How does an AC stick welder current control work?

[systemloc]

I've looked everywhere for a proper schematic for anything similar to a Lincoln AC-225 tombstone. Can anyone explain how the current selection control works? Is it simply switching between more and less turns on the secondary winding?

I'm wondering, if this is the case, how this affects the no-load voltage. At low current settings, the voltage would be low and it would be more difficult to strike a spark.

The reason I'm asking is for info on building something similar to an AC-225 using microwave transformers ala Dansworkshop.com, but I don't care to play with SCR's for current control.

[KEB]

The current control is built into the transformer. As far as I know, there are two methods, one involving a variable reactance (inductor) and one varying the magnetic flux (coupling) between the primary and secondary windings.

The variable coupling approach is easier to understand...it's accomplished by moving a piece of metal in and out of the transformer. The open circuit voltage is determined by the ratio of the primary and secondary windings, and the current is controlled by how much magnetic coupling there is between winding. Variable reactance transformers use multiple taps on an additional (I think) winding on the transformer to control coupling.

I don't have all the design data in front of me to look at this in detail, but basically these are special transformers. That's why some other sort of current control, i.e, SCRs, is required with other types of transformers.

I understand that microwave transformers are current limited, not constant current. In this case, the current limiting is accomplished by shunting some of the magnetic coupling between the primary and secondary windings.

If you're looking for a working welder, try Craigslist. AC buzzbox type welders routinely show up in this area for less than $100, and I've seen a few in the $50 range. If you're looking for a fun project, hope this info helps.

Keith