Battery Tab Welder

[reckerd]

I have found different ideas online and much seems to contradict each other. I would like to ask if anyone has successfully built a capacitor battery tab welder. I would like to be able to vary the charging voltages for the varying thicknesses of metal needed. Also, can anyone tell me why some use capacitors in series and others in parallel? And lastly, are SCR's of the same current rating about equal in other specs?

[jwax]

Welcome, reckerd!
Have a look at: http://www.5bears.com/welder.htm
I haven't built it (yet), but it appears practical, and pretty cheap.
It controls power to the weld by varying the "time-on" of current.
A capacitive discharge circuit will give more control over energy to the weld, but more complex circuitry.
Cap energy is changed by building the bank in all sorts of combinations of capacitance and voltages- series and parallel mixing is common to match what the designer is going for.
Also, check ebay for Unitek welders- great gear!
John

[L. Daniel Rosa]

And lastly, are SCR's of the same current rating about equal in other specs?

No. First off there is the voltage rating, of which several are available for nearly any package. Then there is the power dissipation capability which is often consistent to the package and current rating, but not always. There is also the seemingly minor detail as to whether the tab is electrically isolated from the SCR, which could have some impact on the dissipation.

There are also deeper ratings that can become expensive. The dV/dt and dI/dt have to do with the maximum power frequency that can be controlled and what types of loads will cause mischief. The I-squared-t that is _very_ significant to high power impulses like spot welding or rail guns.