Homemade coil

[Phil]

I am inthe middle of building the P1, in building the coil I plan on placing it in a prefabbed coil structure. I am wondering what the out come would be if I filled the inside with fiberglass,NO CLOTH just the fiberglass.Anybody got any ideas.

[Gary]

Re: Homemade coil

REAL HEAVY, Other than that, OK

[Reg]

Re: Homemade coil

Hi Phil,

I guess part of the key to answering your question is just how fast are you going to try to sample? The reason I mention this is because of something Eric Foster posted on the PI forum concerning potting resin. Here is his quote.

"Two other things that make a difference to the winding capacitance. One is the insulating material (dielectric properties) on the wire. i.e. PVC insulation is different to Kynar. The other is potting material. I am very careful not to let any potting resin creep into the coil windings, as this can have a dramatic effect on coil capacitance."

So, a person should be careful on just how they pot the windings and what they are going to use if they are going to try to sample with very short delays. It can make a difference.

Now, Gary is correct, if you just use fiberglass resin, the coil will be extremely heavy. This will be fine if you are going to use it as a boat anchor also, or only when searchin in the water. If, however, you are going to use it on land, you might want to consider using either one of the light weight resins or a whole lot of microspheres in the fiberglass resin to reduce the weight. The microspheres work quite well, but I prefer to use both a light weight resin and extra microspheres, or simply use a two part rigid foam. The latter does a good job of waterproofing and is very light. The foam works very well for mono coils and can be used for DD's also, but one has to be careful because the foam will have a tendency to expand and possibly move things a little. So, it is a good idea to have the coils somewhat rigidly positioned before using the foam.

Reg

[Robby_H]

Re: Homemade coil

It seems Kynar can have a larger constant than pvc by a factor of between 1.5 to 4. Pvc can be as low as 2.7 but may be as high as 6.5. Teflon epoxy or coating is the perfect choice at all frequencies but rather expensive and the Epirez epoxy that I used to play with has a claimed constant of between 2.6 and 3.2 @ 100khz depending on the type. There was a light weight, white Epirez that claimed 2.6 but it doesn't seem to be available now. Some types of plumbers tape appear to have a low constant but wrapping requires a lot of patience.
Has anyone got figures for the stuff they are using?

[FJ]

Re: Homemade coil

Hi,
Can you tell us where you get 2 part foam from? Where you get microspheres from? And also, if you shield the coil with lead tape, wouldn't that keep the resin or foam out of the windings?
Thanks,
FJ

[Reg]

Re: Homemade coil

Hi FJ,

First, let me say that if you use your coil for a battering ram, then the foam is probably not the best choice as a filler. However, if you want a light weight coil that isn't going to be abused, then the foam will most likely work quite well.

I have coils over 2 years old that are filled with the foam and they are still quite strong and light.

I purchased my two part foam from a place called Iasco-Tesco. Here is the link:
http://www.iasco-tesco.com/

The foam is a rigid 2 lb variety. I have found this to work quite well. I am sure other foams will work the same.

As for the microspheres, I don't remember where I purchased mine, but I think I got them from US Composites. Here is the link:
http://www.shopmaninc.com/products.html

I purchased a light weight resin from a place called Smooth-on. Here is the link to the home page.
http://www.smooth-on.com/

I used the Feather Lite for a light weight filler and still added microspheres.

Yes, normally, using something like a lead foil will keep the resin out of the windings. However, many people use other types of shielding that may not.

Reg

[Al Arens]

Re: Homemade coil

Hello Reg
Would the insulation type of foam used to seal small cracks around the house work? How about silicone sealant? Would these have any adverse effects on the coil performance?
Thanks, Al

[Reg]

Re: Homemade coil

Hi Al,

I tried the household foam and it worked to hold the windings in place but I was not quite satisfied with it. The stuff is not as rigid as I would like so it doesn't really add any strength to the housing. Outside of that, it didn't seem to affect the coil, so it probably should work if just used to hold the windings in place. This stuff does make a great glue so if it used to hold the windings, then make sure to get the other half of the housing on before the foam sets up. This way, it will help keep the halves from coming apart.

One has to be very careful when using Silicone Sealant. There are different types and some are very caustic. The original Silicone sealant has acetic acid as a discharge and this will eat most everything electronic in nature. At least, it will attack the copper. Some of the later silicone sealants are electrically safe. So, a person should definitely select the proper one. Otherwise, at some later point, failures could occur as the result of using the sealant.

The two part rigid foam works well because it does add additional strength to the housing. The extra stiffness keeps things from twisting or moving and causing false signals. When I am not worried about strenght, or trying for the lightest coil, I will use the two part foam very sparingly. This foam expands more than stated, so a little goes a long ways when using it this way. A couple of my feather light weight coils are made with this foam. They are extremely light and still quite stable. The down side of this foam is it almost always has to be ordered. At least, it does around here.

Reg

[Kev]

Re: Homemade coil

Hi Robby,
I'm using an epoxy from RS Components part # 199-1468 It has a dielectric constant of 3.4 I can achieve fairly low delay times, but have often wondered if my coil sandwich was actually one big capacitor.

The RS epoxy is expensive compared to marine epoxies, I expect one of the properties of actual potting compound should be a low constant, whereas marine types would be UV immunity, strength, flexibility etc., but not electrical properties.

Is Epirex a Teflon based product?
I'll let you know if I come up with anything interesting in the marine dept.

Cheers
Kev

[Robby_H]

Re: Homemade coil

Hi Kev,
Thanks for the info.
I once worked for the NSW Maritime Board back in the early eighties and got to use Epirez for free as any left over was tossed out at the end of a job. It is very expensive unfortunately or was last time I priced it. I don't think they make a Teflon based product as this is Duponts (Dumonts?)invention? Eperez has a website but there are other manufacturers out there that may also have data sheets available on the web.
Back in the vlf days it was found that winding a shield around the coil itself caused problems in a DD design. The residual capacitive coupling was very hard to minimise compared to using the method of spraying the foam former itself although it also needed some thought re the design. It seems that any excess capacitive coupling of the tx coil to the rx coil combined with the inductive coupling can also cause problems in hot ground with a pi even though the tx/rx method is entirely different.
I found a post of yours interesting where I think you said that contact of particles of the shield coating with the outside world caused noises.
There are some weird effects caused by coil design such as some magnetic rocks give a very loud noise when only lightly touching a Minelab coil shell closest to the windings but don't if there is a tiny air gap. I find this a bit hard to explain but there must be a reason for it. It seems commercial coil, shield and shell design could do with some further improvements.
Rob.

[Phil]

Re: Homemade coil

THANKS EVERYBODY FOR ALL THE INFO.REG, is given all the info I am using,THANKS AGAIN

[Al%20Arens]

Re: Homemade coil

Reg,
Thanks for the info. How quickly does the two part foam start to expand?
Al.

[Reg]

Re: Homemade coil

Hi Al,

The two part foam starts to expand in about a minute so everything has to be in place and ready to go by then. I mix the stuff in a paper cup for about 20 seconds, pour it quickly around in half of the coil housing, assemble the halves of the coil housing and then I place the almost completed coil in some board clamps to hold the two halves evenly together. I do wear some cheap latex gloves to keep the stuff from my hands. It will expand with some oozing out from the two halves.

No matter how hard one tries, there will always be a little mess, so a person might want to try mixing some and seeing what happens before actually assembling a coil.

Some form of clamping is necessary to assure the two halves do not separate or spread apart as the result of the expanding foam. This clamping should be as complete around the coil form as possible, otherwise one part will rise more than another and the coil will look strange.

On my forms, I have two separate forms each made using two 1" by 5" boards spaced the proper amount to hold both sides of the coil form. I also use some large spring clamps to hold the halves together where the board forms do not reach.

I make a little extra foam and leave some in the paper cup. This way, when it fully expands and has cooled a little, I know I can remove the coil from the clamps, and trim off the extra that has oozed from between the two halves. If done soon enough, this isn't that difficult. If removed too soon, it is a gooy mess, while too late, and it is much more difficult to remove the extra that has oozed out.

The foam makes a great glue so once it has set up, the two halves will not come apart. Although the foam should make the halves reasonably waterproof, I still seal the two halves using a slurry made by dissolving some plastic in MEK to seal the edges of the two halves together.

Reg

[Eric Foster]

Re: Homemade coil

Hi Reg and Al,

When I made coils this way, I used to glue the two halves together first, then with the complete coil in a board clamp, pour the foam in through a 1/2in hole in the top shell. The area surrounding the hole was protected with masking tape first, so that the foam that leaked out did not adhere to the shell itself. With the filling hole at the top end of the coil, the board clamps were angled, so that when the foam was poured in, it naturally ran to the bottom and foamed upwards toward the hole. That ensured complete filling without voids. The trick is to mix up just the right amount of foam, so that when expanded, only the minimum amount emerges from the hole. When hard, the excess was trimmed and a circular label stuck over the hole. Board clamps are absolutely essential, as even quite thick shell material will balloon badly from the pressure of the expanding foam.

Eric.

[bbsailor]

Eric: Coil Question?

Eric,

How did you hold the coil or coils (for DD) in place so that the expanding foam does not move the coil(s) when expanding? Or is that even an important consideration?

Thanks

bbsailor