I have a Lincoln 225 amp "tombstone" AC arc welder. I would like to add DC welding to it. Several websites say all I need to do is put a bridge diode in the output, turn it on, and weld. Looking at the drawing of the theoretical output, instead of a positive lobe and a negative lobe, you now have two positive lobes. The original problem this was intended to solve was the decrease and fall to zero between lobes. I need something to smooth out the output, so it never approaches zero. Theoretically, this produces a smoother weld, with less spatter.

The output at 225 amps is about 35 volts.

When I was playing with switching power supplies, using a capacitor smoothed out the output significantly. The bigger the µF, the smoother the output, to a point. Well, that was 12V and 5V, with, perhaps 20A. Finding capacitors to handle that was a snap.

Can you advise me what I should be considering? Do I need both a choke and a capacitor, just the choke, just the capacitor, and what values? This doesn't have to be aerospace precision, just DIY kludging. I have an industrial surplus yard near me, so BIG capacitors are available.

 

It would have to be pretty huge capacitor value, a heavy soft iron copper wound choke should suffice.
For the gauge of wire required, you may have to create one yourself.
Max.

 

Acme Industrial Surplus (Where Wiley Coyote shops!) has these capacitors bigger than a Fosters beer can. I don't know the voltage and amperage rating, but the bolts look like 10-24. As to self winding, I could take a piece of the welder cable to use for that. 5 AWG multistrand copper. Got lots of scrap steel, and a few iron bars. How many turns of wire? 1.5" bar OK?

I remember Max Headroom. Too bad the show was so short lived. Some of their predictions have come true.

 

1) What is `welder`?? You mean TIG or MIG or MAW or GMAW.
2) I have rather fresh experience with MIG (wire welder in CO2 atmosphere). For 0,6 wire probably my figures will be slightly largish, but I have 0,8-1,2 mm tract, rather large three-phase trafo with 6 diode bridge, 6 capacitors 50V 20 000 mkF each in parallel and choke on about 1,5 kW steel core (5x5 cm) with pencil size wire as much as possible on that core. Works well, up to 4 mm thick plates and from 0,35 mm plates. Capacitor is slightly too big for thin materials and hardly smallish for thickest materials. Coil is rather good however the pencil size wire is overheating thus I applied a PC fan to cool it down. As this machine is for heavy use and must be stable day by day, I vote for non-smps machines except the cases if they must be mobile to change from place to place.

 

1) What is `welder`?? You mean TIG or MIG or MAW or GMAW.
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"Tombstone" usually indicates a SMAW stick welder.
Max.

 

That's right, SMAW. 220V single phase. The thing I worry about the capacitors is melting the connectors. The welding cable is 5 AWG. If I was to parallel 3 of the Fosters beer can size capacitors, I'd still worry about the current damaging the connectors. I look at HVAC capacitors, and their connectors are a lot more robust.

An idea just hit me. If, instead of daisy chaining the wire, I'd T connect the wire, the current would never flow through the wire that would connect the capacitors to the system. The current would, instead, flow into, then out of the array.

 

Have you tried this without the caps or inductor yet? The reason I ask is years ago, late 1960's, I worked at a place that did what you're talking about, without caps or inductor, and it worked out pretty good. Although the welder was a better box to begin with. Stick welding isn't near as fussy as Tig or Mig. Before spending money or time try it first, a lot of what is theory is out weighed in practicality.

 

Tomorrow, I do my first welds with it. I have some 1/4" scrap. Will use 7018, with about 150 A. Hey. If it's smooth and easy, DC is moot. If not, well, hmmm. Materials for a change are about $70.