Detecting weld start event based on time derivative of weld voltage

An apparatus for welding or cutting comprising:

• a power supply configured to provide weld power to an electrode tip extending from a torch to initiate an arc on a workpiece;
• and
• a controller coupled to the power supply and configured to perform:
• receiving voltage values indicative of a weld voltage provided to the electrode tip by the power supply;
• computing a time derivative of the voltage values to produce voltage derivative values that represent a contact resistance between the electrode tip and the workpiece;
• detecting a weld start by detecting a decrease in the voltage derivative values from above to below a time-derivative threshold, the time-derivative threshold being a predetermined value that is mapped to a corresponding predetermined value of contact resistance;
• and
• increasing the weld power supplied to the electrode tip to initiate the arc on the workpiece.