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 Post subject: Shielded PSU inductors
PostPosted: Thu May 17, 2007 5:09 pm 
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There has been a lot of talk about PSU inductors at Wallmart lately, so I thought I'd show how it could be done. Counting out the copper mount gadget, this is no big deal to do the DIY kitchen table way. The steel housing is 1mm thick, the magnet wire is 1.9mm to get a nice low DCR and the inductors shown here are of the 5mH variety :drinkers:

Magura :-)


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PostPosted: Thu May 17, 2007 5:11 pm 
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pic 2


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PostPosted: Thu May 17, 2007 5:12 pm 
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And the inductors......


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PostPosted: Thu May 17, 2007 5:24 pm 
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For better quality pics visit http://www.briangt.com/gallery/magura

Magura :-)


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PostPosted: Thu May 17, 2007 9:24 pm 
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Morten,
Will your steel housing and brass base act as short-curcuited turn around your coil? I'm affraid it can be too much loss. Considering this like a transformer withs shorted secondary. Please argue. :mrgreen:


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PostPosted: Thu May 17, 2007 9:44 pm 
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That would require a fastening bolt through the center of the inductor to cause problems. So in fact there is no shorted turn

This configuration works rather well at keeping the magnetic mess where it belongs. I have found that ferromagnetic stainless steel gives the best results with relatively thin sheets of shielding material. I have not calculated this, but simply used a coil and a DMM and measured the effect of a few different materials. That it turned out to be the ferromagnetic stainless steel that offered the best properties puzzles me a bit, cause regular steel offers higher ferromagnetic density AFAIK ??

The copper base is just a nice way of fastening the inductor, and a fair conductor to chassis-ground.

Magura :-)


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PostPosted: Thu May 17, 2007 10:12 pm 
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That would require a fastening bolt through the center of the inductor to cause problems. So in fact there is no shorted turn


You confused with toroid coil. There are 3 shorted turns - upper and lower flanges and lateral surface. Measure your coils with and without screen - you will be pleasantly surprised.


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PostPosted: Thu May 17, 2007 10:24 pm 
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but how can inductors then be inside a chassis? A chassis would give the same result, no?

I have to admit that I didn't measure the inductors themselves, but the mess around them.

Magura :-)


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PostPosted: Thu May 17, 2007 10:32 pm 
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A chassis would give the same result, yes
Have you ever see crossover inductors on conductive (metal) plate?
You can use only toroid coils in your housings/or make radial slot.


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PostPosted: Thu May 17, 2007 10:44 pm 
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dimitri wrote:
A chassis would give the same result, yes
Have you ever see crossover inductors on conductive (metal) plate?
You can use only toroid coils in your housings/or make radial slot.


Hmm, I'll be back with some measurements after the weekend....something dosn't add up here, cause I usually measure my PSU's when they are mounted in their final position in the chassis, and the inductors worked well in that application.

Are we talking not working at all, or are we talking inductor influenced but still working as supposed? I'm thinking shorted turn on torriod trafo....you are in no doubt if that have been the case.

Magura :-)


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PostPosted: Fri May 18, 2007 12:53 am 
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Quote:
Are we talking not working at all, or are we talking inductor influenced but still working as supposed?

It would be resistor, not inductor. CG, am I right?

Quote:
I'm thinking shorted turn on torriod trafo

Here it is. Use plastic washer to break the current path through the mounting bolt.


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PostPosted: Sat May 19, 2007 2:45 am 
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Oh, this makes my brain hurt on a Friday night.

This is pretty complicated by virtue of the combination of magnetic materials used along with the brass.

Dmitri is right that the brass piece as well as the steel piece will act as shorted turns which will affect the inductance of the whole assembly, even as a function of frequency.

However, I don't think that your coils will turn into resistors. Not quite. Otherwise, you could never shield coils for EMI and other similar pruposes. But, you won't get the 5 mH you're expecting, either. The shorted turn of the brass piece will reduce the inductance, while the steel piece will tend to increase the inductance because of its permeability - this distorts the fields which effects the inductance. You could calculate all of this, but it's much easier to measure. Keep in mind that there will be losses associated as well. That will add to Dimitri's resistor effect. That may be good in some ways for a power supply, since it will dampen some of the high frequency resonances that might be excited by the high frequency components of the rectifier switching (unless you're using tube rectifiers...)

Part of the reason why cross-over networks aren't put on metal plates is because they will couple or radiate to other circuit elements. Another reason is that permeable materials are non-linear. Ask any audio transformer designer. Or even a switch mode power supply designer.

In order to share the brain-pain, here's some germane links:

http://www.classictesla.com/download/emfieldtheory.pdf

Just kidding! :cool2:

http://www.classictesla.com/download/ia99.pdf

This next one is pretty interesting, since it actually is accurate. It also shows how scientists can get things wrong by not "checking their work." Then it becomes accepted, and you end up with the kinds of arguments that you see between "objectivists" and "subjectivists."

http://www.eagleware.com/pdf/apps/2020_ ... lenoid.pdf

These last two are just some interesting pages that I found when searching for a better explanation than I'd give in a short response.

First, here's an example of some of the kinds of mysterious effects that couldn't possibly have an effect on audio equipment, at least according to most people.

emcesd.com/tt2001/tt060101.htm

The second is more general purpose.

Want to become a bona fide expert on a wide range of topics? For free? Look for yourself.

ocw.mit.edu/index.html

Yes, it is just what it looks like.


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PostPosted: Mon May 28, 2007 9:21 pm 
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As promised, a few measurements of the inductance with and without the shield.

Fully enclosed in the box from the above pic, the inductance is appox. 3 % higher than in fee air. As CG predicted, it is even higher with the ferrittic steel cup only.

As these are for the inductors of a CLC PSU filter, a few percent makes no difference. This has though made me scedule a few tests of load-inductors with and without housing.

Magura :-)


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PostPosted: Tue May 29, 2007 12:53 pm 
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Wow! I got one right. I guess even a blind squirrel will find a nut once in awhile... :yawinkle:


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PostPosted: Tue May 29, 2007 10:42 pm 
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No you got both right....the inductance did lower when the inductor just sat on the copper plate =D>

Magura :-)


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