Low noise PSU transformer

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How come the transformer can work correctly with this one turn winding SHORTED. Usually winding turns shorted, induces overheating if not the death of the transformer.
Looking more carefully, I see it is not installed like an additionnal one turn winding. It is a shorted one turn for the stray field only.

See Flux band [Encyclopedia Magnetica] , which explains why it doesn't cause overheating. It's not wound around the toroidal core itself (i.e. through the centre of the core), but is outside the core and windings, where it is just a short to any escaping flux.

If you look at many toroidal transformers manufactured with shields, they look like that DIY effort by SC - the shield goes around the outside, like the custom shielded transformer linked to by Per-Anders in post #12. I think if the shield contacted the mounting bolt at both ends, you'd be in major trouble!

Regards,
Don
 
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a 7 or 11dB noise reduction from a copper flux band proves it works fairly well
Do you know the thickness of the flux band that gave this interesting noise reduction ?
From my understanding of a shorted coil in a variable field:
I presume the noise reduction is directly related to the resistance of the band.
So with a band twice as thick one can expect twice the dB attenuation, in the limit of 10mm where skin effect comes in. At that ultra low resistance, it better be a plain copper ring, not made from a band with a solder joint.
EDIT Skin effect is 10mm for copper at 50Hz, but if the noise has harmonics 100Hz, 200Hz,,,, it goes down to 2mm at 1KHz.
I’ve got some 0.3mm (easier to bend) sheet copper ready to shield them with copper flux bands!
With 0.3mm, you can go with multiple bands as Mark Johnson suggests in post #34, much easier to bend.
 
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Do you know the thickness of the flux band that gave this interesting noise reduction ?

They used 0.5mm copper, but I think that was more a factor of their local metal recycler having some copper sheet of that thickness.

From my understanding of a shorted coil in a variable field:
I presume the noise reduction is directly related to the resistance of the band.

Yes, that may be the case - I think that's why copper works so well, because of its low resistance.

So with a band twice as thick one can expect twice the dB attenuation, in the limit of 10mm where skin effect comes in. At that ultra low resistance, it better be a plain copper ring, not made from a band with a solder joint.
With 0.3mm, you can go with multiple bands as Mark Johnson suggest in post #34, much easier to bend.

That might be so, but I've only got so much copper sheet, and a number of different transformers I want to apply it to! Actually, I'd forgotten, but it was actually 0.4mm copper sheet that I bought, and I've got enough to do about 6 300VA transformers - as it happens, I've got 3 power amplifiers with twin 300VA transformers, one of which I haven't built yet, although I have all the parts. So I'll only have a few small off-cuts of copper left after I finish.

The soldered bands were only possible with EI cores - you can't do it with a toroidal transformer, due to its outer insulation, which would melt. So that was the reason why SC overlapped the copper, and used a screw clamp to hold it in place.

Regards,
Don
 
With an EI or similar shaped core with a flux band, the leakage flux still extends away from the transformer, but re-orients itself to looping out from the winding edge and back to the edge of the flux band on each side of the winding. Without the flux band the flux loops out from one end of the winding and extends around and returns to the other edge of the winding. So the flux band won't stop stray flux from egressing completely, and so I don't think any grandiose plans for multiple or thicker flux bands are going to achieve much.

Like orienting a power transformer relative to an output transformer to reduce hum transfer, there could be benefit in checking the influence of orientation of a transformer with belly band, as it may well be different to what you would choose with a transformer with no belly band.
 
Why would a transformer without at least one screen winding be used in "an ultra low noise instrumentation amplifier" design? Either the designer hasn't done their home work, or penny pinching I guess.

That is assuming conducted noise from the mains is even a problem with a properly designed power supply. In reality, the magnetic field from the transformer is going to be a bigger problem in a small box.
 
The thread started as an ultra low noise IA - similar to a lab grade bench meter, the isolation and grounding would likely be in the same design vein if any old mains AC connection could be used.

But the goal posts have now been clarified to just a standard modern instrument amp.

Still, I don’t see any high end test equipment with linear supplies anymore anyway. For a low power device it’s easy enough to achieve excellent numbers without a screened transformer. The primary to secondary capacitance is of little concern IMO if the rest of the supply is designed well. Certainly, go for the electrostatic screen if available, but even if not I’d still use a toroid over an EI core or split bobbin probably.
 
The lack of an electrostatic shield in the transformer is NOT the issue.
At the mains inlet, it is fitted with chokes and caps in a grounded enclosure.
The rest of the supply is designed well.
There is no trace of conducted noise from the mains.

chris719: Still, I don’t see any high end test equipment with linear supplies anymore anyway.
Interesting. What do they use, is it available off the shelf ?
 
The lack of an electrostatic shield in the transformer is NOT the issue.
At the mains inlet, it is fitted with chokes and caps in a grounded enclosure.
The rest of the supply is designed well.
There is no trace of conducted noise from the mains.

Interesting. What do they use, is it available off the shelf ?

I would assume it’s custom or at least not a standard off-the-shelf part in the case of equipment from the likes of Keysight, Tek, R&S, etc.
 
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