I am building a spot welder in order to combine 18650 battery cells into battery packs using nickel strips. I have viewed several online tutorials that describe the process of creating this tool using a transformer salvaged from a microwave.

One of the first steps of this project is to remove the secondary coil of the transformer and replace it with a six AWG gauge wire turned 2-3 times. During this process, I was using a rotary tool with a cutoff wheel to cut through the secondary coil when I accidentally nicked the primary coil. Please refer to the attached photos to see the damage. I tried to show a few different angles in the photos to give an idea of how deep the nick is.

My question is whether or not it is safe to continue to use this transformer with the nicked primary coil, or should I chalk this up to a learning experience, throw away the transformer, and start over with a new one?

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  • \$\begingroup\$ Just my opinion, but it might be repairable. See if you can scrape some of the enamel off, and solder a piece of copper wire over the cut section. Then try running the transformer at maximum current and make sure that spot doesn't get hotter than the rest of the primary. You would need to add a secondary winding to get the current. Then apply a suitable insulating enamel. \$\endgroup\$
    – PStechPaul
    Mar 23 at 2:03
  • 1
    \$\begingroup\$ It's gonna get hotter than the rest of the primary because part of the cross-section is cut through. But who cares, since it's an extremely low duty-cycle spot welder and the primary probably isn't running all that close to full power anyway. Just go ahead and put it in service and watch for discoloration. Determine which "end" of the winding that one is close to, and make that neutral. Also in the future, here's a tip: hand tools. Hard to do too much damage with hand tools. \$\endgroup\$ Mar 23 at 2:43
  • \$\begingroup\$ Also the enamel is cut through so this spot is no longer insulated. \$\endgroup\$
    – user253751
    Mar 23 at 2:44
  • \$\begingroup\$ what would happen if it failed catastrophically? I suppose the possibilities are that it melts and shorts everything and throws sparks everywhere and catches fire; or it touches the transformer core and energizes that (tripping breaker if grounded; creating an electrocution hazard if not. Even if grounded there is a risk your grounding isn't actually working how you think it is) \$\endgroup\$
    – user253751
    Mar 23 at 2:46
  • \$\begingroup\$ if catastrophic failures can be contained safely, they can be learning opportunities too - then you will know whether it was okay :) I have read of professors blowing up components in front of students so the students will be less afraid of it happening to them \$\endgroup\$
    – user253751
    Mar 23 at 2:46


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