Converting an AC Welder to give DC


Welding can be done with AC but for better welds and for work on thinner sheets of metal, DC is needed.  Lincoln sells a inexpensive AC arc welder that can handle as much as 225 amps of current at about a dollar per amp.  A DC welder of this current can cost twice as much, and a Mig welder of this current is way more.  I decided to buy the cheap Buzz Box and then convert it to do what I want.  First I built this DC arc conversion.  Next I will build a Mig and Tig extension and then I will have a fairly complete welding system for very little money.

I acquired these four 300-amp,200-volt diodes from Ebay for $7.00 apiece.  They originally sold for $90.00 each.  You can get this stuff cheap and there.  The old transformer will be used as the choke coil (see circuit diagram).

Close up of the diode, these suckers are Huge!  The measure in at 3 inches across.

The heat sinks for the diodes and the cooling fans in the back ground.

The heat sinks are prepared.

The terminals and heat-sinks must be connected such that the heavy current (as much as 225 amps) can flow unrestricted, and so that cooling is possible in both air or in a oil bath, and so that the whole is structurally sound.  To insure all of this I used both aluminum and copper strapping scavenged from the transformer shown earlier. Notice how the aluminum is wrapped with the copper.

Below is a diagram of the full wave bridge.  Again, while the circuitry is simple, the actual physical construction is challenging due to the heat dissipation requirements and the current carrying requirements.  

And now some pictures of the completed air cooled rectifier in my welding cart.

  

A view of the completed air cooled full wave bridge mounted on its nylon header.  The header is made from a nylon kitchen cutting board.  The box is an old main-frame computer power supply box.

 

A closer view.

A view of the choke coil and cooling fans.  The choke is 10 turns of 2 separate stands of heavy gauge copper.  The orange wire is 400 amp rated flexible cable.  You can just see the corner of the Buzz box welder that provides the AC.

And there is the final output.  It may not be pretty, but it works great.  Next step is to build a simple polarity switch.

 

...And, what can you do with a DC arc welder?  Anything, and I am going to build a Mig and Tig with it as well.  For now I updated my bikes handlebars from the stock buck-horns to a set of lane-splitter Z-bars.  I got the Z-bars off of E-bay for 20 buck, but they were too narrow for my taste, I like a bit more leverage on the steering.  So I used the DC welder to weld on 3 inches of 1 inch OD pipe to each end of the Z-Bars where the hand-grip would hide the welds.  The arc welder worked great and I got a nice weld on both sides.  You can see the final product here (please disregard the mess of my garage, kids bikes and stuff)