Generally, electric arc welding entails providing current through a welding arc.  The heat of the arc melts metal that fuses together.  When welding with a wire feeder, the arc is formed between the tip of the wire and the workpiece.  If the wire touches the workpiece a short circuit forms, and the arc is extinguished. 
Some welding processes are performed best when there is not short circuits, others intentionally alternate between short circuits and arcs (short arc welding, e.g,), and others are performed best remaining in a short circuit (laser cladding welding, e.g.).  Generally, short circuits result in less heating of the workpiece, but the transition from a short to an arc results in spatter. 
The prior art has attempted to reduce spatter by reducing the current during, or just before, the transition from short to arc.  Because the transition occurs quickly, simply commanding the current to a lower level can result in spatter because the current isn't lowered quickly enough due to system inductance and the system response time.  Early attempts included changing the resistance using a switch in the current path. ( See.  e.g., U.S. Pat.  No.  5,001,326).  These attempts were largely unsuccessful because they still were not fast enough to reduce the current before the arc formed.  A significant improvement was predicting when the arc would form, based on the rate of change of output power (dp/dt).  This prediction provided enough advance time to overcome the lag time in the current command, thus the current was reduced before the arc formed.  This greatly reduced spatter.  The predictive technique is described in U.S. Pat.  No.  6,087,626, which is hereby incorporated by reference.  While the predictive control works well, it is a sophisticated control scheme, and not necessarily consistent with low cost welders. 
Another improvement was using mechanical control of the wire to create the arc.  The arc is formed when the wire is retracted (or the advance is slowed).  Thus, the transition to the arc occurs at a known time and the current is lowered prior to that time.  Alternatively, the current is lowered, and then the wire is retracted.  Because the current is low when the arc is formed, spatter is reduced.  This sort of system is described in U.S. Pat.  No.  6,984,806, hereby incorporated by reference.  While this system performs well it requires a wire feed motor close to the arc. 
Also, the prior art is limited by the system response time.  Whether the command is to change the current or to retract the wire it takes time for that command to be carried out.  In the event of an unexpected arc, or an unexpected change in arc length, the response time for the wire to retract or the current to be reduced can be too long, resulting in spatter. 
Welding processes that are best performed avoiding short circuits occasionally do have short circuits.  It is desirable to transition back to an arc without excess spatter.  The ways to control spatter in short circuit welding can be used in non-short circuit welding, but add to the cost and complexity.  It is not practical to pay for the cost of a reversible wire feed for the occasional short circuit, in most arc welding applications.  Likewise, it is often not practical to use the predictive control when shorts occur only occasionally. 
Another welding process is laser cladding welding.  Laser welding can be performed as a hybrid process where the laser provides the heat and wire is fed into the molten pool.  The wire is preferably resistively preheated.  A welding-type device can be used for this, where the output is intentionally short circuited to the workpiece (often called hot-wire applications).  However, if an inadvertent arc forms, it can create spatter or cause too much iron to become part of the weld (by melting too much wire).  Arcs can be avoided by lowering the output power, but the system response time limits the usefulness of this to avoid arcs and spatter. 
In all three of the above processes (desired all arc and no short, desired repeated arc-short-arc-short transitions, and desired all short and no arc) it is desirable to control the arc and the arc formation in such a way as to avoid spatter and to maintain the desired state (whether it be arc or short).  The control should be simple and fast, so that it can be used on a wide variety of type of welding type systems.  Accordingly, a welding type system is desired that provides arc control, preferably by rapidly changing the current level in the arc, using simple and effective circuitry