I am confused about the use of a step-down transformer in a welding machine. The transformer steps down the voltage, let's say from 220V on the primary to 12V on the secondary. It is said that it increases the current significantly which melts the metal pieces when welding them, but I am not able to understand that.

My understanding is that the transformer only affects the voltage and the current drawn is dependent on the resistance in the secondary circuit, so the current should be very low given high resistance on the secondary side, not very high as described everywhere because the secondary side should follow Ohm's law. I know this is a misunderstanding of concepts on my part. Please help me with the correct analysis of the situation.

  • \$\begingroup\$ Why do you think there is high resistance on secondary? Conductivity of iron is pretty good. \$\endgroup\$ Commented Dec 8, 2023 at 16:19
  • \$\begingroup\$ You mean a 12 volt supply can generate 50-60 amps of current required to melt the iron then why not a 12 volt battery melt the iron piece and weld them? \$\endgroup\$
    – Arsalan
    Commented Dec 8, 2023 at 16:42
  • \$\begingroup\$ The little energy gaps in the AC current from the transformer help to disconnect or move the welding tool. Also the inductance of the transformer helps to establish contact with HV arcing during slack joints. \$\endgroup\$
    – Jens
    Commented Dec 8, 2023 at 17:45

1 Answer 1


The current in the secondary is dictated by the impedance of the secondary. The current in the primary is set by the current in the secondary, divided by the transformer ratio.

So in a step-down transformer, the primary current is much lower than it would be if the load was just connected across the input voltage directly.

12 V batteries aren't used for this because they would drain very quickly. You'd have to swap batteries every few minutes while welding, and have a bank of a few dozen to cycle between as the others charge. A mains-powered transformer can provide power continuously, limited only by your ability to pay the power bills. And it costs less than tons of car batteries.


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