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Old 27th September 2006, 08:10 AM   #1
torzsok is offline torzsok  Hungary
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Default transformer shield newbie q

Dear all,

I have a toroid tranny, and I want to put it quite close to the amplifier circuit, so I want a shield between them. My questions are:
1. is an empty pcboard OK as a material for sheilding?
2. should it be connected to ground? Which one? Is chassis ground an option?

Thanks
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Old 27th September 2006, 08:15 AM   #2
Nordic is offline Nordic  South Africa
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The higher the frequency, the thinner the required shield... for emissions down in the 50/60 Hz area something thicker will be needed.
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Old 27th September 2006, 08:20 AM   #3
torzsok is offline torzsok  Hungary
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A double sided pcb, with the two sides connected on the edges? Or should it be really thick? What about a mesh?
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Old 27th September 2006, 11:39 AM   #4
Nordic is offline Nordic  South Africa
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I don't know if it realy is enough, but I often see commercial amps with 2 to 3 mm thick metal plates.... I think it has to be ferous (i.e. some iron derivative) but I'm not realy sure... google is your friend.
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Old 27th September 2006, 12:31 PM   #5
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Torroid? I've always heard that Torroid's don't throw the big EM field. Maybe you don't have to shield that one?

PCB shield mesh, or complete plane??? Well, you're going to have to investigate what kind of skin depth is necessary for 50/60Hz, to see if there is a shielding benefit at all. If there is a benefit and you do implement a PCB shield, I think a mesh would be more appropriate. The mesh greatly reduces eddy currents that could be generated by the magnetic flux cutting across a contiguous plane. I'm assuming you can generate eddy currents in copper. I don't know. You can, for sure, in aluminum.

Someone else should chime in on this. I've never examined stray fields coming off of a torroid.
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Old 27th September 2006, 01:28 PM   #6
Nordic is offline Nordic  South Africa
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why not assemble it on a piece of wood and see how it does as is, this will give you chance to look at what orientation gives the least interference.
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Old 27th September 2006, 04:46 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
I go with Nordic on this.
Orient the toroid to minimise any audible or measurable effects.

If the transformer lead outs are solid core, then swap them to flexible to enable easy rotation around the mounting bolt.

I have see it stated that there is a stronger field sent out in the direction (axis) of the hole through the toroid, but I cannot confirm this.
If this is the case then keeping the side of the toroid towards the pcb would avoid this potential problem.

Finally, the electrostatic field can be reduced by a non magnetic shield. This shield must completely enclose either the PCB or the transformer. A gap is just as bad as none.
The electromagnetic field can be reduced by a magnetic material and this material must be thick. I do not know if this shield needs to fully enclose.
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Old 27th September 2006, 05:11 PM   #8
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Default Copper works ...

... that's what most Faraday Cages are made of = copper screen or plates.

Be sure to connect the copper planes of the single or double sided board material to a ground, preferable the metal case ground, not the common audio ground.

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Old 28th September 2006, 07:14 AM   #9
torzsok is offline torzsok  Hungary
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Thanks for the suggestions.
To be honest, I designed the look first and ordered the case and the torroid accordingly. Only then started thinking about this shielding.
And yes, the torroid axis pinges the circuit in the heart and turning it is no option.

If mesh is an option I can wrap the torroid in copper mesh connected to the metal chassis.

AndrewT: If a gap is just as bad as none then mesh is out of question, right? Or, the gap should be only 'small enough'?

I can do both: mesh AND board over each other but it seems to be an overkill.

What about those metal cups often seen in cd-players over the torroid? How are they connected to the chassis? They must be separated from the the bolt fixing the torroid, or else they would work as just another turn in the transformer. Not that I could get hold of such a thing, just wondering.
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Old 28th September 2006, 07:23 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
if you are still considering copper or aluminium screening then you can only reduce electrostatic radiation.

These materials will have almost no attenuating effect on electromagnetic radiation.

Transformer hum is much more likely to be a magnetic field effect.
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AndrewT: If a gap is just as bad as none then mesh is out of question, right? Or, the gap should be only 'small enough'?
a mesh will work, but I have no expertise in this field. EMI attenuation is a science for which expertise is hard won.
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