Mounting question

[Atilla]

It turned out that the enclosure I picked wasn't as high as I thought when ordering it. So now I've an interesting issue with mounting the toroids. They fit perfectly on their own. However when stacked with the rubber pads and the top bracket, they are the exact height of the case. It closes but the metal bracket touches the top of the case.

I was trying to find a more detailed explanation about toroidals and the shortened loop effect, but I wasn't too successful. What is it that I should avoid exactly? If the bracket and the chasis are insulated with something in between, isn't that going to be enough to prevent problems? I don't want to close the lid and discover that things are going to explode.

[Nico Ras]

Pour a bit of epoxy into the toroid (about half full) then drill a hole in the centre and use a much shorter bold. Now there is no chance for the bolthead to touch the cover.

The rubber disc under the transformer is to insulate mechanical vibration.

If the cabinet is steel you may find a bit of mechanical hum coming from the cover but electrically it should not be a problem.

[chatziva]

you could drill some holes on the chassis and use cable ties to secure the trafo

[lanchile]

As long as the transformers have the Rubber pads before the metal wash(top/bottom),everything is fine.bare wires from transformers should never touch the metal chassis(just in case of the coating of wire from transformer peels off and touch the metal chassis).Always isolate transformers from chassis (if it is metal).if you check with an ohmmeter you will see that the bolt and metal bracket touch the chassis but, the bolt and metal bracket DOES NOT touch the wires of transformer because of the rubber pads.

[Atilla]

The epoxy is a really nice idea. I've seen toroids with center screws, that'll probably work out perfect.

There are some ventilation holes as well, I could try to use them and secure the toroids with cable ties, that should probably work out nicely as well.

Good thing you reminded me to leave an extra layer of insulation between the trafo and the chasis, it's always a good idea to keep things insulated better.

[Atilla]

That reminds me that I was wondering about something else. I've a small circuit made on a prototyping board - inrush current limiting. The problem there is that the bottom side of the board is covered with uninsulated tracks. Any ideas what I could use to coat them with, so I can be sure nothing would accidentally touch the board and short something, or bring potential to the chasis?

Of course, the chasis will be connected to safety earth and there are fuses as well, but I don't want to leave any chance for mistakes.

[wakibaki]

Originally posted by Atilla
...the shortened loop effect...

That's shorted. This is where a part of the metal structure of the box or transformer mount forms a complete conducting loop that passes through the hole in the toroid as it would if for example you used Jubilee Clips (metal hose clamps) instead of cable ties to secure the TX to the chassis. The same thing can happen if a securing bolt touches both the top and bottom of the enclosing box. It then becomes a secondary winding.

For mounting PCBs you can use a standoff. You can get self-adhesive ones, they have a harpoon like spring clip that passes thru a circular hole in the PCB and then latches open.

w

[Atilla]

Sorry - shorted, of course. I read it wrong and then started wondering ... what on earth is a shortened loop. *sigh*

Conductive material around the toroid core, no load - bad situation. Thanks for getting my brain working right again.

[lanchile]

You can use "double side stick tape" for your inrush current limiting board or use "five" drops of silicone one in each corner and one in the middle.

[Atilla]

All good ideas, thank you very much!

Soon, it will sing.