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Old 8th January 2007, 09:31 AM   #1
Wynand is offline Wynand  South Africa
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Default How to shield a transformer?

I want to put a metal plate around my toroidal transformer so that I can mount the rest of the PSU on top of it.

I'm thinking I'll get a bracket made covering the three sides exposed inside the casing. The other sides will be towards the side, back and bottom of the case.

Is this a clever thing to do, and do I need any specific metal type e.g, aluminium, steel?

This will greatly help the space I've got in my box since I want to make a 5 channel amp with all the preamp goodies all inside.

Thanks
Wynand
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Old 8th January 2007, 10:04 AM   #2
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If you want to sheild the magnetic field you must use something that is magnetic, iron! If you have a toroid sheilding is hardly necessary. Do you have any problems with magnetic fields?
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Old 8th January 2007, 10:38 AM   #3
Wynand is offline Wynand  South Africa
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I have no problems at the moment, but do not want to rebuild the amp another time if it does interfere since I've stripped the internals 3 times already.

Currently I've got a Mauro design in with a soft-start circuit and another transformer for the s-start.

I want to make it 5 channels so the whole thing has to be moved inside

Just for clarity. I want to mount the circuit on top of the transformer.
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Old 8th January 2007, 11:42 AM   #4
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My source for Mu-Metal (MuShield in New England) suggests that before purchasing their product (or anyone elses), try shileding the emitting device with inexpensive steel. All metals are going to have some shielding effect (even silicon) -- but this is where the electromagnetics get tricky. Most importantly, and least expensively, consider the orientation of the transformer to the circuitry --

Here's a PDF from MuShield:
http://www.mushield.com/documentatio...sign_guide.pdf

Bryston has a less-than-technical discussion:
http://www.bryston.ca/newsletters/85_files/vol8is5.html
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Old 10th January 2007, 02:09 PM   #5
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by jackinnj
Bryston has a less-than-technical discussion:
http://www.bryston.ca/newsletters/85_files/vol8is5.html
That's a joke! Alright, it's not actually lies, but it's an unusual representation of some selected facts. And they didn't mention the most important factor: Toroids are cheaper for manufacturers to buy (reduced core size means less material cost and reduced transport costs) and cheaper to sell (reduced transport and packaging costs).

To go back to the original question, distance is your friend. Induction from a transformer drops with the square of distance, and if you get even further away, by the cube. It takes an awful lot of expensive screening to equal that...
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Old 12th January 2007, 08:26 AM   #6
AndrewT is online now AndrewT  Scotland
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Hi,
Bryston's paper on EI vs Toroid is OK,
I disagree with the noise paragraph. The point should be shared, both can be equally bad.
I disagree with Bryston's attitude when the EI genuinely wins a point. The put down is undeserved. Give EI the point.
Some points are awarded for the same characteristic but in different paragraphs.
The overall conclusion shows toroid way ahead of EI, I believe they are closer, in these selecected operating conditions.
There are some occasions when the EI actually wins out as the better performer. Bryston have chosen to ignore these.

Shielding.
Sugden fit a steel ring around the toroid that almost fills the gap between bottom and top faces of the chassis. The toroid is laid flat, off to one side of the circuitry.

Radiated field.
Rotating the toroid around it's central bolted fixing can reduce the radiated field in one particular direction and "pointing" this away from a sensitive part of the circuitry can reduce noise. I suspect the problem is the edge at the end of the winding of the core. It (in Bryston speak) probably "sprays" stray field out from that unterminated edge.
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Old 12th January 2007, 10:11 AM   #7
Nordic is offline Nordic  South Africa
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I think he said he wants to put the boards on top of the transformer... A toroid will be a problem as the field is mostly on the axis going through the hole like a spear, isn't it? -|-

I have an EI core for my ref_c (which by the way doesn't like being close to the transformer) it is ******* huge unfortunately, its not going to fit any regular case.... maybe one of those BIG amp cases yeah...
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Old 12th January 2007, 11:05 AM   #8
AR2 is offline AR2  United States
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Default Re: How to shield a transformer?

Quote:
Originally posted by Wynand
I want to put a metal plate around my toroidal transformer so that I can mount the rest of the PSU on top of it.

I'm thinking I'll get a bracket made covering the three sides exposed inside the casing. The other sides will be towards the side, back and bottom of the case.

Is this a clever thing to do, and do I need any specific metal type e.g, aluminium, steel?

This will greatly help the space I've got in my box since I want to make a 5 channel amp with all the preamp goodies all inside.

Thanks
Wynand
There are no problems in mounting plate on the top of toroid with PS electronics. Only one thing to worry about. Do not have central bolt that holds torroid going through or touching top and bottom plates where those plates are connected or where they would form U shape. It has been reported that torroids act wierdly and overheat in these conditions.
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Old 12th January 2007, 11:22 AM   #9
EC8010 is offline EC8010  United Kingdom
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Default Re: Re: How to shield a transformer?

Quote:
Originally posted by AR2
It has been reported that toroids act weirdly and overheat in these conditions.
Yes, that's a shorted turn!
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Old 12th January 2007, 10:05 PM   #10
lndm is offline lndm  Australia
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The EI's beneficial lossyness seems to be a fortunate coincidence. Apart from the stray field pattern, would there be any benefit to, say, a toroid that has been gapped to be equally lossy, ie in regards to the transformer's performance and behaviour?
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