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Offline Paul A
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« Reply #30 on: November 09, 2010, 02:57:28 am »
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Hi Sean,
If you strip a ferrite from an old radio DON'T take the coil(s) off!
Try connecting them...... you might just be in for a pleasant surprise!  Great


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Offline GoldDigger1950
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« Reply #31 on: November 09, 2010, 12:35:17 pm »
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Quote:Posted by Mudflap
Thanks GD. Ordered one of these puppies today. Now have to find a chunk of ferrite for the pinpointer loop. 40 or so years ago I would have a bunch of am radio ferrites laying around. Now seems only place to find them is from Russia on eBay. Could strip one from my Sony 2010 SWL or maybe the PRD5.... nah... No no!..... 

Two words. Garage sale.

The only problem with finding one at a garage sale is that it might work and I personally hate to tear something apart that works. Know what I mean? On the other hand, ferrite rods are available here:

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http://www.goldmine-elec-products.com/prodinfo.asp?number=G8905

Those are very small but they occasionally do have larger ones.
These are bigger. The 2000u would suit your needs.

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http://www.stormwise.com/page26.htm

You can glue them together, end to end, to make longer rods. The ferrite rod itself is fine ground iron filings crushed together with adhesive. You are only interested in the ferrite rod's ability to focus the field, not in conductivity. When you have them glued to about 4 inches in length, wrap them with two layers of 30AWG magnet wire making the coil windings very tight with the start and ending wires at the same end. When you finish, coat with several coats of varnish or spray polyurethane clear coat. Soak it on the first coat to bind the wires. More coats later to keep it from corroding. I sometimes use wire wrap wire instead of magnet wire because it is Kynar coated instead of varnish coated like magnet wire.

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http://www.radioshack.com/product/index.jsp?productId=2062640


Don't worry at all about your inductor's value. With a ferrite core inductor, close is good enough.

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Offline seanengman
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« Reply #32 on: November 09, 2010, 12:42:04 pm »
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Wow, thanks... I'll keep that in mind. So if a guy wanted to do something like a Sunray, it would be that difficult at all.

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Offline Mudflap
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« Reply #33 on: November 09, 2010, 03:35:30 pm »
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Most of the ones I see on eBay have permeability of around 400. Probably more suited for higher freqs than metal detecting freqs. I can bridge check the inductance. For a pinpointer probe thinking of a ferrite w/ coil potted into a PVC pipe. At detecting frequencies can probably run the leads parallel down a 12" pipe without too much of capacitance problem. Maybe too long of a pipe and without a balanced feed could be a problem...although a couple pf at that freq shouldn't hurt.

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Offline GoldDigger1950
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« Reply #34 on: November 09, 2010, 03:45:40 pm »
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Mudflap, because we are talking about audio frequencies at a power level high enough to drive a 6" or better coil, the smaller ferrite coil won't really matter. The purpose of the ferrite is to focus the energy at the end of the coil on both ends. When you wrap your own coil, go from one end to the other and back again so that the strongest field will be at the tip away from the wires. If you keep the coil a short distance from the board, the components themselves won't be too much of a factor. If they are, a bit of aluminum foil in a square inside of a plastic baggie should take care of that.

I would certainly pot it inside of PVC or other plastic. In fact, my first of these will be a pinpointer so we will be doing this in tandem.

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« Reply #35 on: November 10, 2010, 03:16:18 am »
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Hi,
You can get the ferrite rod (with coils!) pretty cheap from "Maplin" here in the U.K.

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I reckon one of those coils would likely as not actually work o.k....... saves a lot of effort!



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« Reply #36 on: November 10, 2010, 08:18:24 pm »
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Thanks GD. Andy said to use around 350uH.
Jim

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« Reply #37 on: November 11, 2010, 03:22:08 am »
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Just had a look ... the inductance of a medium wave coil is around 350 - 370uh!!!!
So..... if you can find a ferrite with a m/w coil on...... Job Done!!!


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« Reply #38 on: November 11, 2010, 06:31:08 am »
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Thanks for the info Paul

Regards Xavier

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So many questions so little time
Offline Mudflap
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« Reply #39 on: November 11, 2010, 03:06:04 pm »
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Thanks Paul. That is interesting.

Posted on: November 11, 2010, 02:19:47 PM
Thanks Paul. That is interesting.

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« Last Edit: November 11, 2010, 03:08:44 pm by Mudflap, Reason: spelling » 		Logged
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