Toroid winding question.

Not very much. You can look at it as being the sum of two current sheets: one wrapping tightly around the core, and another annular one whose inner layer cancels out the first sheet and whose outer layer represents the actual windings. The annular one doesn't enclose the core, so the field in the core is the same as with an equal current per unit length in a tight winding.

There will be some additional field outside the core due to the outer annulus, which will increase the leakage inductance of the transformer a bit.

Cheers

Phil Hobbs

Thanks, that's put my mind at rest.

Shouldn't matter. Use that thermal-strip pseudo-Litz wire maybe. Or the thin-skin Teflon insulated stuff.

Can you use a pot core? Toroids are a pain to wind and mount.

Thanks. High vibration, so no pot cores, high temperature so yes, Teflon. Thin skinned? Is that special or just the normal class A?

Cheers

This sort of stuff. Military, fine strand, thin teflon insulation:

Huh, how is a toroid easier to mount (with respect to vibration) than a pot core?

Low profile pot cores are available, too, and half a dozen other low-profile shapes (for bobbin and planar use).

Tim

What bandwidth? Impedance? Leakage?

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

Beware if you put a metal screw through the center of a toroid.

Why?

Huh, what happens? It's only ~1/2 a shorted turn. I've seen big power supply toroids with metal screws (through the center) holding them in place.

George H.

You can't have half a turn on a toroid.

I had a toroid power supply transformer held in place with one screw through its centre, mounted inside a flat aluminium box. The box was so flat in fact, that the cover would sometimes press on the top of the screw, thus completing a shorted single-turn winding. It made the transformer pretty hot and the short-circuit current caused severe hum on the analog signals. A layer of tape on the inside surface of the cover over the transformer fixed it.

Jeroen Belleman

Metallic fasteners thru pot cores can royally screw up the works. So I've always resorted to the spring-clip style of mounting. ...Jim Thompson

I measured a pot core with and without a ss screw through the hole. No apparent difference.

We use ss screws to mount pot cores and toroids; no problems.

True ss screws are good. Sometimes hard to find at the onesey/twosey quantity.

Most ss appliances aren't ss. ...Jim Thompson

Not easier, but less susceptible.

Toroid is one piece, so no separate parts to jiggle about. It gets strapped down and potted. This is serious vibration, outside my normal comfort zone, so I'll go with the guys who've done it before.

Cheers

Home Depot and Lowes carry them, onesey-twosey. If you want more, they're readily available at the usual places.

They're SS, just not the 30x varieties we normally associate with SS.

There is a difference, depending on whether the core is gapped. A ferromagnetic screw is OK, if the core is not gapped. The screw messes the field in a gapped core.

A toroid can be mounted with a screw through the hole as long as there is no path around the screw on te outside of the core.

If the toroid is wound without a reverse-layer return, you get a percent or two of the field induction axially on the screw. That screw is longer than the toroid is tall, so it has longer range coupling. 300 series SS is nominally not magnetic, but screw-rolling puts internal stresses in, so you can't count on as-annealed condition from the factory (nor after the box jostles a few dozen times).

It's credible that a screw could dominate the field leakage.

OK, that makes sense. So the warning is not against a metal screw, but against a magnetic screw. (Unrelated but I have some brass

George H.