I have a toroidal ferrite core which has an outer diameter of 15 cm, inner diameter of 11 cm and height of 4 cm. I want to use it to make a transformer which takes in mains voltage (240V) and outputs high current (>50A) at any voltage (even half a volt will be okay). I will be using wire of 2.5 square millimeters for winding the primary and secondary of 10 square millimeters. How many turns of each do I need to make? Will this be safe? Is there anything else I need to know?

  • \$\begingroup\$ Sounds dangerous to me. Why don't you make a guess at how many turns of each. How much current from the 240 VAC, if your output is 1 VAC @50A? Do you think all the wires should be the same diameter? (look up the relation between wire size and current capacity.) \$\endgroup\$ Commented Oct 3, 2014 at 12:38

1 Answer 1


Any transformer connected to 240V AC has to have sufficient inductance in the primary so that it isn't taking a large magnetization current - just think of the primary and ignore the secondary for now - imagine you are only making an inductor to connect to the AC - you don't want it taking ten amps just by itself.

Of course there is a technical reason for not taking ten amps - this will almost certainly saturate the core and fry.

So, armed with the details you have on the toroid such as the \$A_L\$ value, this will help you understand how many turns are needed to obtain an inductance of (say) 10 henries. 10 henries will have an impedance of about 3000 ohms at 50Hz and will take a current of about 80mA when connected to the 240V AC.

Then you need to decide if this will saturate the core and fry. You Used \$A_L\$ and a target inductance of 10 henries to tell you how many turns you need to wind then, you can calculate the magneto-motive force (ampere-turns). Then, divide MMF by the net length around your toriod to get H (magnetic field strength, ampere-turns per metre) and you are nearly there.

Referring to the BH curve in the toroid's data sheet and using the value of H just calculated determine what magnetic flux density (B) you will be getting from the graphs normally supplied - if it's more than about 0.4 Teslas then you will likely run into saturation problems.

I'm not doing the math but it's going to be a close run thing as to whether this toroid is big enough to tolerate mains voltage across the primary - ferrite toroids are not normally used as regular AC transformers - they are better suited to a much higher frequency (as per what you get in an off-line switcher) because of this issue.

If you don't know what the ferrite is made of forget it (and I mean that).

Winding the secondary is a cinch in comparison but before any advise is given, technical detail of the toroid material is needed.

  • \$\begingroup\$ The permeability of the toroid is 3000μ \$\endgroup\$
    – AvZ
    Commented Oct 3, 2014 at 13:26
  • 1
    \$\begingroup\$ Andy, actually toroidal power transformers are very common in certain applications, such as high-end audio gear, variacs and gun-style soldering irons. But the cores are usually coiled steel sheeting, not ferrite. As you say, getting enough primary inductance and saturation are going to be issues with ferrite. \$\endgroup\$
    – Dave Tweed
    Commented Oct 3, 2014 at 15:14
  • \$\begingroup\$ @DaveTweed I did mean ferrite toroids - I'll correct. \$\endgroup\$
    – Andy aka
    Commented Oct 3, 2014 at 18:06
  • \$\begingroup\$ @AvZ - realistically, without a data sheet this isn't going to be a workable solution - do you have a link to the data sheet? I'm not sure what 3000μ is meant to be - units? or is it meant to be relative permeability? \$\endgroup\$
    – Andy aka
    Commented Oct 3, 2014 at 18:09

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