Dr. Livingstone, I Presume WELDWISE 2400 - Manual (Page 15)

0428-INS-400 Rev. E
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size of the loss if they've mushroomed 2, 3, even 4 millimeters! A constant current control or a weld
stepper may be used to regulate the amount of current used, but a controller or stepper does not track
the change in surface area. So, even though the current is regulated, the current density is overlooked.
Unfortunately, inadequate current density usually produces inadequate welds. Following proper
preventive maintenance schedules can help ensure sufficient current density by ensuring that the
electrodes remain in good condition.
As proven in the example above, it is crucial to have the proper current density at the area where the
weld is to be made. Depending on the materials being welded, however, 'proper' current density is
actually a range, rather than one specific amount. Welding engineers call this range the weld lobe. Each
parameter involved in making the weld (current, voltage, resistance, etc.) has its own range, or lobe.
Quality welds are made when the weld process stays within the lobe. The next chapter will discuss
weld lobes and tolerancing, which is a way to ensure that the weld process does not fall outside of the
lobe.
Ohm's Law and Joule's Laws
The following laws are widely thought to be what make or break resistance welding. While it is true
that these laws are very important to resistance welding, there are a few details that should be clarified.
Ohm's Law states that V (Voltage) = I (Current) x R (Resistance).
What does this mean in real-world terms? Returning to the pipe example, the more water pressure
there is in a pipe (more voltage), the more water can flow through that pipe (more current). If the size
of the pipe decreases (more resistance), then the water flow will decrease (less current) but the pressure
drop along the pipe will increase (more voltage).
Joule's Law states that H (Heat) = I (Current) x V (Voltage) x T (Time the current is allowed to flow).
Or, written differently,
H (Heat) = I
2 (Current squared) x R (Resistance) x T (Time the current is allowed to flow).
Note: V (Voltage) = I (Current) x R (Resistance), so the two equations are the same, just stated
differently. The second version of this law is probably more common in the field.
Joule’s Law is an equation that gives the amount of heat (energy) delivered to something. It would
seem sensible to assume that it's the amount of heat delivered to the weld. However, it is important to
consider all the factors in the equation: Current, Voltage, and Time. Joule's Law assumes that each of
these factors remains constant in the secondary of the welding transformer. A weld controller or weld
timer may indeed provide a constant amount of current at the electrodes, but recall Ohm's Law:
Voltage equals Current times Resistance, or written differently, Current equals Voltage divided by
Resistance. Factors like pitting or mushrooming of the electrodes, dirty workpieces, changes in force,
etc. all have an effect on the surface area (the area of contact) between the electrode and the workpiece.
Since changes in the surface area affect the contact resistance (resistance of the surface area), it is
reasonable to say that the resistance at the workpiece is not constant, but rather a factor that can change
depending on a number of other conditions. If Resistance is not constant, then according to Ohm's