Saurav
Last Activity: 7 Years ago
The reason is a combination of resistance and available power. You're absolutely correct that power is current squared times resistance or voltage squared over resistance, but think about the resistance of what you're welding - it's highly conductive, and you have a large contact area (the ground clamp, that is), so we're talking a few ohms at worst. If I apply a large voltage to that, according to V=IR, it will attempt to pass a large current as well, which means either I've got a huge voltage and a huge current (which means waaaaay too much power) or, far more likely, I'll hit the current limit on my power supply and the voltage will drop anyway. Since the resistance is small, though, we don't need much voltage to overcome the resistance of the surface being welded, and can pass a huge current with a comparatively small voltage, which step-down transformers are perfect for obtaining. The precise opposite logic is used when doing high-voltage power lines - there, we want to dissipate the least amount of power, so we jack the voltage up to several kilovolts at very low currents. Hope this is useful. Best way I've found to think about it is try to think through the consequences of what would happen if you did use a high-voltage power supply.
