Toroids, mounting bolts, and shorted turns

[Saurav]

Hi,

Just getting ready for BrianGT's Gainclone kit to get here, and I have a question. My toroidal transformer arrived from PArts Express, and it has 2 sets of mounting hardware (bolt, dished metal washer, foam/rubbery washer). I looked up shorted turns, and if I understand it right, I'll have a problem only if the bolt touches both the top and bottom plates, right? If I use both metal washers (one under the transformer and one on top), that won't cause a shorted turn. I think I have that right, but I'd just like to be sure before I connect power

I think it'll help to use the two foam washers, it'll probably isolate the transformer's vibrations from the chassis better? I'm not sure if there's any advantage to putting the second metal washer under the transformer though.

Thanks,
Saurav

[andy_c]

Hi,

There's a good illustration of the shorted turns issue here http://www.bcrn.com/Media/ToroidalPowerTransformers.pdf on page 6.

[Saurav]

Thanks. A picture always helps. I think I'm OK with using a metal mounting plate both on top of, and under, the transformer.

[BrianGT]

Originally posted by Saurav
Thanks. A picture always helps. I think I'm OK with using a metal mounting plate both on top of, and under, the transformer.

I am using a metal plate on top of my transformer, with a bolt going through it, touching both the plate and my chassis bottom. This hasn't caused me any problems.

I guess it would become a problem if you grounded the top plate someway besides the bolt, creating a loop like in the picture.

--
Brian

[Saurav]

I was thinking of sending the earth connector of the IEC plug to that top plate, since it's a nice thick nut/bolt and should make a good star ground point. That should still be OK though. The IEC earth prong isn't connected to the chassis anywhere else, and it's the bolt that connects the chassis to earth here. So, that shouldn't create a loop either.

I hope

[AudioFreak]

Basically, if you cant map a path of electrically conductive material that forms a loop going thru the centre of the toroid then you should be safe from shorted turns external to the toroids insulation. Doesn't matter if both the top and bottom washers are conductive as long as the only path of conduction between them is thru the mounting bolt going thru the centre of the toroid.

[lgreen]

In my GC, the tansformer bolt is very close to the top cover. but the transformer is isolated from the metal chassis by a rubber pad, and the top plate of the mounting assembly is isolated from the transformer via a rubber pad. Neverless, when I push down on the chassis of my GC (when "on"), the top metal cover hits the transformer bolt, this causes a very loud vibration and the area of contact immediately gets too hot to touch.

Is this "shorted turn?" The issue has been solved by using a shorter xformer bolt.

[tja]

Yup, that's the shorted turn. The bolt through the center of the transformer is forming a loop with the chassis, with almost no resistance I should think, therefore the current can be quite high, thus generating the heat.

Take