(Arc welding) Why does voltage drop when current is increased? I hope I'm asking in the right place!
I worked with automated TIG (GTAW) welding equipment. I'm confused about how the welding power supply controls the current applied through the work piece. If the entire circuit was a simple static load, with a known resistance, it makes sense that voltage across the load could be controlled to keep the current constant. If I want more current, I increase the potential.
In this case, the arc voltage will drop with increasing current. This can be seen clearly on a diagram of voltage vs. amperage for a constant current welding power supply and is consistent with my own findings. If, during the middle of a weld, I command the machine to produce more current, will the machine increase the potential across the output terminals? I have a suspicion that the dynamic nature of the arc welding aspect of this comes into play. I.E. the conductive plasma region becomes larger once it reaches a steady state at a higher current?
Please let me know if my question makes any sense.
 A: Arcs display what is sometimes called negative resistance- which is where the voltage across the arc decreases with increasing current, and whether or not you experience it in a welding rig depends on the design of its power supply. Here is what is going on:
Imagine we establish an arc in air and limit the current through it with either a clever power supply or a series resistor. This limits the amount of air that can be maintained in an ionized state within the arc volume, and the size of the arc is stable.
Now imagine we want to push more current through the arc, so we dial up more voltage. This makes the current rise and as it does, more air gets ionized- and the arc's cross-sectional area increases. This area increase reduces the electrical resistance of the arc, allowing more current to flow, and as it does, the voltage across the arc goes down- because its resistance has gone down.
This picture will get more complicated if the arc welder's power supply is designed to deliver constant current, constant voltage, or constant power, but the basic fact to remember is this: pushing more current through an arc causes its electrical resistance to go down.
