Diode question-referred here by a welding web member.
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  1. #1
    fronty_78 is offline Plastic
    Join Date
    Jun 2011
    Ottsville, PA

    Default Diode question-referred here by a welding web member.

    Hello to all,
    This is my first post here. Machining is extremely interesting to me, I find it daunting, trying to figure out where to begin to learn the skill. I hope to someday start on the quest. My question for this evening is not about machining (just a heads up.)

    It is this:
    I recently acquired two Lincoln idealarc 250s that my school was going to scrap. Both machines only weld AC. To try to find the best way to solve this problem I started a thread over at Weldingweb.com. I was referred here for more insight.

    It seems that the older style diodes go bad on these machine often. There is an existing thread that shows how the owner of another idealarc replaced the older style diodes with a more current version. He used 150A 600V diodes to fix his machine. I am wondering if anyone here has ever modified an idealarc like this by chance. If so, can you walk me through it. If no one has, can someone tell give me a part number for the correct diodes to be used. And still if not, can someone point me somewhere that I might find more information?

    Thank you for your time,
    Marc Leonard

  2. #2
    MichaelP is online now Titanium
    Join Date
    Dec 2008
    IL/WI border


    Send e-mail to PM user Macona. He is a specialist in welder repairs and a very nice guy, in general.

  3. #3
    Heavey Metal is offline Titanium
    Join Date
    May 2011


    First you need to verify that it is a diode and not just a connection.

    When a diode goes out usually it trips the breaker. (it goes to ground)

    Do you have the schematic?

  4. #4
    magneticanomaly is offline Stainless
    Join Date
    Mar 2007
    On Elk Mountain, West Virginia, USA


    Lincoln had been VERY helpful to me in troubleshooting and documenting very old Lincoln machines.

    They would be my first stop.

  5. #5
    JoeE. is offline Titanium
    Join Date
    Aug 2006


    I may be missing something here, but he says this is an AC only machine.

    Diodes are used to convert AC to DC.

    If I missed something, like maybe this could be the reason they only weld AC.............never mind!

    Call Larry @Weldmart online and get diodes...............

  6. #6
    USMCPOP is offline Titanium
    Join Date
    Jul 2005
    Midlothian, Virginia


    If these are very old AC/DC machines, they may have selenium rectifiers, which are not as reliable as silicon rectifiers. They can be replaced with silicon diodes if the selenium rectifiers are shot.

    Even if they are AC-only machines, you can make them DC, but you'd want to add an inductor to get the right welding characteristics. Then you'd need a means of switching from AC to DC.

    Download the appropriate manual from Lincoln and check the schematic to see what you have.

  7. #7
    eKretz is offline Titanium
    Join Date
    Mar 2005
    Northwest Indiana


    I second the recommendation to PM Macona, he used to run a welder repair shop and has shown several times in the past that he knows his way around a welding machine's guts.

  8. #8
    millalot is online now Plastic
    Join Date
    Apr 2007

    Smile Idealarc 250S

    This is an AC/DC machine. So the diodes or associated circuit is faulty. If you have no ac output at all then all diodes are faulty, but this is unusual for all 4 of them to fail,??
    Your 150Amp 600V diodes will do the job as this is a bridge circuit they are operating at a 50% duty cycle.
    You should check the condition of the change over switch for burnt out contacts.
    Parts can be scourced here.
    Lincoln Idealarc 250 AC/DC Welder Replacement Parts - U.S. Welding & Safety Supply Co.
    Licoln part Nos are listed.
    You might obtain cheaper ones on the internet search but prices from this company seem fair .

  9. #9
    Pipes is offline Cast Iron
    Join Date
    Feb 2007
    Rhode Island, USA

    Default Get manual

    Definetly get the manual and please be very, very, very carefull if you undertake this repair yourself. There are lethal voltages in many areas of these machines.

    Lincoln has a great library of manuals. I was able to get documentation dated 1947 for a machine I was repairing.

  10. #10
    hendryfinn is offline Banned
    Join Date
    Nov 2011

    Smile Diode

    Yeah, the diodes can make the difference based on the characteristics... I got some idea with your thought... I'm looking for more information to hand the help to you.... Thanks for the innovative idea...!


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