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A Metal Melter is a kind of home-made low voltage (less than 40 volts, mine is 3v) welder (small-scale) made from a microwave oven transformer (MOT). I left the primary intact (the thicker wire, which connects to house current), and replaced the secondary (the high voltage winding -- the much thinner wire) with 2.5 turns of 4 guage which I purchased from Walmart in the form of the thickest auto jumper cables I could get. It's pretty safe already because the jumper cable insulation is very thick, and it works great, but if it overheats, the insulation might melt, so...

I want to make it as safe as I can make it. I plan to add a fuse on the line side, ground the steel body of the transformer, add a fan, and also build a ventilated non-conductive enclosure for it.

In particular, I want advice on how best to line the secondary with robust insulation as described below:

The secondary is completely separate from the primary, in it's own slot, and I want to line the secondary slot with an approximately one-eighth inch thick non-melting insulator that would make it extremely safe. I was thinking of glass with Kapton on both sides to maintain its integrity even if it breaks or shatters. I could take a torch and bend the glass around the transformer corners before covering it with two layers of Kapton. So, is this good enough, and is there a better solution?

The transformer can get very hot, because of the low voltage (I'm thinking of increasing the windings and therefore voltage), so the insulating solution must withstand high temperatures due to accidental overuse of the transformer. That's why glass was my first thought. Thanks.

Here is a picture of one of my first tests with it.

Picture of Metal Melter with Sparks

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    \$\begingroup\$ If you want something safe, you buy the tool you need, and let the full weight of your country's product safety laws back you up. \$\endgroup\$ – Scott Seidman Jan 7 at 14:48
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    \$\begingroup\$ If you want to make a high-current secondary, you should be looking at copper sheeting, not jumper cables. A 1.5" wide strip of "16 oz." (22 mil thick) copper flashing (available in any hardware store) has the same cross-sectional area and current capacity as your AWG 4 wire, and is a lot easier to manipulate. Combine turns in series/parallel as needed. Use Kapton (polyimide) tape for insulation between turns. \$\endgroup\$ – Dave Tweed Jan 7 at 15:05
  • \$\begingroup\$ @DaveTweed - I was actually thinking of copper tubing, which I could then water-cool. But I think your way allows me to use multiple layers and significantly reduce the resistance, and therefore, the heat production. I might combine the strategies. \$\endgroup\$ – MicroservicesOnDDD Jan 7 at 17:18
  • \$\begingroup\$ @ScottSeidman - I eventually want to be putting those labels on MY products, certifying them, but I have to start learning somewhere. I still want advice on how best to do it. I've taken many things apart, and some do it better than others, and they're all labeled. As a manufacturer, how would I make it safe? And I've narrowed down the question to lining the secondary. Thank you, Scott. I appreciate your advice. \$\endgroup\$ – MicroservicesOnDDD Jan 7 at 17:30
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    \$\begingroup\$ You don't want to take anybody's word from the internet on this. Frankly, I'd start by hiring a product liability attorney. Another way to do this is to start by trying to get product liability insurance, and see what insurance will cost. Then, figure out how to insulate yourself so that when some idiot passes 20 amps through their heart, your personal finances can't be touched. \$\endgroup\$ – Scott Seidman Jan 7 at 17:55

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