AC to DC conversion
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  1. #1

    AC to DC conversion

    What is involved in converting an AC stick welder into an AC/DC stick welder?

    The welder in question is rated at 30-200 amps AC. I would like to have the ability to weld with DC as well as AC. Any suggestions?

  2. #2
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    Re: AC to DC conversion

    Quote Originally Posted by beaualex View Post
    What is involved in converting an AC stick welder into an AC/DC stick welder?

    The welder in question is rated at 30-200 amps AC. I would like to have the ability to weld with DC as well as AC. Any suggestions?
    You can make a bridge rectifier with four large diodes. It won't give a really pure DC, but it's better than nothing.

  3. #3
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    Re: AC to DC conversion

    Quote Originally Posted by beaualex View Post
    What is involved in converting an AC stick welder into an AC/DC stick welder?

    The welder in question is rated at 30-200 amps AC. I would like to have the ability to weld with DC as well as AC. Any suggestions?
    Sell it and buy a DC machine.

  4. #4
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    Re: AC to DC conversion

    Hello beaualex, unless you're just interested in doing something on your own and have the time and resources, buying a DC ready machine makes more sense. rlitman gave you some of the mechanics of making this happen, but the reality says trading off or selling your AC machine and buying a DC or AC/DC capable one is probably much simpler and likely safer. My $.02, good luck and best regards, Allan
    aevald

  5. #5
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    Re: AC to DC conversion

    It can be done, but can it be done well? Years ago my brother did this and it worked, but it was an experimental, temporary setup. The solution was to buy a DC welder.

    Here's a link to another post on the same topic:

    http://weldingweb.com/showthread.php?t=41119

    Jason
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  6. #6
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    Re: AC to DC conversion

    It ain't that hard. I added a circuit in my Lincoln PowerArc4000 to give me DC. Run some heavy wire from the AC outputs (inside the case) and bring it to an aluminum heat sink (you need 2 layers separated by insulated stand-offs. Add some big-*** diodes from Ebay to make the full wave bridge. Run the rectified DC wire (either the neg or the positive, but not both) with some wraps around a nice hunk of steel to make an inductor and smooth out the arc and then get both + and - to some plug-in lugs on the outside of the box. That way you can have your choice of AC or DC (electrode positive or negative) just by plugging the stinger and work cable into the appropriate plugs. This took me about a day to do everything. Of course, when you rectify AC to DC you will lose some amps, but it will get a nice smooth arc with less spatter. Mine has been working just fine for years.
    Here is an article that might give you some hints:
    http://forum.doityourself.com/weldin...dc-welder.html
    Doug
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  7. #7
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    Re: AC to DC conversion

    Take a look at the "wiring diagram" (the manufacturers generally don't provide complete schematics) for a basic transformer-type DC or AC/DC machine and you see that they basically do what is proposed by conversion hobbyists. I'm looking at the wiring diagram of the output end of the Miller Dialarc250 that I happened to have stored in my computer. I see a diode bridge with capacitors paralleling each diode for voltage spike protection, a varistor and a resistor paralleling the output of the bridge, also for voltage spike control, and an inductor in series with the positive output lead for arc stabilization. No data on values. Additionally, the "wiring diagram" may be a simplified diagram but is probably complete for this section of the welder.

    Sure you have to be sure to use adequate heat sinks and select diodes with adequate voltage and current ratings and get an inductor with adequate current rating and inductance, but none of it is rocket science. Note that it greatly simplifies packaging and mounting your home-brew rectifier if you obtain two forward and two reverse diodes of the same family. The reverse diodes are usually recognizable by an "R" in the type number but test them. Using these pairs you can mount two forward diodes on one heat sink and two reverse diodes on a second heat sink without using insulating hardware on the diode stud. Then you can mount the heat sinks on insulating hardware and take the rectified current directly off the heat sink.

    Regarding paralleling diodes which RickV says in the cited earlier thread that the manufacturers do all the time - they do it to save money by using cheaper diodes. The cost tradeoff is that they either buy presorted diodes from the manufacturer or sort large quantities of diodes in house into batches by forward voltage under a variety of loads and temperatures so the current sharing of paralleled diodes will be relatively equal. In the absence of this sorting or selection on your work bench it is a pure gamble whether your pair of diodes will work or blow under high current. In fact, manufacturers often parallel diodes inside the large packages of high current diodes but in this case the individual diodes are almost perfectly matched because they are processed on the same semiconductor wafer. In my opinion, for the hobbyist it's better to obtain adequately rated single diodes than to try to match inadequate diodes.

    But I agree that you have to want to do a project of this type to make it worthwhile. Depends a lot on what your time is worth. In my case, I'm mostly retired so time is not a major issue and I have one of the largest junk collections in the area, including a bucket of large stud diodes and some high current inductors. I have two inverter DC machines but still intend to add a rectifier to a huge USAF surplus AC stick welder that I've been using for 30 years or so. Any day now.....

    awright

  8. #8
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    Re: AC to DC conversion

    . . . and take it one step further from Doug's modification description and add 100-120,000 uF or capacitors after the rectifier to stabilize the output voltage and make it even better yet.
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  9. #9
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    Re: AC to DC conversion

    For what good used DC machines are selling for right now, you could prolly almost pick up a decent Miller Dialarc 250 AC/DC for what your purchase of the materials (retail) & TIME putting it together will cost you.

    See them go for under a few hundred if you look. Even used inverters like the the Miller Maxtron 450 go for under $700 if you look around.

    Unless you're building it simply for the experience & not practicality, you'll end up with something that has literally no resale value & will prolly perform to a lesser standard than a purchased unit will.

    IMHO of course.
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  10. #10
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    Re: AC to DC conversion

    Whats your location? I have an old "missing link".

  11. #11
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    Re: AC to DC conversion

    Whose location and what missing link?

    awright

  12. #12
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    Re: AC to DC conversion

    The OP.

  13. #13
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    Re: AC to DC conversion

    I did this to a 230A buzz box a long time ago (and also made a DIY HF circuit, which eventually got used for TIG). There's a reason I ended up buying a Dynasty 200 DX, BUT, if you're really wanting to do this, I just found this, and it's pretty much everything you need.
    It's got your insulated mounting base, it's already set up as a rectifier so you just need to hook your wires up to the screw terminals and go:

    http://cgi.ebay.com/ebaymotors/HIGH-...Q5fAccessories

    It is set up as a three phase rectifier, but if you look at the schematic, if you just hook your A/C up to any two of the three phase terminals ignoring the third, and your DC to the DC terminals, it is the same as a single phase rectifier.
    That's got WAY more than enough current and reverse voltage capacity for your needs, and you won't need to worry about paralleling diodes (which is a very bad idea).

    Your other option (which is what I did at the time, because I got all four diodes for under $30) is to get four stud mount diodes like this:
    http://cgi.ebay.com/ebaymotors/300A-...item3a5ba23289
    and mount them to some copper bus bar (1/4" aluminum should work too, but you'd need to deal with oxidation at the connection) (oh, and did I mention its a total pain in the a$$ to drill a 1" hole in 1/4" copper), and then mount them to an insulating base (I used wood, but hdpe or bakelite would be safer), and make your own screw terminals (couple more bucks for stainless screws and nuts), and then house the whole thing in a shoebox sized metal enclosure that can't short the terminals, and offers adequate ventilation.
    But once you've finished that, and discovered not only how much money you spent on the project, but also how much time, you'll be very disappointed when you compare the results to someone else's chinese inverter welder, that runs rod SO much more smoothly (and draws less current too).

    Oh, and if you're shopping for diodes to rectify A/C, you'll need something that is rated for just about double the A/C voltage in reverse (technically around 1.4 times the nominal A/C voltage, but 2 times is safer), and at least half the current rating of your welder (YES, ONLY half the current, because in a single phase bridge, only half the current flows on each side, and truth be told, current ratings for diodes are really sketchy at best. Diodes can safely run more than 10 times their current rating for very short durations. The current rating is for continuous use, and has to do with heat dissipation) at a minimum.
    Last edited by rlitman; 11-16-2010 at 11:18 PM.

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