One of my old mig welders stopped working some time ago.  It is a Clarke Weld 100E.  I had previously fitted an internal fan to keep the transformer and rectifier cool.  The fault that developed, however, was to do with the wire feed, which stopped working.  The fault was the Thyristor (the 3 legged black thing with the metal tab which has a hole in it).  I replaced the Thyristor and the welder was working again.

One it was working again, I thought it would be useful to make the welder portable, by converting it to work from batteries as well as mains power.

The first problem to overcome was the wire feed controller that I had just repaired.  Unfortunately this controller is not capable of working off batteries since it’s design relies on AC power.  I designed a simple pulse width modulated motor speed controller to replace the original one.  The controller first regulates the voltage to 12 volts so that the speed of the motor is not affected when the arc strikes and the draws a large current.

I connected a couple of thick cables to the rectifier outputs and passed them through a grommet.  A couple of car battery connectors  were fastened to the other ends.

I connected up a couple of old batteries in series to give it a try.  If it works well, I plan on buying two new batteries and put it to some use.