Instruction monocoil for minelab GP series

[bigrex]

?

Is the coil length usually 0.25" ?

[Largesarge]

Rex, as far as coil length is concerned think of the coil as wound on a form and since you're going to be winding overlapping turns I usually use .25". Below are the results from Gary's calc. for an 11" coil.


Inductance0.302 mH, DC Resistance0.69 Ohms, Wire Gauge20 AWG, Wire Diameter33.4 mils (1 mil = .001 in), Coil Length0.25 in, Coil Inner Diameter10.75 in, Coil Outer Diameter11.0172 in, Average Turn Diameter10.82 in, Wire Length67.98 feet, Copper Weight0.21 pounds, Turns24, Levels3.21, Maximum Turns Per Level7.5 (Based on wire diameter), Nearest British Wire Gauge21 SWG, Metric Equivalent0.848 mm, Includes Insulation Thickness, Current Handling Capability1.46 at 700 cm per Amp.

This falls in pretty much with what Reg said cept for a couple more turns, but all the other info is right there.

[bigrex]

It seems to me all the calculators ask for variables I don't yet know. I only know the desired coil outside diameter of 24 or 25", possibly the inductance (300-350), and that's pretty much it, the gauge wire I guess could be 18 to 20+ AWG But I don't know the inside circumference, the number of turns, the width of the coil, the coil inside diameter, or the coil length, which seem to be the main variables these calculators ask for. Therefore, I can't seem to use the calculators. I did some math using a forumula:

no.of turns of coil (squared)=[Microhenries(18xdiameter)+40(coil length)]/
diameter squared

I assumed coil length would be 0.25"?

so, TOC2=300(18(25) + 40(0.25)/25x25

TOC2=135,010/625

So i need approximately 14.7 turns of the coil

I just wanted to see if I came up with the same thing as the suggested

(25" coil is 18 turns of 20.5 gauge wire). So now I'm confused again and would appreciate any advice. I'll try to make this my last question as they are probably becoming a nuisance.

ps-I've now posted messages on the Aussie sites that were mentioned, thanks again.

[bigrex]

Thanks for the reply largesarge,

I'm still not sure I follow you so I prepared this image and all one has to do is respond A., B., or C. or none of the above to answer my question about coil length. Thanks. Sorry if I seem dense. I imagine it would have to be B. if it can be as small as .25", either that or it is something else I don't have shown here.

Note-I got the formula from the previous post at:

https://www.geotech1.com/thunting/te...-L_formula.jpg

RCB

[Largesarge]

Rex, If you look at the data you see it says 3.21 levels of 7.5 turns. Those 7.5 turns or .112" is the coil length (you're looking edge on). I use .25 to start my calculation but the program usually changes that.

[bigrex]

Do you mind if you give me a link to the calculator you are using? Since there are several I would like to make sure I'm using the right one, or the one that you are. Thanks.

I used one at:http://www3.telus.net/chemelec/Calcu...Inductance.htm

and came up with the following while trying to keep inductance around .3 mH and resistance around but not above .5Ohms, does this look right?:


Inductance0.3033 mH DC Resistance0.4676 Ohms Wire Gauge16.5 AWG Wire Diameter49.6 mils (1 mil = .001 in) Coil Length0.25 in Coil Inner Diameter24.71 in Coil Outer Diameter25.0076 in Average Turn Diameter24.8084 in Wire Length103.9171 feet Copper Weight0.7318 pounds Turns16 Levels3.1744 Maximum Turns Per Level5 (Based on wire diameter)

[Reg]

Hi Rex,

Normally when working with formula's designed to calculate the inductance of the coil, such formulas are designed for single layer coils wound in an inductor fashion where they may a length dependent upon the number of turns.

Now, with the website I use, the calculations are such that the length is asked for and I simply use .22 inch or .25 inch depending upon the wire used. These numbers are basically the thickness of the windings which, will be either the length or the width of the winding itself.

I use a different website which I have found to be quite accurate when buiding coils. I have found the calculations to be quite accurate when I check the coil with my BK 878 inductance meter. On this site, you only need to know the diameter of the coil, use an estimated coil length of .22 to .25 inch and initially simply guess at the number of turns. Adjust the number if the guess is way off and with a couple of guess can zero right in on the correct number.

That site is:
http://my.athenet.net/~multiplx/cgi-bin/airind.main.cgi

Using this site, and selecting a diameter of 24 inch, a length of .22 inch and guessing a number of turns as 11, the coil calculates to be 258 uh. Increasing the number of turns to 12 will predict a coil of 307.52 uh. (Always use the .1% accuracy answer)

I prefer this site because it is so easy to use and so far, I have not found it to be more than a turn off when using the same size wire. Now, if I use a totally different wire, then it might change by another turn or so, but still will be quite close.

Reg.

[bigrex]

Thanks Reg,

That calculator page leads me to another question. I noticed the ohms are in ohms per 1000 ft. How does that compare to the target of 0.5 ohms (ohms of resistance) that was suggested for the coil? I also noted the other calculator page was in mH while this one is in uH I suppose the mH one is in microhenries? For example when I entered 25" diameter 11 turns and .25" length I got 6.386 ohms per sq ft.(for 18AWG), should I move the decimal over one place? Thanks once again for all the tips and advice.

Rex B.

[chemelec]

Bigrex,

uH is Micro-Henries
mH is Milli-Henries.

[nncunew]

bigrex

the coli's length is the width.

in your picture is the 2nd case. I have the same question last month but after some analisis i determined what does means COIL LENGTH.

In order to obtein the real coil length (total wire length) you need to calculate a liner integral that depends of the diameter variation (initial radio and numbers of turns) and the wire diameter, with this value you can obtein aproximate DC coil impedance.

newly, sorry for my bad english... i speak spanish

Thank,

[email protected]

[bigrex]

On my last post I meant per 1000 sq ft.:

Thanks Reg,

That calculator page leads me to another question. I noticed the ohms are in ohms per 1000 ft. How does that compare to the target of 0.5 ohms (ohms of resistance) that was suggested for the coil? I also noted the other calculator page was in mH while this one is in uH I suppose the mH one is in microhenries? For example when I entered 25" diameter 11 turns and .25" length I got 6.386 ohms per (1000) sq ft.(for 18AWG), should I move the decimal over one place? Thanks once again for all the tips and advice.

Rex B.

[robby_h]

Hi Bigrex,
I built many coils back in the vlf days and lately for homebrew pi designs but none for Minelab pi detectors mainly because of the abundance of coils here in Oz. It may seem a waste of time but maybe you should start by making a copy of one you have access to first in order to acquaint yourself with the problems. ML uses a front end that differs noticeably from other pi detectors so you will have to carefully “fit” the coil to the design. There are some knowledgeable guys around and you’d do better if you can contact them.
Do a search on the pi classroom for posts by Eric Foster and Stef on making a mono coil for the ML pi. Stef has made quite a few and seems helpful.

http://www.findmall.com/list.php?34

Good luck,
Rob.

[bigrex]

Actually, that's where I began my search, I couldn't seem to see any way to personal message Eric Foster or find any contact info for him on the forum though I noted posts he had made, nor did I see his contact info. on sites that talked about the goldscan 5. I'll look again though. Thanks. I did make a post which recieved no attention in the PI forum. I've posted in 6 forums total (chemelec, geotech, detecting OZ, an Aussie MSN detecting group, an Aussie Yahoo detecting group, and the PI forum). One australian forum did give some help, and chemelec offered a bit, but this forum has been by far the most helpful. Update-I was able to find this time how to send a personal message to Mr. Foster. Thanks again.

[bigrex]

Here's some info. I recieved on the prospecting OZ Yahoo group. I'll post it here so anyone can look it over just for their general information:

For minelab coils you need an inductance of about 300uhenry and an impedence from memory for about .4 ohmns. The inductance is fairly critical as ML machine don't like if the value deviates too much from 300uh. Use a winding method that gives you the lowest possible interwinding capacitance and the highest resonant frequency.

[robby_h]

http://www.findmall.com/read.php?34,...935#msg-134935

http://www.findmall.com/read.php?34,...947#msg-134947

I think Eric suggested 22x0.2 stranded hook up wire was ok for starters.
Rob.