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Offline xavierTopic starter
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« on: January 05, 2013, 01:32:32 pm »
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I have been working on some coils for a TGSL (DD Coils) the induction should be TX 6.0 mH and RX 6.5 mH. I'm not going to go into the details of how we go about making coil but I have just found out that once I apply super Glue to the coil to render it ridged, the inductance drastically changes  (View photos). What I did notice though is that the inductance seem to be going back down, Now the question is by how much will it go down and will it get back to it original inductance (I very much doubt it). So for those of you who are planing to make coils in the near future be aware of this phenomena  Wise I will be monitoring these coils to see what will the final outcome be.       

All the best
Xavier

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Offline GoldDigger1950
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« Reply #1 on: January 05, 2013, 01:38:09 pm »
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When a coils is loosely wound, it tends to flex during operation. The atoms of metal heat and cool (that means expand and contract) during the oscillations. It's only the tiniest amount but it does happen. Changing the shape of the coil this way means a tiny inductance change from each wire.  This, my friend, is why I have said over and over again, when designing a metal detector, the coil is always designed first. In your case, trial and error will succeed but remember one other thing. A de-tuned coil is needed to build a field, not a a tuned one. So if you design fairly close to tuned and wind your coil on your specs, glue it up and pot it, you're have one that is detuned simply because you cannot possibly make it perfectly.

Try your coil. See how it does. Get back to us.

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Offline xavierTopic starter
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« Reply #2 on: January 05, 2013, 01:49:04 pm »
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GD I completely agree and understand what you are saying but here is another theory that I have. The glue has removed all (Or most of) the air in the coil and therefore making the coil like if it was tightly wound. I will see what the inductance will be in the morning, then I could wind a new coil bearing in mind the increase in inductance so that once the glue has been applied I will have a coil of the correct value. What do you think?   

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« Reply #3 on: January 05, 2013, 01:56:32 pm »
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I potted a coil in Epoxy and it took a good few weeks for things to quit moving.

It Contracts.

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Offline xavierTopic starter
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« Reply #4 on: January 05, 2013, 02:06:17 pm »
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Yes I think that it will come back a bit to the required inductance but I'm damn sure not all the way. And so my friend the thinking cap comes back on. Now I'm going to test these coil just for the hell of it LOL   

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Offline GoldDigger1950
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« Reply #5 on: January 05, 2013, 02:18:44 pm »
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Quote:Posted by xavier
The glue has removed all (Or most of) the air in the coil

This is also true but a tightly wound coil that is potted doesn't have nearly the drift.

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« Reply #6 on: January 05, 2013, 02:36:41 pm »
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Well I tested the coil and it'd a total disaster  Angry So now I guess that it's time for a glass of wine and to relax   

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« Reply #7 on: January 06, 2013, 07:38:51 am »
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OK I'm back. Made a new coil but with a lower inductance, once the glue was applied the inductance went to 6.4mH but once the glue had set the inductance went down to 5.8mH. Check out the photo to see how I got it to 6mH. I'm just hoping that it want cause havoc with the frequency (still need to check that out)   

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« Reply #8 on: January 06, 2013, 02:06:50 pm »
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Fancy Tune O Matic ya have there!

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« Reply #9 on: January 07, 2013, 01:39:23 am »
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Hi Mr X
you have an interesting problem there,
thats a big variation.

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