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I found 2 ferrite rings in a power supply I was stripping for its magnetics. I was wondering why is it that I've never seen these employed as transformers.

I've read that for a number of reasons toroidal cores make superior transformers.

Why couldn't I employ these toroids in a half bridge or push-pull topology as opposed to the E-cores in the photo?

enter image description here

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Why couldn't I employ these toroids in a half bridge or push-pull topology as opposed to the E cores in the photo?

Three reasons that spring to mind: -

  1. It's a lot easier to manually wind turns onto a bobbin then add that bobbin to the cores compared to manually winding a transformer on a toroid.
  2. With core-sets, you can apply gapping to give you superior performance in many cases. Of course that's virtually impossible for a toroid. (It can be done but with difficulty).
  3. An E-core set will have more volume of ferrite and be able to handle higher saturation currents for a given floor-area of a toroid.

I've read that for a number of reasons toroidal cores make superior transformers.

That's not usually the case.

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  • \$\begingroup\$ re: 1) There ARE toroidal transformer wiring machines. As mechanical engineering goes, they are high art! \$\endgroup\$ – Brian Drummond Nov 21 at 15:36
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    \$\begingroup\$ @DKNguyen that article misses a very simple thing and, in that respect, it makes the article broadly wrong; adding a gap does reduce the risk of saturation. Here's why: say you added a gap and the permeability dropped 4 times and say you needed to keep a certain inductance on the primary; well you'd (only) need to double the turns to restore the inductance to the ungapped value. If the ungapped core was close to saturation on a certain supply voltage then quartering the perm and doubling the turns has an overall effect of halving the ampere turns hence, it unambiguously prevents saturations. \$\endgroup\$ – Andy aka Nov 21 at 17:25
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    \$\begingroup\$ @S.s. it doesn't matter if there is DC or not; you can add a gap to a transformer core, reduce the permeability, increase the turns to restore the required inductance and then you will have a net reduction in the effects of ampere turns. The math is simple and the effects are fundamental on any transformer type ferrite or not. \$\endgroup\$ – Andy aka Nov 21 at 22:55
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    \$\begingroup\$ @Andyaka I absolutely love the clarity in your responses. Direct and simple. Together with your answer the whole is a +1 to my mind. So be it. You explain the value of the gap very easily and well. Nice! For those interested in a more detailed exposition, I recommend this site. It covers some additional, interesting details such as Hanna curves. \$\endgroup\$ – jonk Nov 22 at 7:13
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    \$\begingroup\$ I agree, but you've actually inadvertently answered a few other questions I've had for a long time now, because I've harvested a bunch of huge E cores from a some digital HID ballasts that someone gave me. Most of the core halves have the center leg cut down, so I thought I was stuck with using the few that weren't cut to wind transformers and the rest as low ue inductors. Now I have 21 potential transformers as opposed to the 7 that I thought I had, thank you very much! \$\endgroup\$ – user14828 Nov 23 at 19:28
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Sure Toroids make good transformers .In your case be careful .Cores that are for EMC often are made of materials that are deliberately designed to be more lossey to absorb high frequencies .Such cores can give lower radiated EMI .If you use these as a transformer core heating will be a real problem and Power supply efficiency will be down.

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