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Author Topic: How can I block a magnetic field?  (Read 9943 times)

Kevin Harris

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How can I block a magnetic field?
« on: 02/08/2008 17:23:55 »
Kevin Harris asked the Naked Scientists:

Hi Chris. I listen to the Redi Direko Show on Talk Radio 702 here in Johannesburg and wanted to as a question about magnetism.

The magnetic field created by the transformer in my DVD player causes a colour distortion (reds turn to green etc) on my television.

The DVD receiver is mounted above the TV. How can I insulate the magnetic field so that it doesn't cause that irritating distortion on the TV?

Thanks,

Kevin

What do you think?
« Last Edit: 07/06/2010 16:47:23 by chris »

syhprum

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Re: How can I block a magnetic field?
« Reply #1 on: 02/08/2008 18:20:16 »
The best solution if you have the required skills would be to modify the DVD player to use an external power unit, here in the UK DVD players with an external power unit are available from many stores such as Sainsbury,s for less than 20.00 

http://www.sainsburys.co.uk/shoppingandservices/entertainment/instore_electricals/recorder_players/1.htm
« Last Edit: 02/08/2008 19:22:00 by syhprum »

lightarrow

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Re: How can I block a magnetic field?
« Reply #2 on: 02/08/2008 18:22:04 »
Kevin Harris asked the Naked Scientists:

Hi Chris. I listen to the Redi Direko Show on Talk Radio 702 here in
Johannesburg and wanted to as a question about magnetism.

The magnetic field created by the transformer in my DVD player causes a colour distortion (reds turn to green etc) on my television. The DVD receiver is mounted above the tv. How could I insulate the magnetic field so that is
doesn't cause that irritating distortion on the TV?

Thanks,

Kevin

What do you think?
It could seem a silly answer, but the better way to solve the problem is to position the mag. field source a bit farther from the TV: mag. fields are quite difficult to shield. Anyway, there are materials, as mu-metals, which can  shield, a little, mag. fields:
http://en.wikipedia.org/wiki/Mu-metal

chris

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Re: How can I block a magnetic field?
« Reply #3 on: 03/08/2008 09:17:04 »
So this is quite a tricky one to solve isn't it?

How do devices that do contain transformers / produce magnetic fields internally screen them out?

Chris

RD

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Re: How can I block a magnetic field?
« Reply #4 on: 03/08/2008 09:30:30 »
The magnetic field generated by transformers is shielded by steel, (although any ferromagnetic material would do).

Quote
All commercially available magnetic shielding materials are ferromagnetic. This means they are attracted by a magnet just like iron or steel. Ferromagnetic materials are necessary because shields work by pulling the magnetic field towards them and away from what needs to be shielded. The magnetic field will actually become concentrated within the shield itself, but the field will still exist.
http://www.magnetic-shield.com/faq/interference.html#q4

A cheap baking sheet may be worth a try, provided a magnet will stick to it.
« Last Edit: 03/08/2008 12:51:42 by RD »

chris

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Re: How can I block a magnetic field?
« Reply #5 on: 03/08/2008 09:32:54 »
So maybe the answer to Kevin's problem is a layer of steel between the DVD player and the TV?

Chris

RD

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Re: How can I block a magnetic field?
« Reply #6 on: 03/08/2008 09:44:30 »
Quote
Mild steel can shield magnetic fields and is appropriate for applications requiring a small attenuation of a high flux density field.
http://www.mushield.com/faq.shtml#q10

syhprum

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Re: How can I block a magnetic field?
« Reply #7 on: 03/08/2008 09:45:40 »
Many small devices that require a few Watts DC do not use mains frequency transformers but rather first convert the mains supply to DC with a bridge rectifier and then convert this DC supply to about 20KHz with a semiconductor device and then use a small ferrite core transformer operating at this frequency.
This is much easier to shield and to filter the rectified DC output, this is the way all modern TV's computers etc are powered.

chris

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Re: How can I block a magnetic field?
« Reply #8 on: 03/08/2008 11:24:33 »
Thanks - why is the higher frequency much easier to shield? Is it because of hysteresis losses?

techmind

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Re: How can I block a magnetic field?
« Reply #9 on: 03/08/2008 22:19:33 »
The magnetic field created by the transformer in my DVD player causes a colour distortion (reds turn to green etc) on my television. The DVD receiver is mounted above the tv. How could I insulate the magnetic field so that is doesn't cause that irritating distortion on the TV?

I might question your assumption that it is the transformer that is causing the colour-distortion. Can you confirm that the distortion goes away when the DVD player is unplugged from the mains electricity supply, and thus that it is not a permanent-magnet in the player which is causing a problem.

If it were a mains-frequency transformer then I would expect the distortion (geometric or colour) to drift over a number of seconds because these days the 50Hz TV is not locked precisely to the 50Hz mains, so you get a low frequency beat. The picture may slowly weave, or the colour cycle - sometimes faster, sometimes very slowly.
If it was a high frequency (eg  around 20kHz) transformer ("switch mode power supply") then firstly the transformer design would normally contain the flux quite effectively (not enough leakage to affect a TV even if very close), and secondly I imagine the visual effect of a 10's kHz field on a TV would tend to be more like horizontal streaking (because the transformer frequency is comparable to the 15.6kHz TV line-frequency).

I agree the simplest solution is just to move the DVD player further from the set.

Chris:
I think you'll find that higher-frequency fields (kHz upwards) are probably more easily screened because you can better rely on induced electrical currents in a metal sheild (even a non-magnetic one, like aluminium) to repel/cancel the field. For low-frequency (50Hz down to DC) you need real magnetic screening.
« Last Edit: 03/08/2008 22:21:34 by techmind »

lyner

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Re: How can I block a magnetic field?
« Reply #10 on: 04/08/2008 08:29:16 »
You could always buy yourself a nice new flat screen TV! Go on go on - you know you want to.

chris

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Re: How can I block a magnetic field?
« Reply #11 on: 07/08/2008 21:10:03 »
Thanks Andrew. Excuse my ignorance but why is the higher frequency better at inducing a current in the screen?

Chris

lyner

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Re: How can I block a magnetic field?
« Reply #12 on: 07/08/2008 22:19:03 »
Skin depth a 50Hz is about 3mm for iron and about 11mm for aluminium. For 50kHz it would be 1/30th of those values, so the screening effect of a metal case would be significantly better for the higher frequencies.
Screening depends on both conductance and permeability of the metal which you make a screen from.
As far as I remember, there is also the difference in transformer designs for 50Hz and 50KHz. A transformer for high frequency use involves less 'iron' and  there is less external magnetic field for a given power transfer.
This is one of the reasons  that they use around 600Hz for  aircraft electrics, rather than 50Hz: less iron / weight.

daveshorts

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Re: How can I block a magnetic field?
« Reply #13 on: 09/08/2008 09:40:39 »
High frequencies are easier to screen because of an effect discovered by michael faraday. If you apply a changing magnetic field to a conductor a voltage is induced in the conductor, that is proportional to the rate of change of field, the higher the frequency the greater the rate of change of field so the larger the voltage.

These voltages cause currents to flow which produce their own magnetic fields which counteract the original field. This means that the higher the frequency the larger these currents will be per thickness of metal so the better the fields will be screened.

If the magnetic field isn't changing at all this strategy doesn't work. In this case you have to use soft magnetic materials such as some steels, or ideally mu metal, these act a bit like short circuits for magnetic field, so if you surround a magnet with them very little field escapes.

 
« Last Edit: 13/08/2008 08:13:33 by daveshorts »

jackiechan

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Re: How can I block a magnetic field?
« Reply #14 on: 26/09/2013 02:52:48 »
Well, this sounds like a nice learning opportunity. Just get yourself some magnets and try using various materials to shield it.  ;D

[Mod: Please don't link to a specific source (probably your own).  People can find a place to buy a magnet (if they don't have one laying around the house)]
« Last Edit: 26/09/2013 05:06:58 by CliffordK »

syhprum

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Re: How can I block a magnetic field?
« Reply #15 on: 26/09/2013 04:02:09 »
How nice to see discussion of a simple technical problem which of course should be in the technical session not Physics and Cosmology.
This problem is a blast from the past and would not arise with current equipment i.e. a flat panel TV and a modern DVD player .

PS take care putting permanent magnets near a CRT type display they can induce permanent fields in the structure that the degaussing system is unable to remove.   

alancalverd

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Re: How can I block a magnetic field?
« Reply #16 on: 26/09/2013 22:00:04 »
Much more likely that the cause is the permanent magnet in the DVD drive motor. In that case you will need a fairly thick piece of steel (not stainless - the ordinary rusty stuff) to shield the CRT. Better to move the DVD player to some other location.

CliffordK

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Re: How can I block a magnetic field?
« Reply #17 on: 26/09/2013 22:31:11 »
LCD and Plasma screens are also much less susceptible to electric and magnetic fields than CRT monitors.  Of course it is an expensive choice (but cheaper every day). 

In one office where I worked, we had two offices that had bad electronic interference.  One beside some large electronic transformers outside, and another near a stack of microwaves in the break room.  I couldn't convince the users to move their desks, but an LCD monitor fixed their problems.

evan_au

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Re: How can I block a magnetic field?
« Reply #18 on: 27/09/2013 01:28:21 »
Quote
How do devices that do contain transformers / produce magnetic fields internally screen them out?
The idea is to direct the magnetic field just where you want it, without it "leaking out" to other places.

You do this by using a material with a high relative permeability; the magnetic field passes through this material much more easily than through air/vacuum (which has a relative permeability of 1). You shape this magnetic material so it forms a closed path with the magnetic source (eg a coil of wire, or a permanent magnet).
  • Mains-frequency transformers (50-60Hz) use an iron core
  • The smaller and lighter transformers used at high frequencies on modern electronics use a ferrite core.
  • Mu-Metal mentioned above has a relative permeability of up to 100,000; however, it works best for small magnetic fields (low coercivity), its properties are affected by mechanical shocks, and by exposure to strong magnetic fields.
  • Forming the magnetic material into a closed path is much harder with an electric motor, since some parts of the magnetic path have to move. This results in an air-gap.
  • All magnetic materials have problems with "saturation" at low frequencies (especially with DC fields such as a permanent magnet  electric motor). This effect can be reduced with an air-gap.
Economical magnetic materials carry magnetic fields about 1000-10000 times better than air. In contrast, economical electrical conductors carry electrical currents billions of times better than air (and plastic is a better insulator than air). So I guess you could say we are much better at directing electric currents where we want them than we are directing magnetic fields where we want them.

evan_au

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Re: How can I block a magnetic field?
« Reply #19 on: 27/09/2013 01:36:55 »
If you decide to stay with your old CRT television, you may find that the colour distortion remains even after you have inserted a metal shield, moved the DVD player further away, etc.

Most CRT TVs already have an internal magnetic shield around the TV tube. However, this can become permanently magnetised by being exposed to the external magnetic field.

So CRT TVs have a "degaussing" circuit which can eliminate this permanent magnetic field by applying an oscillating magnetic field which reduces gradually to zero.

To save energy, the degaussing circuit is only activated ocasionally. I understand that some TVs only activate it when the power is first turned on; try unplugging the TV from mains power, wait 10 seconds and then plug it in again.

If this does not work, you may need to consult the manual to find out how to activate the degaussing circuit.

 

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