Tweet
As a suggestion: Self adhesive copper foil, which is available in various widths rolls from Stained glass suppliers as well as in wider widths and sheets from some electronics suppliers seems to work out really well for shields. I find it easier to work with than aluminum foil (all thumbs), and it is solderable. It is a bit pricey though. Also the rolls of self adhesive aluminum foil used for muffler bandages might work out also. The foil is much thicker and the adhesive as I remember was very good.
-
-
Re: copper foil
Hi Russ,
One has to be careful using thick copper or aluminum foil for shielding. It will be detected and will cause a droop in the output of the preamp. This can effect several things. Depending upon the delay, it could be sufficient to saturate or nearly saturate the differential or sample hold amp.
One way to overcome this a bit is to not over damp the coil, this way the output of the preamp is such that the sensing of the shielding can be compensated for somewhat. It may not be proper but it works.
Now, for a really cheap shielding material, I just tried some aluminized mylar ribbon. I picked up some at Wal-mart for about $2.00. It measures about 1.5 ohms per inch. Surprisingly, this worked quite well, about as good as the copper mesh tape I mention below. You have to tape the ground lead to the surface of the mylar tape.
Electronic Goldmine did have some extremely thin copper foil tape. Actually it is a form of mylar coated with an extremely thin copper mesh. This is thin enough to be used ok. It sells for about $10 a roll, if they still have it. You have to tape a wire to the copper rather than try to solder it though since the copper is so thin.
One way to do compensate for the shielding is adjust the damping so there is a very slight dip when the coil decays to 0V when looking across the main damping resistor. This will be reflected as an extra hump in the signal as seen at the output of the preamp. The signal will decay from the opposite polarity than what would be seen when the coil is over damped.
As for other shielding materials, I have used conductive rubber tape that I would expect to work about the same as using carbon based paint. What I noticed is this doesn't seem to reduce the noise as well as the copper or aluminized mylar tape.
Reg -
Re: copper foil
Hi Reg
Thanks for the information. Is the effect you mentioned true for IB as well as PI coils. I am currently working on only IB designs though I intend to get into PI in the future. This has been a great learning experience for me since I decided to build my own. Having been an electronics professional for 30 years the various circuits that I have seen really don't present a problem for me,, but the coils are the real challenge as I think most would agree. This forum is a great help. So many good folks sharing their experience and information. Thanks again.
RussComment
-
Re: copper foil
Hi Russ,
I really cannot comment about IB coils since my main experimenting has been with PI coils. I know that most manufacturers use a conductive spray or now a conductive housing for a Faraday shield, but whether the coil would benefit by using a more thorough shielding or rather it would cause problems, I cannot say.
If I were to be working with an IB design, I would probably build two identical coils, one with a paint shielding and one with a tape shielding to see.
RegComment
-
Re: I forgot
Hi Russ,
I forgot that a friend of mine used a some aluminized mylar foil a while back on a TR and it worked fine with no negative side effects.
In his case, he cut a piece to fit in the bottom of the coil housing rather than wrap the coil windings.
If you decide to try this method, I would recommend you shield both bottom and top if the detector is very sensitive.
RegComment
-
Re: copper foil
Hi Reg
The conductive paint would certainly be easier. I was thinking of trying aluminum spray paint. However that method would seem to require some sort of channel for the coil which could be coated on the inside similar to the molded ones. I suppose I could mill the appropriate slots for the coils out of some suitable material, ABS probably. There is much opportunity and need for comparative testing. So much to do so little time, and so many variables. It seems ever juncture in the quest for the best design brings with it a new set of questions. When I got interested in building a really good machine I thought it would be an interesting diversion for a couple weeks. Now it seems I may have opened a can of worms I may never get the lid back on. Perhaps a warning should be displayed on the top of the site. " Building Metal Detectors May be Addictive, Enter At Your Own Risk"
RussComment
-
Re: copper foil
Hi Russ,
I am not sure the aluminum paint is conductive. I would spray some on a test board and check it. The shielding has to be conductive to work. Dave Johnson posted the brand of one of the graphite sprays available on a previous page.
Strange you should mention about milling the slots in a suitable material. You might consider some rigid foam.
That is basically what Minelab does on some of their coils. Except they use a rigid fiber material instead of foam as a coil form. The form has grooves built in for the windings. These grooves are filled with a clear epoxy of some sort to hold the windings solid.
Their shielding consisted of a piece of some type of tough paper coated with a conductive paint, taped to the bottom of the winding form. This assembly is then mounted in the plastic coil housing. Some spray was applied to the top of the form also.
If you do not have a coil housing, any simple thing can for for experimenting. I used a piece of masonite cut to the general shape. However, for the final design, I opted to purchase a coil housing. Both Bill Hays of Hays Electronics and Rick Schroeder make and sell coil housings. Unfortunately, the selection of sizes and shapes are limited.
RegComment
Comment