I have a microwave oven transformer which I had damaged in the process of removing the high-voltage secondary. I will be using this for spot welding 18650 cells. I have all the required figures.

enter image description here

I wish to know if changing the wire gauge would affect the current output in the secondary; I need the maximum current output.

As far as I understand, the heat loss and resistance depends on the primary winding gauge and the current draw in the secondary depends on the width of the cemetery core and the gauge of the secondary winding, correct?

With that said, 22AWG would be a little less thereby creating more heat in the primary but would help adding more turns on the secondary thereby increasing current draw.

Link to MOT dimensions

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    \$\begingroup\$ 1 T flux density?! That's possible with silicon steel, but even there it'd typically pretty close to saturation \$\endgroup\$ Mar 21, 2017 at 10:56
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    \$\begingroup\$ the excel sheet was take from here ludens.cl/Electron/trafos/trafos.html \$\endgroup\$
    – George
    Mar 21, 2017 at 10:56
  • \$\begingroup\$ The transformer did not have anything on the spec to check the T Flux so that was the default value. Would it possible to determine the flux ? \$\endgroup\$
    – George
    Mar 21, 2017 at 10:57
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    \$\begingroup\$ @MarkoBuršič I have something like 0.3 T in my head for saturation of powder cores, and something like 1.5 T for high-quality steel cores, so that's where my remark comes from \$\endgroup\$ Mar 21, 2017 at 14:19
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    \$\begingroup\$ hmmm I could see one of my existing transformers secondary wave form to be a little distorted ? does that mean the core is saturating and dumb it ? \$\endgroup\$
    – George
    Mar 21, 2017 at 16:42

1 Answer 1


For the given core size, you have the nominal power. Compute the primary winding turns with respect on required flux density. The primary cross section is defined with primary nominal current. For secondary compute the the turns using ratio formula Ns/Np=Vs/Vp. Use secondary wire thickness with respect of nominal secondary current and/or check that the window is completely filled.

  • \$\begingroup\$ hmmm compute ???? Could you suggest a primary wire gauge and number of turns based on the pics attached in my post, i would really appreciate it. \$\endgroup\$
    – George
    Mar 21, 2017 at 16:43
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    \$\begingroup\$ At the time of the answer, there were no pics, just "I have damaged the secondary..." . Well you scarped all, you have a bare core. Do you have a bobin where you will put the winding? You should change your question in : I have a core...how can I calculate a transformer. \$\endgroup\$ Mar 21, 2017 at 17:11
  • \$\begingroup\$ Sorry if this thread was leading. I've change the heading now. Was the pics helpful to determine the wire gauge and the number of windings. \$\endgroup\$
    – George
    Mar 22, 2017 at 9:08
  • \$\begingroup\$ @George Is your primary winding still alive? Do you have a bobin? If not how do you plan to wind the transformer? Do you have insulation material? \$\endgroup\$ Mar 22, 2017 at 14:41
  • \$\begingroup\$ I have the damaged coil so with that size and comparing with the transformer core i've made a jig to wind the coil. I will be using this as a insulator ebay.com/itm/… and the wire ebay.com/itm/… \$\endgroup\$
    – George
    Mar 22, 2017 at 15:09

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