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DC conversion for AC welder using a Bridge Rectifier & Capacitor or Inductor

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I am working on a simple conversion for my AC stick welder, to add DC welding capability to it (instead of paying an extra $400 for a pre-built one).

I have most of the wiring worked out, but need help properly sizing the smoothing capacitor.

The design is simple, 48-53V AC (single phase) @ 30-225A (selectable) runs through a 300A bridge rectifier to produce a "bumpy" DC waveform. I need to smooth out the wave to a ripple, so a capacitor needs to be added.
example: http://www.antonine-education.co.uk/physics_a2/options/module_9/Topic_3/capacitative_smoothing.htm

So here's my dilemma, I have no clue how to properly size the capacitor to this application.
Any help would be greatly appreciated.
Thanks ;)

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Capacitors are better at smoothing lower currents, especially at higher voltages.
You'll need a huge capacitor to smooth 225A to say 5V ripple, think somewhere in the region of 0.5F.

Yup, that could get expensive real quick :-\
They do sell 1.0F capacitors for audio systems, but I'm not so sure the voltage/amperage I'm looking to use would match properly. I think I read that they are rated around 24 volts, I'm looking at using about twice that.

Inductors are better at smoothing higher currents.

Sounds like a better match.
Plus, I'm guessing I could build one myself from scratch.

Any suggestions on wire size, coil size, amount of coils, size of core, etc?
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TO be honest I've never actually done this before so I wouldn't be able to tell you exactly how many turns wound over what core, I'm remembering a suggestion made my someone on another forum I no longer post at. All I can say is, get a thick bar of ferrous metal and wrap as many turns of thick welding cable round it as you can.

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What makes you think that you must smooth out the ripple on the full-wave rectified waveform coming out of the rectifier bridge for welding?  Why would a smooth DC voltage/current work better than the raw rectified wave?  The heating value of the arc will not be significantly affected.

Now, there IS good reason to provide an inductor on the output, but it is generally referred to as an arc stabilizer because it helps maintain the arc across the nulls of the rectified waveform.  It also affects the handling characteristics of the arc, but not, I believe, because the DC is smoother (except that the nulls are rounded out).

Since you are not necessarily trying to "smooth" the ripple out, conventional L-C filter design concepts are not particularly applicable.

Wire size should be selected for the highest average (actually, true RMS of the AC+DC ) current you expect to use over any, say, ten minute period, taking into consideration the fact that you are dealing with a coil, not a straight wire which would have better heat dissipation.

You may want to provide an air gap in the magnetic core to avoid magnetic saturation of the core but I'm not sure that is really necessary.  An "air gap" is normally provided by inserting a fiberboard shim between the "E" stack and the "I" stack.

I think your best bet is to look at the inductors in commercial welders of the same current capability that you want and try to roughly copy the inductor core size, wire size, and coil size that you see.  I don't believe the inductor value is particularly critical to the quality of the arc or the resulting weld.


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  • 10 years later...
  • 8 months later...
On 5/15/2021 at 6:57 PM, Csgati said:

"Use a full wave bridge rectifier say 300 amps and a large inductor. Caps make it pop and splatter. Inductors make it smooth."

Ok, I know this was bumped last year from 2010 but I gotta bump it again because there's so much wrong info. 

Capacitors smooth voltage. Inductors smooth current. 

Both are needed for better dc welding and are always used on every iron core transformer based welders other than the cheapest models, or ac only welders (usually the cheapest models lol). ALWAYS.

Pretty sure they are also included on other welder types (inverters, other tech) too. The whole point is for fine tuning of voltage and current, and that is impossible to do without those two components.





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