I just want to know, what is the difference between open center and closed center toroidal transformer. Open center meaning the toroid has a metal rod in the middle while the open center doesn't have anything in the middle.
Open: https://my.element14.com/vigortronix/vtx-146-050-215/50va-toroidal-transformer-2x15v/dp/2817652
Closed: https://my.element14.com/multicomp/mcta500-35/transformer-toroidal-2-x-35v-500va/dp/9532951
What is the difference between open center and closed center toroidal transformer?
There is almost no difference. Just one is ready for mounting.
Large toroidal transformers are usually bolted this way simply because it is convenient. Smaller ones may be glued into a base, either horizontally or vertically, with nothing in the center.
Consider that the toroid itself is the "conductor" of magnetism. It has a high magnetic permeability, so the vast majority of magnetic flux is contained inside that.
Whether a rod is placed in the center of the toroid makes little difference to the operation inside the toroid. This rod is also at a right angle (perpendicular) to the flux inside the toroid (circular), so it has even less of an effect. So the rod does not act as a "shorted turn" or anything and is largely "invisible" to the toroid.
What you call it closed is a mounting accessory. You drill the plate where you want to fix the transformer and then you bolt the transformer, the metal cap is a retainer.
