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I'm working on a device capable of soldering wires to tiny components (3mm brass rings). I've used a PSU of a computer to make a resistance soldering device. Using the 5V line and a graphite stick of 5 mm I'm able to generate enough heat for the process. The only disadvantage is that the PSU has a short-circuit protection and therefore shuts down when the soldering process is repeated.

Now I've bought a transformer 230V --> 6V rated at 50VAC to use as power supply. On the primary winding the current was limited by a power controller (KEMO M160) rated at 300W. The power controller was initially able to control the current. After a few test it didn't have any effect. I suspect it's broken now.

I suspect the power controller might not be the best way to control the current for this application.

My question is, what is a good way to limit the current drawn by the transformer, for the application of resistance soldering?

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    \$\begingroup\$ Do you mean soldering or welding The two are very different, but you seem to use the two words as though they are the same. Also, I've never heard of "resistance soldering" although "resistance welding" is a common expression. \$\endgroup\$
    – JRE
    Mar 4, 2019 at 11:43
  • \$\begingroup\$ I'm using it for the process of soldering. Edited the title. \$\endgroup\$
    – Piet
    Mar 4, 2019 at 12:15
  • \$\begingroup\$ Hello and welcome to electronics.stackexchange. Have you considered to sacrifice a transformer soldering iron for your purpose? Then, you would only need a relay to on-off. To come back to your question: to control primary wingdings of the transformer you would need a IGBT with some control electronics. \$\endgroup\$
    – smajli
    Mar 4, 2019 at 12:34
  • \$\begingroup\$ If you have the budget I would recommend a suitably (over-)sized Variable transformer (Variac) for the input side. These are tolerant of transformers as loads. \$\endgroup\$
    – KalleMP
    Mar 4, 2019 at 12:34

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If you have the budget I would recommend a suitably (over-)sized Variable transformer (Variac) for the input side. These are tolerant of transformers as loads.

If stepwise adjustment is acceptable then you could use resistors to reduce the available current. Either on the input (need to be aware of the mains voltage dangers and switching issues) or on the secondary.

If you have a number of larger 50-75W 12V automotive filament headlamp bulbs you could connect them in series and parallel combinations to provide durable limiting resistors.

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