, you might leave a message asking for help in building a ML coil on the Finders forum or possibly on the Prospecting in OZ forum. Hopefully, Robby H or another of the people more knowledgeable about ML coils will jump in and provide a little more info. There are quite a few guys over there who have built coils so, hopefully, someone will jump in and help.

Reg

[Unregistered]

Comments

Thanks for a comprehensive reply Reg,

I do have one more question though, sorry if I seem to be asking the same question twice. You mentioned that it's not really necessary to dismantle a coil but that I do need good testing equipment that is accurate. However, it was my understanding that to measure things such as capacitance, I would have to attach the meter to the end wires of the coil which is inside the shell or coil housing. I suppose I could use the calculators for figuring most of the specifics but I suppose it should be coupled with the equipment tests as you suggested. Please let me know what I'm missing here when you have a chance.

Regards,

Rex B.

[bigrex]

Question

Now I'm just wondering what "winding length is" Is it the total width of the bundled wire after all the "turns" have been completed or wound? Or is is the circumference around the wound coil?, or is it the total length of the wire if it were unwound? and is coil length the same as winding length. (A short response of a couple sentences to this question will be sufficient), Thanks again.