Pondering layout - Noise considerations

[tdvt]

Working on the general physical layout of some small projects, which generated a couple questions regarding the generation of noise/interference & the proximity to other conductors.

• Is partially filtered DC noisy/hum generating? Specifically wondering about the proximity of other wires to early B+ nodes, not the ripple current in the actual power rail.

Second, I recently used a toroidal transformer in a project with no issues, but while researching things I read a thread (here I think) where the OP recommended never using a through-bolt to secure the transformer to the chassis.

In my instance, I used a threaded plastic insert which I epoxy potted into the center & used a corresponding plastic bolt which works great, but may not have been necessary.
Low noise is supposed to be one of the advantages of toroids & they are even supplied with a steel bolt for attaching to the chassis. But, I also read of some instances of toriods generating hum a being replaced with traditional transformers as a fix. So..

• Is it possible to generate  magnetic interference/hum from mounting a toroidal transformer using ferrous or even non-ferrous metal fasteners?

[glass54]

Hi tvdt,
"Second, I recently used a toroidal transformer in a project with no issues, but while researching things I read a thread (here I think) where the OP recommended never using a through-bolt to secure the transformer to the chassis."
This is Incorrect. As a a user of hundreds/(thousands) of Toroidal Tx, in both SS and Tube assemblies, I have had no problems.
In new builds or replacement of EI tx's, I use the standard mounting kit ie large steel top washer, rubberised washer, the toroid and then finally the chassis side rubberised washer. In the 30VA to 200VA range, the "through bolt" is typically M4 or M5 with small washer and LockNut.
You can even do a partial shield over the toroidal  body but NEVER a complete magnetic path.
Also NEVER over-tighten Toroidal installations, leads to disasters 
In a 1RU chassis, make sure the Nut and end of screw have sufficient clearance to avoid magnetic short ( )
I have never actually done the test, (I could one day), but I have been advised that a torodal Tx has "highest EMI" when loaded with a low percentage of max load,
Trust this helps
Mirek

[tdvt]

Mirek,
Thanks for these insights, it sound like the all-around voice of experience... , exactly what I was hoping for.

It may have been my own (extreme) interpretation regarding the attachment of the toroid, but after looking at some drawings of toroidal EMI fields, I was flashing on building electromagnets in grade school; the rod through the coil of wire, but maybe not even applicable with AC.

Anyway, with this last build, I erred on the side of caution & didn't use any metal fasteners on the transformer mounting.

But this is why I posed the question & I appreciate getting information from someone that has successfully used so many of these.