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In this video a high-current transformer secondary coil is shorted with numerous thick pieces of metal - pieces of rather thick wire and a knife blade. All those pieces either get red-hot or just melt. The video lasts about five minutes. The transformer doesn't seem to have any kind of active cooling yet it doesn't melt and even its windings insulation doesn't burn.

Why doesn't the transformer get damaged when shorted?

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What you are observing here is the basis for resistance soldering.

Resistance soldering is the name applied to a technique where the heat to melt solder (or strip wire) is instantaneously generated by passing a high amerage electrical current through a resistive material. There are three key components of resistance soldering:

A specialized step-down transformer that will generate the appropriate current
A resistive material to generate the heat
The ability to complete an electrical circuit

The transformer and the leads coming out of it are copper which conducts very well. The items being placed across the copper leads are of materials that do not conduct as well as copper. That is why the various items tend to get very hot (and even melt) while the copper does not. I rather suspect that if they had placed a heavy copper bar across the leads the results might have been different.

Here is a paper Build Your Own Resistance Solderer which uses a battery charger.

Finally you may have seen one of those little "cold heat" soldering tools that run on a couple of AA batteries which came out a few years ago. They work on the same principle as described here.

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Summary:

  • May be within ratings if aggressively designed transformer

  • May be lower voltage than stated so higher current OK

  • May be fully fraudulent.

The transformer WILL be being heated. As long as the currents drawn are below the design current then the transformer will not be damaged. The transformer looks to be not more than say 400 VA in rating. He says 12V output but the current to heat the knife blade or thicker wires looks to be more than the 40 or so amps that a 400 VA 12V transformer would provide.

It may be that the transformer is more like 2V output - where a 400 VA transformer would provide 200A "safely" [tm].

And it MAY be that there is a 12V car battery hidden out of sight.
Why should or would there be?
I know not - BUT many people make many claims on the internet that are suspect or just plain untrue.

Maybe I have the Wattage wrong - if the designer pushed the iron hard and did not care about some core heating from magnetising current as the core went partially into saturation he MAY manage 1000 Watts, That's 12V at 80A+. Or 2V at say 500A :-).
The wire appears to be the winding wire - not good for 100's of amps in normal use.

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It depends on the ratio of resistance of the load (the thing doing the shorting) to the resistance of the secondary. Since the current thru both is the same, they will each be heated proprotional to their resistance. High current windings have deliberately low resistances for this reason.

This effect is the basis of a soldering gun. Open one up and you will see the secondary is a single loop of quite a thick conductor. This is why when you connect a few inches of #10 copper wire to it, the #10 wire gets a lot hotter than the pipe-looking conductor that is the secondary.

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