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I have made a circuit to detect the material of a metallic cylinder. It is able to discern aluminum brass and steel targets, and I used a hard drive voice coil for this. This is for a machine I am developing for my small business, and I need to find a source of coils. I don't want to wind my own by hand for quality control reasons, but I'd like to get as close to the hard drive coil as possible:

inside diameter: .5 to .65 inches
length: .1 to .2 inches
number of turns: 150 to 200

My GoogleFoo is failing me. I can't seem to find anything in this range.

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  • \$\begingroup\$ how about coplanar portal.groupkos.com/images/8/89/… \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 31 '16 at 19:08
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    \$\begingroup\$ In case you get stuck building your own: gingerybookstore.com/CoilWindingMachine.html \$\endgroup\$ – Scott Seidman Oct 31 '16 at 19:27
  • \$\begingroup\$ What gauge wire? I don't think they would be very hard to make with a moderate investment in tooling. \$\endgroup\$ – Spehro Pefhany Oct 31 '16 at 19:46
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    \$\begingroup\$ usually any transformer company has a catalog for coil forms to make in any size. in this range and you'll be wise to use mm as most are int'l , look for coil former (aka bobbin) 12~16mm D bobbin x 5mm W for example \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 31 '16 at 19:57
  • \$\begingroup\$ What frequency do you use? \$\endgroup\$ – Andy aka Nov 1 '16 at 9:43
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Most coils of this sort are custom made as there is very little of a general market for them. You need to find a coil winding service. If you live in a metropolitan industrial area, you will probably find a local one. They will guide you thru the specification and build process. In general this is not a complex or costly proposition.

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You may be in luck here with wireless power coils coming to market now and in stock. ( and cheap)

http://www.digikey.com/en/articles/techzone/2015/apr/advanced-wireless-power-parts-make-designing-chargers-easier-than-ever

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  • \$\begingroup\$ That is a really good idea, but my calculations show I need about .5mH, and the ones I see max out at about .05mH. Everyone says to make my own, or have them made. I'm checking various services now to see how much that would cost. \$\endgroup\$ – Brian Michalk Oct 31 '16 at 23:12
  • \$\begingroup\$ I've done what you are doing at much lower L 50uH and higher f, What f are you using? The common method is to use 3 frequencies or sweep it so one can discern ferrous permeability from Aluminum eddy current loss only \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 31 '16 at 23:43
  • \$\begingroup\$ They are cheap enough. I'll order one and see how it does. Thanks. I'm using a touch interface, and it is running at about 270Khz. I'll dig into the library code and see if there are any options to tweak the frequency. \$\endgroup\$ – Brian Michalk Nov 1 '16 at 14:18
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    \$\begingroup\$ 50uH @270kHz is in the 200 Ohm range so for series resonant you need very low Z driver and parallel resonant a current source. We used a differential coil approach for eddy current sensing in steel. This is far more sensitive than a single coil and is used in all metal detectors. Then we measured with PLL's the quadrature currents with very high resolution. Get the tri-coil as well \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 1 '16 at 14:55

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