Toroidal Transformer Shield

[Helsing]

I want to make a bent metal cap to put over a PC mount toroid transformer. It is inside a mic pre enclosure that just doesnt have room for a shield wall. The noise is very minimal but I would like to improve it.

I was thinking that I could put some flexible adhesive between the top of the transformerand the inside of the cap to hold it in place. The current draw is very low and the thing barely gets warm at all.

Does this shield/cap need to be grounded? Any other things I should know?

Helsing

 

[jensenmann]

Is there a reason to do this?
Usually the magnetic field of a torroidal is very small outside. That´s the reason for building trannies thist way. Are you sure that the tranny is the reason for your humproblem?
If you put a box around it you will have a coil with one winding around your tranny. This will cause a current by induction in your box. If you ground it this current will modulate the ground. Then you have more trouble. A wall between tranny and circuitry might be a better choice (grounded to chassis).
cheers
Jens

 

[Guest]

[quote author="Helsing"]Does this shield/cap need to be grounded? [/quote]
Toroisal shield is simple big toroid, in which small {shielded} toroid
is enclosed.
There are {for measurement mumetall toroid} other
shieldnig systhem. It is four toroids, which surrounds your
torrois completelly and it {after bandage} looks like big toroid.
I have one at home, if someone want fotos, I may make it.

There is also simple mumetal can, it may be usefull for you.

... And ground everythink. {but only at one point!}

xvlk

 

[VacuumVoodoo]

Mu-metal foil, tape etc can be bought from

http://www.lessemf.com

I am having great success with their products shielding receiver transducers in reverb tanks.

 

[Guest]

[quote author="VacuumVoodoo"]Mu-metal foil, tape etc can be bought from
[/quote]

Yes, to wind mumetall strip on the finished {and bandaged} toroid, like it is "tercially" winding... But you must wound insulated strip to
prevent shortwound...
xvlk

 

[pmroz]

...can't find it, but there is a thread around here that suggests re-orienting (turning) the toroid until hum is reduced. Thought i'd mention...

 

[pmroz]

... helsing, didn't realize was your topic... thanks for the AA card and trafos... schem almost ready to post...

 

[Helsing]

pmroz,

You're welcome.

This is a plastic encased Amveco PC mount toroid. There is no mounting bolt, completely enclosed. That is why I was thinking I would adhese this "cap" to the plastic case and was wondering if grounding was necessary.

I spoke with a metal house that said they build transformer shields out of something called CRS TIN Plate. Does that ring a bell?

edit: DUH CRS "Cold Rolled Steel"

Helsing

 

[PRR]

> CRS TIN Plate. Does that ring a bell?

Cold Rolled Steel, tin-plated.

Basically a tuna-fish can. Much better than air, not as good as Mu-metal.

I would try a tuna-fish can first. If the tuna-can works, but you need it neat, then get a custom can.

If it does not work well enough, find two cans that fit over each other; two CRS shells may work well and will be cheaper than one Mu-metal.

Distance is by FAR your "cheapest" shielding. Never build things too tight.

 

[Helsing]

Thanks PRR. I spaced the TX as far away as I could. Right now things are very quiet but I would like to improve it if possible.

The transformer is completely encolosed in plastic and the shield would just be adhesed to the plastic. Tuna can doesnt work because it is a square case. Would I be asking for trouble grounding or not grounding this shield?

Helsing

 

[Ethan]

That's strange... Isn't steel ferromagnetic?
I believe the shield should be grounded. If it wasn't, wouldn't it be just like an antenae?

 

[Helsing]

Cant rotate a PC Mount Toroid. Down side of the convenience.

Helsing

 

[CJ]

Ground the shield.
Bending mu metal takes away some of it's shielding properties.
This is why mu cans are RF heated/anealed after they are formed.
Try two pieces of regular metal with a sheet of thin brass or copper in between.

Torroids cancel hum only if the windings are perfectly spaced, but there is probably always going to be a little hum.

Make sure the hum isn't from poor filtering, ac filaments if tube amp.

I don't know but can filter caps radiate hum ?

 

[VacuumVoodoo]

Filtercaps are subjected to current pulses and as they do have some inductance so these pulses produce non-static electromagnetic field which can induce voltage pulses in surrounding circuits. Electrolytics are housed in metal cans though which provides some shielding. We always remember to twist the heater supply wiring but it is also good practice to twist the HV and correspondning GND wire. Just make sure the wires have isolation rated for the peak voltage they will carry. HV wires from the power transformer should also be twisted. Together with proper grounding scheme this will give a really hum-free circuit.