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Aluminium foil as AC hum shield

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 10 June, 2014 » Aluminium foil as AC hum shield « Previous Next »

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atferrari
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Post Number: 1533
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Posted on Sunday, 27 April, 2014 - 02:46 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Aluminium foil lining the enclosure of an audio amplifier circuit, is it good as AC hum shield?
Agustín Tomás - Buenos Aires - Argentina
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zeitghost
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Posted on Sunday, 27 April, 2014 - 06:13 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

As long as you ground it properly.
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rob_guyer
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Posted on Saturday, 03 May, 2014 - 08:49 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

An external electromagnetic field applied to an aluminium surface will induce eddy currents which oppose the applied field. But the foil should be of a reasonable thickness or a small fraction of the induced current will approach the inner surface and produce a radiated field inside the enclosure. Experiments by Mr J.J. Thomson at Cambridge estimate that 4cm of copper will reduce a 50Hz applied field to 0.2 percent or about 40dB. If a single layer of foil proves unsatisfactory, a second layer spaced a mm or so from the first will give an exponential improvement.
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gabriel
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Posted on Saturday, 03 May, 2014 - 10:16 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hard to solder, i would take a tin can, Altoids box etc. You have to ground it, so you have to solder it...I think this is a better choice. Good luck!
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bowden_p
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Posted on Saturday, 03 May, 2014 - 04:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Rob,
Is aluminium foil effective at screening a 50/60Hz magnetic field? (e.g. the leakage flux from a transformer.) I can see how it would screen an electrostatic field, by connection to a ground, but I thought only something like mu-metal was effective for a magnetic field? ( As used once on sceening 'scope electron guns.)

Wouldn't a 50Hz field go straight through thin aluminium foil? This thread has raised my curiousity now!

With regards, Paul.
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atferrari
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Posted on Saturday, 03 May, 2014 - 04:38 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola Gabriel,

Sometimes, Altoid boxes are small.

Yes Paul, that's why I posted my question. Expecting to complete the building of my enclosure today and will see the outcome.
Agustín Tomás - Buenos Aires - Argentina
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rob_guyer
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Posted on Sunday, 04 May, 2014 - 07:50 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,Paul,like you I was intrigued by the choice of aluminium foil to screen against 50Hz EM fields. The author of my old textbook (*) gives some practical maths based on the fundamental electromagnetic theorems to explain screening of magnetic fields. The formula can only be approximate because the field is not entirely parallel to the metal surface- and the maths are in MKS units which do not directly translate into the SI system.
Paraphrasing the author; As the magnetic field propagates through the metal, its amplitude diminishes exponentially, (eddy currents as per Flemings RH Rule), to a fraction of its original value, given approximately as [e raised to the minus (m times x)] where x is the penetration distance --and m is [2 times pi times SQRT of the product of permeability, frequency and conductivity]. Because the m for iron is 3-4 times that of aluminium for about the same conductivity, back-of-a-beermat calculations show that, at 50Hz, tinplate screens can be one quarter the thickness of the non-ferrous alternative.
Gabriel was quite right to point this out. My double-screening suggestion improves screening only a little.
At 50Hz, permeability dominates attenuation, being supplanted at higher frequencies by the eddy current effect.

(*) Radio frequency Measurements. L Hartshorn, D.Sc
Chapman and Hall, London, 1942.
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ianjoh
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Posted on Sunday, 04 May, 2014 - 11:17 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have seen an enclosure made from offcuts of double sided board. A heavy duty iron will be required though!
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bowden_p
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Posted on Sunday, 04 May, 2014 - 12:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Rob, ianjon,
Thanks for the theory. This explains why plated steel "cans" are used in PC's, TV tuners, etc. Radio hams use PCB boxes to try out experimental r.f. circuitry, and it also gives a good ground plane too.
With regards, Paul.
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zeitghost
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Posted on Sunday, 04 May, 2014 - 01:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

So, to sum up, aluminium foil is effective as a screen against electrostatic fields but is ineffective as a screen against alternating magnetic fields at 50Hz or so.

Whereas at higher frequencies, 50kHz and above, it becomes pretty good again.

I've never seen an IF can that's not aluminium, apart from some tuning coils in an ancient Philips Superinductance TRF set, where they were made of copper.

And where the screening for the tuning condensors was made of tinplate.
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alec_t
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Posted on Sunday, 04 May, 2014 - 11:33 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In the good old days tinplate food cans could be cut open and flattened out easily as a convenient source of screening material. Modern cans have corrugations and plastic linings which unfortunately make them much less suitable.

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