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Shorted IGBT K50T60 and 1N4744A In………

By on December 6, 2019
k50t60 igbt shorted











My brother entered my shop carrying his old welding machine and explained that his machine sometime weld and most of the time does not. I have worked on that machine many times and from what I experienced, there are a lot of joints that are connected together with wires.

how to repair welding machine

The green board has to come off and check every soldering joints and connection and make sure all are solid.

how to fix welding machine

After taking the printed circuit I ended up with the big heat sink that has all these isolation pieces on.

shorted 1n4744a

As I was checking all the joints and connections, I came across many parts that are loose and when I checked with my multi meter, the diodes were shorted and some resistors were open.

I was shocked to see these bad parts and in my mind I knew that the main IGBT’s are shorted too because they are part of the same circuit.

shorted 1n4744a and repair

That is a view from the other side of the printed circuit and you can see the damage there as well.

In order to replace all the shorted part, the wires connected to that part have to come off too. I really had a hard time working on that machine but it was a challenge set before me to repair it.

how to fix and repair welding machine

More parts are bad and had to be replaced on this side as well. The yellow and the black wire are there to complete the connection between the IGBT and the main resistor.

k50t60 shorted and replaced

These were the total damaged parts: four diodes 1N4744A. A two 10 ohm resisters that were open and two K50T60 that were shorted out and all were replaced with the exact part numbers.

k50t60 datasheet

You may click on the above photo to check on the IGBT datasheet

1n4744a datasheet

You may click on the above photo to view the Zener Diode datasheet

Finally I got the machine up and running. I was getting the 25 volts at the negative and positive terminals of the machine.

I do not know what happened with this welding machine. According to my brother, it was welding on and off and when I opened it, it was shorted and completely dead.

welding machine repaired

And as we know by experience, getting voltages at these terminals is just an indication that the machine us up and running but that does not mean it will weld. The best is to try it out so my brother took it to his shop next door and gave it a try and it passed with high stars.

I gave him an advice to buy a new one because it could fail any minute due to the fact it was falling apart inside.

Mission Accomplished.

This article was prepared for you by Waleed Rishmawi, one of our ‘Master Authors’ and currently working in the Bethlehem area of Palestine repairing electrical and electronic equipment.

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Note: You can check his previous repair articles in the link below:




  1. Paris Azis

    December 6, 2019 at 4:34 pm

    Hi Waleed,

    What was the buying price of that transistor? I recently needed to replace a similar mosfet (50N60), but locally I found only the 47N60 for the price of 14 euros per piece and I lost my mind!

    Best Regards

    • Waleed Rishmawi

      December 7, 2019 at 2:39 pm

      Paris: I buy them from Ali Express for a cheap price. They are very hard to be found in my home town and if I find some, they would be so expensive. I have tested out these parts from Ali Express and found them to be fit for such repair. have a blessed day

      • Paris Azis

        December 7, 2019 at 7:06 pm

        Hi Waleed

        The relevant videos i have seen in YouTube about this parts' supplier are very disappointing! Anyway i have no personal experience since i never bought parts from there. If not found in our local market, i buy from England. At least it's the easiest way as long as that "Brexit" is delayed!
        I noticed in a photo above that the voltage of the output of the welder is only 25 volt. This voltage is theoretically too small even to start an ignition spark. Do you know if this voltage stays in this level at the stand by stage only, that is, just before the first welding attempt, and then goes to a higher level (50 or 60 volt or even higher)? If your brother has tested it, will you let me know the result? (Not about the output voltage of course. Only about the "sticking" of the welding electrode).

        • Waleed Rishmawi

          December 9, 2019 at 5:03 pm

          Paris: yes, I usually buy ten parts from any sell and when I am content with the quality of these parts I buy later on more from the same seller. if a seller ran out of these part (so far so good) I do the same process.
          about the 25 voltage reading, this is like a stand by voltages. once the welding process starts, the voltages usually go up and stay steady till the welding process is done.
          on the other hand, there are welding machine that actually give you the actual welding voltages and not a stand by voltages. have a blessed day

  2. Albert van Bemmelen

    December 6, 2019 at 4:45 pm

    I hope that the isolation pieces on the big heat sink were all okay because one looked a bit damaged. Since you worked on that machine many times before it has no doubt been a troublesome machine. Seeing the damage on the semiconductors this time I wonder how much was damaged the other times?

    • Waleed Rishmawi

      December 9, 2019 at 3:59 pm

      Albert: yes, the isolation piece is in good shape. it looked bad because the part was shorted on top of it. I always check these out in the final process when adding a new part and adding thermal paste to it. have a blessed day

  3. Parasuraman S

    December 6, 2019 at 7:01 pm

    Never ending welding machine failures, and there is one who will never give up on putting all of them back on track! Good job there, the only one unmatched!

    • Waleed Rishmawi

      December 7, 2019 at 2:40 pm

      Parasuraman: thanks man. you are such an encouragement to me. have a blessed day my friend.

  4. Justice

    December 10, 2019 at 9:12 pm

    Excellent article Walleed thank you.

    • Waleed Rishmawi

      December 12, 2019 at 2:30 am

      Justice: you are most welcome. Have a blessed day

  5. Yogesh Panchal

    December 21, 2019 at 3:49 pm

    Good JOB! Waleed a welding machine specialist.


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