Faraday Sheilding - Shorted Turn?

[Charles (Upstate NY)]

I have kind of a basic question, if I spray the entire coil shell with conductive paint and also the bottom, will this create a faraday shield type of shorted turn on a double D? I have been making sure I have a break in the shielding but I'm wondering if this is really required.

[Reg]

Re: Faraday Sheilding - Shorted Turn?

Hi Charles,

I don't think I would worry too much about the shorted turn effect if you are using the carbon based paint. Normally, the resistance value is too high to be much of a problem.

I am not sure about the nickel based paint though. I guess it would depend upon just how much was used as well as other factors.

I would make sure to keep the shielding a constant distance from the windings and make sure they can't move with respect to each other.

I tried to link to a page mentioned over on Carl's Metal Detecting Form. The site has some technical info written by George Payne. Unfortunately, I couldn't get the link to work.

On the upside, I did have a copy of the info I was referring to about shielding with carbon based paint. This is what was mentioned by George:

"Sometimes there are very simple answers why some manufacturers use paint and others use foil or paper for the loop faraday shield. Usually it is related to ease of construction or cost, not necessarily to performance. Each technique has its advantages and disadvantages. And, you may only determine what those factors are by trial and error. My background has been in using paint shields. This process can be a little tricky if you don’t know what to avoid. For example. It’s best to have a very smooth surface on which to place the paint. Painting a shield on an irregular surface can cause excessive noise in the detector. Generally its best to have some distance between the shield and the coils. However, I have seen many designs were the shield is place directly on the wire. A foil shield for example. The conductivity of the shield must be low enough to not interfere with the detector operation. My personal experience has suggested that resistances around 10K ohms per square (inch) are optimum. However, the resistance can vary quite a bit without effecting detector operation. When this value drops below 1k then you can have pickup problems. "

I hope this helps,

Reg

[Carl]

Re: Faraday Sheilding - Shorted Turn?

The commercial coils I've seen that use a spray-on shield, seem to use a black coating, what Reg referred to as a carbon-based paint. I suspect it's more like a big resistor.

If you use a highly conductive paint, then I think you should keep a break in the shielding. Chip makers are now putting spiral inductors on chip, and often use a conductive metal shield under it, that is radially patterned specifically to break up eddy current generation. In your case, with a DD spider coil, I think a simple gap is enough.

- Carl