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Old 13th February 2007, 10:23 AM   #1
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Question Toroid mounting getting hot ...

Hi there,

I've this strange problem with mounting my toroidial transformers form my class A amp. The mounting gets really hot:

I've two identical 160VA 18V toroids wired in series to give me postive and negative supply rails.

These toroids are mounted with metal bolts trough their centers. Now comes the funny (and worrying) part:

When I clamp down the toroids by means of 1 piece of metal (be it steel or aluminium) with two holes trough which the bolts go then the metal gets very hot very quickly.

When I use 1 piece of metal per bolt (so each toroid has its own piece of metal clamping it down), then these stay nice and cool.

The worrying part is of course that these bolts screw into my heatsinks which are HUGE. So in my tests of an hour or so these don't get hot.

I don't sleep well on this. Something is going on which I don't understand and it seems to be rather powerfull. The speed with which the one piece of metal get's really hot worries me.

Can anybody explain to me what's going on and if I do need to worry.

Thanks, MArco
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Old 13th February 2007, 10:38 AM   #2
es44 is offline es44  Denmark
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You don't want to make a short circuit like that. When you clamp the toroids down that way, you make one winding, similar to the real windings on the toroids.
That means one BIG short , as that winding has no ends, and is therefor seen from the primary side as a short circiut, drawing all the current i can, only limited by the resistance in your bolts and the clamp.

Best regards
Ebbe
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Old 13th February 2007, 10:48 AM   #3
Elvee is online now Elvee  Belgium
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Quote:
Originally posted by es44
You don't want to make a short circuit like that. When you clamp the toroids down that way, you make one winding, similar to the real windings on the toroids.
That means one BIG short , as that winding has no ends, and is therefor seen from the primary side as a short circiut, drawing all the current i can, only limited by the resistance in your bolts and the clamp.

Best refards
Ebbe
Correct analysis.
I can add that in your case, a solution does exist: you simply have to reverse the primary of one of the transformer, and the problem will disappear: the voltages induced in the bolts will have opposite phases and cancel each other.
LV
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Old 13th February 2007, 10:59 AM   #4
es44 is offline es44  Denmark
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Quote:
Originally posted by Elvee

Correct analysis.
I can add that in your case, a solution does exist: you simply have to reverse the primary of one of the transformer, and the problem will disappear: the voltages induced in the bolts will have opposite phases and cancel each other.
LV
Also correct

Or maybe use some non-conductive material for the clamp, a piece of plastic instead of metal.

Simple solutions to something that could have had serious consequenses. At least for the toroid.
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Old 13th February 2007, 11:27 AM   #5
cviller is offline cviller  Denmark
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I have two toroids I need to mount, so I would like to understand this problem correctly in order to avoid it. I have drawn very rough sketch on how I understand deduikertjes connected his and how I would connect mine - is the drawing correct?

I won't be using any metalbar on top of the toroids - so they will only be connected on the bottom plate.
Attached Images
File Type: png toroids.png (15.6 KB, 264 views)
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Old 13th February 2007, 11:44 AM   #6
es44 is offline es44  Denmark
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You are exactly right cviller. you will have no short with the way that you plan to mount yours.

All the best
Ebbe
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Old 13th February 2007, 11:56 AM   #7
cviller is offline cviller  Denmark
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Cool, thanks!

And I'll be able to melt metal between the bolts to impress friends!
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Old 13th February 2007, 11:59 AM   #8
es44 is offline es44  Denmark
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Yep, you don't need tubes to get that glowing in the dark, just big fuses. hehe.
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Old 13th February 2007, 12:01 PM   #9
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Well,

Thanks for the quick and informative answers. I do understand now how to clamp down those toroids but not yet completely what is happening.

Quote:
You don't want to make a short circuit like that. When you clamp the toroids down that way, you make one winding, similar to the real windings on the toroids.
Do I understand correctly that a tension (voltage) is created in the bolts by the electro-magnetic field in the toroids. Although the clamp is a non-magnetic material the electric coupling of the two bolts results in the bolts and the clamps working as a sort of transformer winding with very little resistance (am I lucky that these bolts were very dirty ...).

Connecting the bolts on one side only does not break the electrical connection but does prevent the assemblage working as a transformer winding.

MArco
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Old 13th February 2007, 12:39 PM   #10
es44 is offline es44  Denmark
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Quote:
Originally posted by deduikertjes

Do I understand correctly that a tension (voltage) is created in the bolts by the electro-magnetic field in the toroids. Although the clamp is a non-magnetic material the electric coupling of the two bolts results in the bolts and the clamps working as a sort of transformer winding with very little resistance (am I lucky that these bolts were very dirty ...).


MArco

You are kind of right....The clamps don't have to be magnetic, as long as it's electrically conductive, then it will act as a single winding seen from the transformers point of wiev.
If yo use non-electrically conductive material, there will be no problems, Elvee's solution is also useful, and cviller's as well. Just no short circuits

best regards
Ebbe
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