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Old 17th September 2009, 10:23 AM   #1
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Default Mounting Inductors with ferous bolt?

I am mounting some copper foil inductors to a pcb with a stainless steel nut and bolt. The inductors were supplied with a brass screw, but my method will be more secure.

Is there a problem with using ferous (magnetic) fixings through the middle of an inductor?

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Old 17th September 2009, 10:26 AM   #2
Magura is offline Magura  Denmark
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Originally Posted by dublin78 View Post

Is there a problem with using ferous (magnetic) fixings through the middle of an inductor?
Oooooh yes, that's a BIG! no-no.



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Old 17th September 2009, 10:33 AM   #3
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Is stainless steel magnetic?
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Old 17th September 2009, 10:54 AM   #4
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Some stainless is more magnetic than others. It would be easy to test with a magnet.

If there are any superior properties in using a foil inductor over a ordinary wire inductor, the superiority would be completely ruined by using a magnetic core in the center.

I see another problem in your photograph. The inductor with the red tape has its axis parallel to the bigger foil inductor. There will be coupling of magnetic fields--one coil inducing a signal in the other. Inductors should be lined up with their axis at right angles to each other to minimize the coupling.

Also, the nearer one coil is to the other, the more coupling there is. I would place the coils as far away from each other as I could on the board.

I am a physics teacher and I have demonstrated this effect many times to my students.
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Old 17th September 2009, 11:21 AM   #5
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Its perfectly fine to use a ferrous bolt as long as there is no galvanic connection between the top and bottom of the inductor or toroid. In your case the top cover seems to be plastic so it should be fine.
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Old 17th September 2009, 02:09 PM   #6
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Originally Posted by richie00boy View Post
Its perfectly fine to use a ferrous bolt as long as there is no galvanic connection between the top and bottom of the inductor or toroid. In your case the top cover seems to be plastic so it should be fine.
You're confusing the effect of creating a shorted turn, with that of having a different/wrong magnetic path.

The bolt will certainly destroy the advantage of using an air-core coil.
Firstly, you've added core losses.
Secondly, the value of the inductor, and therefore the crossover frequency, will change!

Last edited by pjp; 17th September 2009 at 02:12 PM.
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Old 17th September 2009, 02:23 PM   #7
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Yes there will be a big problem Pick up your multimeter, measure the inductance of the air coil, then put the screw in and measure again, it should raise quite a lot. I have already seen the measured difference at the xovers of a commercial loudspeaker where they used ferous screws to mount the coils.
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Old 17th September 2009, 03:55 PM   #8
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Quote:
Originally Posted by Mario Pankov View Post
Yes there will be a big problem Pick up your multimeter, measure the inductance of the air coil, then put the screw in and measure again, it should raise quite a lot. I have already seen the measured difference at the xovers of a commercial loudspeaker where they used ferous screws to mount the coils.
Not to mention, it's more than a static shift- the screw will saturate, so your inductor will lower in value with higher-power impulses- just when you need a lowpass to do it's job the most, to prevent breakup in metal cones, for example. So you can't design around it.
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Old 17th September 2009, 03:58 PM   #9
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Hi Guys

I have just tested it with a LC meter, and my stainless steel bolt does not make any difference.

I don't know why I did not try this before - my bad.

I pulled out the bolt and tested its magnetic characteristics against a speaker driver magnet. In this case, the stainless steel bolt would not support its own weight, and indeed gave little indication of any ability to be magnetised. Some other stainless bolts in my box were mildly magetisable however.

In the interests of experiment, and of this thread, I inserted a hex key (which was easily magnetised to the driver magnet) into the inductor core, and it increased the inductance from 1.4mH to 1.85mH. Beware.

Cornelis: I am aware of the problems of placing inductors close to each other. There is a good article here: http://www.troelsgravesen.dk/coils.htm

The PCB and the XO have been designed by an experienced engineer, by calculation and by ear. He has sold many kits, so I asume that all is good: AOS Studio 12: http://www.aos-lautsprecher.de/indexE.html

I will however mention this to him when I am next in contact. It had already concearned me.
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Old 17th September 2009, 04:30 PM   #10
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The web page showing the placement of the coils shows very little change in the value of the inductance. I would expect that and it is irrelevant to the point I was making.

The point I make is that two coils close together acts as a transformer, where one coil, the primary, induces a current in the other, the secondary. In the usual transformer an iron core is used to get the coupling is close to 100%, and in your situation is is closer to 1%. The effect will be small, and may even be difficult to detect by listening, but is so easy to eliminate the magnetic coupling altogether by simple placement of the coils with axis at right angles to each other.

Some manufacturers use hot melt glue to mount coils which seems fine to me, as it nonmagnetic and gives great flexibility in positioning the coils. Using a bolt to secure a coil for the sake of mechanical security seems over the top for me unless you intend to throw your crossover around the room.
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