Shielding a Toroidal Transformer - Which is the trick ?

[kagliostro]

As far as I can know when using a toroidal transformer as to avoid problems we must take care on the fixing method, solution like this creates a shortcut loop



But I've seen shield for Toroidal Transformer use like this





I've a nice ex candy iron box of the right size that I would like to use as a shield for a Toroidal Transformer

Can I use it or not ?

Where is the trick ?

Thanks

K

[PRR]

Are you sure you need to shield it? Donut has low leakage. A little distance is as good as a lot of iron.

AFAIK, you can put it in a can but you can NOT run a conductive bolt through the center (and back around via conductive can). I am not sure your bottom picture is safe unless that center post is electrically insulated.

[kagliostro]

Are you sure you need to shield it?

No, absolutely not

it was more a matter of aesthetics, not a real shield problem

So the use of a nylon screw is admitted

What about the use of an iron screw that don't touch the upper side of the shield, can be a solution ?

Thanks

K

[PRR]

If you do it wrong, the candy box will get very hot very fast.

Did you save some candy? Leave a small piece on the box. If it melts, shut-down fast.

[HotBluePlates]

A more typical method is to use a plastic or hard rubber plate on the top of the transformer, with a bolt through the plate to hold the toroid to the chassis.

[DummyLoad]

you can run a conductive bolt through center - it's a single turn winding when you do - secure the bolt only at one end. sandwich the toroid between two rubber washers and use a large flat washer for the top. do not allow top of bolt or top washer to contact chassis - think of it as a single turn winding with one end grounded and the other end floating.

some toroids come with the center filled with epoxy and a have a nut cast in the epoxy - you mount these with a single rubber washer and since the bolt doesn't exit the core, it's considered safer.

best way to mount toroid is to fully encase in mu metal with insulation layer (usually epoxy) and secure the case by edges or bands over the case to chassis. problem here is encapsulated iron has to be derated - IOW, a 1KVA rated toroid used in free air would have a lower KVA rating if encapsulated, IIRC, it's 15-20% less than free air rating and that can be even less with high/continuous duty cycle use.

--DL

[kagliostro]

@ PRR

Did you save some candy?

No, I ate the entire contents of the box

@ HBP & DL

I was thinking to do so:

Drill the bolt and wires holes on the box cover

use the box cover upside down, connected to the chassis

fix the transformer to the chassis with a bolt and washer through the hole in the box cover

then fix the box upside down on his cover, paying attention that the bolt and washer are not touching the box now used as a cover

K

[DummyLoad]

@ PRR

Did you save some candy?

No, I ate the entire contents of the box

@ HBP & DL

I was thinking to do so:

Drill the bolt and wires holes on the box cover

use the box cover upside down, connected to the chassis

fix the transformer to the chassis with a bolt and washer through the hole in the box cover

then fix the box upside down on his cover, paying attention that the bolt and washer are not touching the box now used as a cover

K

franco, two rubber pads, a large fender washer, nut, bolt and lock washer. tried, true and tested. many highly regarded pieces of audio gear are equipped with toroids without a shield. you may be over-complicating things, try it without the shield first, then if you think hum is a problem then shield.

btw, i don't think your candy tin is mu-metal. mu-metal is nickle-iron alloy.

--DL

[kagliostro]

No, really, I wasn't thinking to that as a real shield

I was thinking to use the box only for aesthetics purpose

Thanks for the info

Franco

[Merlin]

As far as I can know when using a toroidal transformer as to avoid problems we must take care on the fixing method, solution like this creates a shortcut loop

All you have to remember is not to create a complete loop of *metal* (an electrical circuit!) around the toroid, i.e., through the centre hole and around.

It is quite alright to have a loop of *non conductive* material, so the plastic box in your post isn't a problem. It is also OK to put the whole thing in a metal box as in your first photo, since there is no connection through the hole.

[kagliostro]

Hi Merlin

At the beginning I didn't noticed that the black box is not made of metal and therefore will give no problems

It is also OK to put the whole thing in a metal box as in your first photo, since there is no connection through the hole.

I think that the bottom of those shield is fixed to the chassis, than an hex head screw, a rubber pad and a metal washer are used to fix the transformer to the treaded central hole of the bottom of the shield

at the end the upper part of the shield is positioned on his base to cover all, without that the bolt or the washer touch it

Thanks for your explanation

Franco

[darryl]

An illustration of some toroidal transformer covers, used on MI Audio amplifiers. Note the hole in the top of each cover, which removes the possibility of creating a shorted turn via the transformer mounting bolt.



Disclosure: This image originally appeared here:  http://www.guitargear.net.au/discussion/index.php/topic,35643.0.html

[kagliostro]

Hi Darryl

Thanks for the link

Franco

[darryl]

When you say conductive bolt, are you describing a ferromagnetic bolt.? or would an Al or Cu bolt be acceptable?  (Be?)

The bolt's magnetic properties are irrelevant. The problem is created by using a bolt made of any electrically conductive material, and having the ends of that bolt connected to each other by a conductive path outside the toroid.

[PRR]

What Darryl said.

The transformer is turns of wire with separate ends for the power to come out.

Wrap one turn, then connect the ends together.

That's a DEAD short.

It matters very little that the turn is not tight on the primary. Main difference is a longer length-of-turn which is higher resistance which is a less-dead short. Still a short.

Instead of one turn of wire, try one turn of ribbon. Wide ribbon. Ribbon wider than the circumference of the donut. That's still a shorted turn, and because of its width it is a very good short.

Yes, tough to get such a wide ribbon through the center. Replace the center with a conductive bolt (which means the "ribbon" around the outside now looks like a candy-can with holes). Brass, iron, zinc.... hardly matters. They are all likely to be exceptionally good shorts, even silicon-iron which isn't nearly as good a conductor as copper. (I take most of my power through an iron wire, it's OK.)

[kagliostro]

(I take most of my power through an iron wire, it's OK.)

PRR do you mean the electric line in your area is build using iron wires instead of copper wires 

I've seen that solution only on the est of Europe and it was many years ago

 

I really think I've misunderstand you

K

[DummyLoad]

iron wires instead of copper wires

the electric lines on the telephone poles in your neighborhoods and high-lines are aluminum conductors over steel for reinforcement. copper is too expensive. in the USA the service entrance to residences is commonly the same.  most of the time you only see copper from the meter-loop to the main panel(s) and house/building wiring - even then, aluminum is still common.

--DL

[kagliostro]

Ciao DL

Aluminium over steel, I can understand, but I'm not sure about what they use here in Italy

I know that aluminium nowadays is often used instead of copper, but here in Italy that didn't seem to be used largely instead of copper, I must ask about that to a friend

However I recovered the transformer of a broken Telwin arc welder (I want to rewind the secondary as to use it as a spot welder) and the existing primary and secondary windings are in aluminium instead of copper

K

[PRR]

My overhead power line is very long.

Copper or Aluminum is too weak to span that far. (They could have installed more poles, but that costs money, and this land is poor for poles.)

Steel can do it; but conductivity is 1/7th the conductance of copper.

There's copper-clad steel. There's steel wrapped around insulated Cu/Al. There is now a hi-strength Aluminum alloy. But this was a while ago and far out in the woods. Keep It Simple.

They ran a steel cable and wrapped two insulated Aluminum conductors around it.

Here the power is 240V Center-Tapped. Since 1913 the CT must be connected to dirt. So in theory it is safe to touch (DON'T!!).

The center-tap of the power is run on the steel wire.

There is a lot of voltage sag. When I turn on the microwave oven, the lights dim.

The sag on the 120V circuits is related to the resistance of that steel condutor.

However after much figuring, I decided that was OK. The naked steel is about the size of the insulated Aluminum. There's a lot of insulation, these cables are only about 1/4 Aluminum and 3/4 rubber. Aluminum is about 4 times better than steel, but there's 4 times more naked steel than insulated aluminum. The steel line has about the same losses as the Aluminum cables.

All three conductors should be bigger. But simple guessing of metal price and labor costs says $5,000-$10,000 to run a new line from the street. Alternatively I could have transformers at both ends and run the existing cable at 440V, which might cost $5,000-$10,000.

Since the lamp-dimming problem is not worth $999 to me, I have not bothered to get better cost estimates.

I did worry that something was WRONG. That I might have a burning connection someplace. No, when I add-up the lengths and sizes, the resistance (voltage drop) numbers I get are very close to the voltage drop I observe.

[PRR]

Steel is 15 times cheaper than Copper, and 7 times lower condutivity. I'm surprised we don't use more steel for power lines. Perhaps in large sizes the increased weight on the pole means more cost of supports. Also more cost to truck heavy rolls of steel to the job.

[kagliostro]

Is a very long time I've seen in the ex Yugoslavia some electric line (on wooden pole) build with galvanized iron

but I remember that often the line was Humming (I don't know the reason for that), there, some people, told me that it was a large waste of energy because of the higher resistance of the wire if compared to copper wire

Your line with the rounded aluminium wires on steel is much more sophisticated and surely much more efficient than those old iron lines

K