Shielding, and more...

[Anonymous]

Hi Reg
Are you saying that the 'boing' happens on ground weed stubble even when the shielding has passed your hand test? This is interesting as I have been out again this morning and there is very little difference between all round shielding and top'n'sides only. I did forget to split the shielding on the all round one so there might be some leg room available yet.
I am a bit unsure of what a DD type coil is? maybe I call it something else. I have looked at the messages and still I am not sure. Maybe I am having a stupid day, could be the rain!!!
My fig 8 coil: Tx=14turns 7/0.2 wire 24AWG about 220mm diameter. Rx=18turns 7/012 wire 28AWG after folding just fits inside Tx by 12mm.
Standard round coil: Tx=same. Rx= 18 turns 7/0.12 wire 28Awg 208mm diameter.
Target example= M3 nut in A2 grade s/steel. 10 of them weigh 3.2g. Any orientation. I have other s/s targets that are even stranger as they are huge and undetectable by the round coil and detectable by the fig8 at way over 100mm, but I do not know the composition. The M3 is detectable only at about 50mm.
The delay is 10usec, and all parameters unchanged, and just a straight swap.
I will go and try 5usec if I can stabilise the round coil. As I can have 8 samples at 5usec intervals, I can see the whole curve, and it is into the noise by the 3rd and so gone completely by 15-20usec. Test was done in air only.
Many thanks Reg I for one am learning heaps thanks to you and Eric. Still can't find the thread on MOSFET heat drift! Please help someone.

 

[Anonymous]

Hi Roy,
My coils that pass the hand test do not introduce any noise or "boings" when passed over weeds.
However, I have a coil that does NOT pass the hand test, that will produce a strange "boing" when passed over certain weeds. It is strange, only a couple of different weeds seems to cause the "boing". Normally, the coil seems to work fine even when brushed up against weeds or grass. However, I have found a couple of small weeds that have a small flower that do cause the strange response.
When it first happened, I wasn't sure what was going on since it was just an occassional "boing". The offending weed was reasonably scarce so I didn't pass over them too often. To make matters worse, it didn't happen all the time even over the same weed. I found the weed or my guess is, the flower part would have to flip up and hit the coil on the top where there was little shielding to cause the audio response.
I tried to simulate the condition intially but couldn't not matter what I tried. I had to actually be around the certain weeds to have the audio "boing" occur.
On a different note, thanks for the info on the SS parts. Now, I have to modify my detector so I can use a separate transmit and receive coil. Unfortunately, at this time, it is not set up that way.
I should be able to track down some of the SS nuts you mentioned also.
The only figure 8 coil I have worked with is one I made quickly and it was both a transmit and receive coil.
I am curious if you ever tried a single smaller receive coil, say about the same diameter as one of the two coils you create with the figure 8 design, to see if it would allow the detection of the SS items any better than your larger receive coil?
Now, a DD design is basically called such because the coils vaguely resemble two D's with one facing the opposite direction. They basically have the straighter portion's overlap. Most coils I have seen made today using this concept actually use two equal sized elliptical coils that overlap about an inch or so along the vertical center axis of the coil housing. One coil is the TX and the other the receive. The positioning or overlap is adjusted to there is little or no signal out of the receive.
As for the knowledge I have on PI's, I have to thank Eric for that. He has taught me more this last year about PI's than I thought I would ever learn. Quite frequently, I veer off track, but fortunately, Eric will cover the subject and put me back on the right track.
As for any knowledge you may obtain from me, I would have to say you should take it more as "food for thought" rather than a standard. I will write what I think is correct, but that is not to say that there isn't a much more accurate answer.
Now, as for stupid days being related to rain, I am fairly certain there is no direct correlation. I have stupid days all too frequently, while rain is becoming nothing but a faded memory.
As for the thread regarding the FET's, I missed it so I can't help you there. What I have noticed from reading the data books is, depending upon the FET used and the drain current, the "on" resistance can increase from about 1.75 times to over twice its original value, when the FET temperature rises from 25 deg C to 100 deg C. Again, these are only approximations and my limited ability to interpret the Data book info.
Reg

 

[Anonymous]

Hi Roy,
The discussion on a constant current TX was all around 6th January this year. Thomas Breuer of TB Electronic posted a CC circuit.
Not raining just south of Oxford, nice warm sun. Where are you?
Eric.

 

[Anonymous]

Hi Reg
The trouble with too much sun is that you forget how stupid everything is in the rain! Believe me, its raining now! And I'm as thick as 2.7182 relatively short planks!(English joke? it wasn't funny anyway)
I have another question that has now emerged. I have been making different coils with different wires. Lets say I want to output 10V and a limit of 0.1A. The V is regulated and the current rises from zero to 0.1A in a certain amount of time depending on the coil characteristics. Is it better to have less turns and have a shorter pulse width, or more turns and a longer pulse width? The final output power being the same in both cases. Which is the more desirable, or are there applications for both?
I shall have to try a DD! Is the eliptical shape in order to get both in a round shape housing? Or is there a more mathmatical reason? Both seem equally plausible.
Thanks for your needless concern regarding the quality of your advice. I won't blame you when I blackout London I promise. In my opinion, we all know what works for ourselves, and if someone else wants to hear about it then all is good, but if it doesn't work for them, thats tuff! But at least you've learnt something! I say...Question all information, be critical of all 'facts', and if possible never take anybodys word for it! Find out for yourself!
Rgds
Roy

 

[Anonymous]

*Originally posted by Roy*
1) Shielding of the coil! why is it so critical? I get drift and interference if its not very even. I am using nickel shielding spray, any better suggestions? Can I make it less critical?
I make my own sheilding compound from Graphite powder mixed with a water based clear gloss laquer "paint".
I'm not sure what paint types are available in the States, however not all clear gloss mixes work as they can insulate the particles...some experimenting is in order <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT="">
However it is a lot cheaper alternative than Nickel sprays.
Most Nickel sprays I have come across have a surface resistance of around 5 ohms over the coil surface diameter.
Graphite will produce between 500 and 2k/ohm depending on your mix. (it doesn't seem to be too critical for the SD series.) I have found it works perfectly.
I completely cover the coil in blotting paper and secure it firmly with paper based packing tape... then paint it all over with the screen mix...no gaps.
Ensure the top and bottom "Earth" wires from the coax feeder inside the coil body (copper hookup wire)are the length of the coils inner diameter and are laid inside and approx 2/3 the way around the casing diameter.
Tape them securely to the outer Screen mix using paper based tape. (plastic is not good in any coil area)
I never get "boings" in highly mineralized ground or "hot weed" areas.
Hope this helps. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">

 

[Anonymous]

Hi Roy,
Boy, your question about the different coils/pulse width's is out of my league. It is better if I leave this question alone and have people think I am stupid, than try to answer it and remove all doubt.
Since I am not quite sure of your question, you might refer to Thomas Bruer's post on or about January 4, 2002 along with the one mentioned by Eric. Maybe the timing charts and the discussion just preceeding them are part of the answer you are looking for. That is the best I can do.
This question is definitely one for Eric, Thomas Bruer, Dave Johnson, or one of the other more knowedgeable people who frequent this forum.
Now, as to your question about the DD coils, my knowledge of them isn't much greater. I can only guess that your answer of, so they can get them into a round housing is probably one main reason. Also, I think it is probaby easier to mass produce a consistent sized elliptical coil using a simple standard form than try to build one with multiple bends, etc. I am fairly certain they designed the initial shape with more of a mathmatical concept, than using my technique of bending, forming and mutilating to see what works.
Hey, if you do find a PI technique that blacks out London, I would be interested in some of the details. I would be interested in trying such an experiment, but probably wouldn't recognize some of the results since I am operating in the dark most of the time anyway.
Your advice is excellent. I will keep it in mind. Generally, I try to follow a path similar to your advice but get sidetracked too often. Unfortunately, my knowledge has limits, but my ignorance has no boundaries.
Reg

 

[Anonymous]

Hi Gaz,
I admire your creativity. I tried to make a similar mix a few years back but couldn't find the graphite powder so I tried an alternative. It didn't work as I had hoped so I abandoned the idea. Instead, I started using a conductive picture tube coating. That seemed to work quite well.
I probably should have mentioned that the "boing" I mentioned did not occur on my home made coils, but on a C Scope factory coil I was experimenting with. I noticed sometime later that the top of the coil is either not shielded or very poorly shielded.
So far, I haven't experienced any adverse reactions with any of my coils, even when I used Scotch 13 semi-conducting tape. Like you, I have a small conductor glued to the tape to assure a consistent resistance around the circumference of the coil. The resistance of this tape is quite high. However, I haven't seen any problems and it seems to totally eliminate any obvious capacitance effect. This tape is a rubber base, rather than plastic.
What I don't know about this tape is the consistency of the resistance of the tape from roll to roll, so I may run into problems later. Anyway, it sure makes the coils lighter.
Reg

 

[Anonymous]

****I tried to make a similar mix a few years back but couldn't find the graphite powder****
I used Dixon's Air-Spun Graphite which is made by Thomas Grozier+Son PhAustralia) 612 94273635 in 450g tins.
If you give them a call perhaps you can find a dealer near you. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
*****What I don't know about this tape is the consistency of the resistance of the tape from roll to roll, so I may run into problems later. Anyway, it sure makes the coils lighter.*****
It's my opinion that it's the "Sticky" glue on the tape that is more the problem than the tape type itself as it is more likely to be conductive.
It would appear the paper backed tapes tend to "dry out" when applied to the Sheild thereby decreasing any "moisture tracks" that may be creating paths for spurious signals/noise/static around the coil. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
...and yes, most likely all over screening should fix the probs with your C Scope factory coil. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">

 

[Anonymous]

Hi Gazza
I find it interesting on your comment that plastic is not good in the coil area,I have felt this also,and I try to use as less possible plastic in the coils as possible.
Regards Frank Wallis

 

[Anonymous]

Hi Reg and Gaz,
I seem to remember some posts a long time ago, from Oz, refering to static buildup on the detector and/or person. This happens particularly in hot dry climates and results from swinging the plastic coil housing over very dry ground. Occasionally, the static will discharge as a spark to a conductive plant, creating a boing.
The electrical shielding/grounding in a metal detector is floating with respect to a true earth ground, so potentials can easily exist between the two.
Eric.

 

[Anonymous]

Hi Eric, it's nice to be amongst such a knowledgable bunch! <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
I think I have encountered the type of "boing" the gentleman you are refering to mentioned.
With the SD's at least, there is a major need to ensure that the coil connectors are properly secured to the detector due to the very low impedances involved.
Even physical contact with your person from the lead while "swinging the stick" can cause odd calls.
Notwithstanding, if the boing was definately occuring due to contact with "hot weeds" etc, it's usually a breakdown in one of the following areas.
1)The shield coating in the Minelab/Coiltek coils due to internal movement (largely resolved now).
2)One of the feeder earth wires from the coax sheild actually breaks where it's bent to start contact with the sheild itself... (I don't think they should be using <STRONG>annealed</STRONG> copper wire imho.)
3)Water /dust entering the coil via the seam where the glue has not withstood the hard knocks due to normal wear and tear, this wets the paper inside causing all sorts of dramas.
Coiltek here in Aussie will normally fix any of these problems. (if a user isn't too keen to play "doctor" themselves. <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT=""> )

 

[Anonymous]

I have been out of comms but alas I am back! I will try a 3 coil fig of erm... 80?
I have also found that the connection to the sheilding is a problem area, and can quickly break down. I now use lead tape with conductive adhesive and then paint over with nickel paint.
seems to work a treat. Can you tell me what thickness the lead tape is you use for your shielding?
Many thanks
Roy