took apart factory coil and

[Joe]

first noticed bunched up wire beside the RX coil. It looks like it leads to the TX coil. Is that correct and what purpose does it serve? Also, noticed what appeared to be no shielding on coils,just foam to hold them in place. This is a concentric coil and is that normal or is the shielding built into the plastic housing? Joe

[Jeff Kinzli]

Re: took apart factory coil and

Hi Joe,

I'm not sure what type of coil you took apart, but the wire could be a few things, such as extra wire used to balance (null) the coils, or, it could be coming from the TX coil (usually will have a high value resistor on it as well, which is used for resistive nulling, and also is used to get rid of the resistive component of all sorts of stray targets and capacitances). There's usually a smaller (~50ohm) resistor in there as well to move the Q of the coil around.

I suspect that the shield is in the casing for the coil, rather than applied directly to the coils themselves. Some manufactures use a conductive plastic-type material for their coilforms which acts as the shield.

Hope that helps,

-Jeff Kinzli

[Joe]

Re: took apart factory coil and

Jeff,
Thanks for the info. My guess is the coiled up wire is to null out the RX coil sence no resistors were present. If I wanted to make the same coil but larger/deeper seeking how do I keep my inductance around the factories. What do I have to change so it will work on the same detector? Any ideas on how to figure the nulling coils length? Joe

[Jeff Kinzli]

Re: took apart factory coil and

Joe,

As you grow the coil diameter, you'll need more turns for a given inductance. As I said before, it's nice to try to keep the inductance along the lines of what your reference coil is, but not necessary. You'll also find that for any given inductance coil, you can vary the phase response, Q and other factors by simple addition of a resistor, and vary the nulling by very simple passive components.

Depending on the type of detector this is, the parameters of the TX coils may or may not be part of final oscillator of the detector (as is the case in Whites 6.5kHz machines, etc.), which means that there will be a capacitor across the coil to tune it (it's the final part of the colpitts oscillator) to the right frequency.

If not, then the TX just has a maximum output voltage for a certain resistance/inductance, and that is easy to find with a scope and signal generator (or detector).

I'm kind of running off at the mouth. I have been thinking of publishing some of the stuff that's in my head but in a more controlled and readable format. I wonder if there's much interest in that? Seems there's a collective knowledge out there about coils and detector design, but it's held by a very few folks, and the experimenters don't always have access to it. Just thinking outloud...

[Sean Goddard]

Re: took apart factory coil and

Go For it!

99% of the people on this forum are willing to learn all they can about beepers. I try to help but I don't know it all.

We need to fill in each others gaps.

I posted a motion machine design earlier down the page, and hoped people would take it up.

If we ALL work on it, perhaps we can make a "REFERENCE" design, which we can then expand and glue PIC micros on and stuff like that.

Could turn into a VERY interesting project (that was the original idea anyhow).

[Joe]

Re: took apart factory coil and

I 2nd that motion!
Your right, there are plenty of people like myself that are struggling to get information on "How to build coils correctly" and piece the bits they get here and there and make sense of it all. By the way, thanks for help. Joe

[Frank Wallis]

Re: took apart factory coil and

Jeff
I think you mean as you grow the size of the coil less turns are required for the given inductance.
Regards Frank Wallis

[Jeff Kinzli]

Re: took apart factory coil and

Yes, Frank, you are correct. I'm not sure how that got typed out but I'm learning that sometimes it pays to slow down in typing and proofread before posting Thanks for the correction.

[Robby_H]

Re: took apart factory coil and

Frank's right. A general rule of thumb for frequencies around 10khz and under is that the DC resistance is the same for different coil diameters but this only applies if using the same wire size. In this case a large and small coil would use the same length of the same wire resulting in less turns as the coil size increases.
Most of the junk inside commercial coils isn't needed if copying one or making different sizes.
It is far better to make measurements of it's characteristics and make these the same but balance the coils manually when setting the epoxy. Slight trimming on the field can be done with a ctitically mounted and selected small ferrite slug and is easy with manual GB as balance is achieved when there is no audio change when rotating the GB pot. This last bit was an advantage here in Australia when we used vlf for gold as the GB pot was often adjusted every couple of steps and could be quite hard on the ears if the sample window was looking at the wrong part of the sine wave.