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Blown Varistor But Mosfets Were Good

By on August 15, 2020
blown varistor in welding machine









A welding technician brought this tiny welding machine for repair. The cause for this dead welding machine was due to the fact it was powered by a power generator. A few seconds after the welding process started, something blew up inside the machine and it was dead.

how to repair welding machine

The welding technician knows that this machine cannot be powered by a power generator but the one who hired him to do that welding job insisted on doing that. In the process, the boss promised to buy the welder a new welding machine and if this machine is repaired then he will have an extra machine to work with.

I do not accept such repair cases because the damage is too much and usually I would waste my time trying to fix it and if it was fixable then it would be too expensive. I was willing to help this guy out because he is a good friend of my brother.

how to fix welding machine

It is a two board welding machine and I was expecting the worse when I took the cover off.  To my surprise I did not see any physical damage on board and that is a good sign.

blown varistor in welder

The only thing that I saw that was damaged and actually burned was the varistor near the capacitor area and that put a smile on my face. I think that varistor protected the machine from fully being damaged and that gave me some hope.

blown varistor photo

This is what was left of the damage varistor which shows the damage force that was imposed on this machine.

welder circuit board repair

To my surprise, none of the Mosfets were found shorted and when I say “to my surprise” because I have seen machines being powered by power generators and get damaged beyond any repair.

I replaced the damaged varistor with a new one and made sure that all was well with this machine; I put everything back together and tested the machine.

hobby 170e repaired

Power came one and that is a good indication that all was well. If the Thermal light was on then I would be worried but it was not and to that I could say…

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. Albert van Bemmelen

    August 15, 2020 at 7:01 pm

    The customer was lucky twice. For one thing because you decided to help him, and second for the fact that the damage was only minimal.

    • Waleed Rishmawi

      August 17, 2020 at 6:06 pm

      Albert: I appreciate your comment. have a blessed day


    August 15, 2020 at 7:37 pm

    I always enjoy reading your repairs. Can I ask how you decide the welder is OK. I presume you do not weld with it but measure the output voltage? The only control on the Front Panel is the 'current-control' is there anyway you can tell this is operating OK?
    Thanking you in anticipation and for educating the masses.

    Kind regards,

    Anthony Paterson

    • Waleed Rishmawi

      August 17, 2020 at 6:09 pm

      Anthony: I am glad to hear that. I know the welder is fine when I see the light coming on, voltage reading at the output terminals and I always test the machine for confirmation. sometimes it comes on and there are voltage readings at the output but sometimes with that present the welding does not take place because of voltage dropping. so I always test the machine to make sure all is well. have a blessed day

  3. Yogesh Panchal

    August 15, 2020 at 10:04 pm

    Good Job!

    • Waleed Rishmawi

      August 17, 2020 at 6:09 pm

      Yogesh: thanks man. have a blessed day

  4. Parasuraman S

    August 15, 2020 at 10:50 pm

    Yes, like Albert van Bemmelen said, once you say you would accept a job, the customer is indeed lucky! Many thanks for sharing such interesting cases!

    • Waleed Rishmawi

      August 17, 2020 at 6:10 pm

      Parasuraman: you are most welcome my friend. thanks for sharing your articles with us as well. have a blessed day

  5. Henrique J. G. Ulbrich

    August 16, 2020 at 2:45 am

    Thanks for sharing this different maintenance. I did not know anything from welding machines, just beginning to know these devices due to your maintenance articles. I find it very interesting. Thanks a lot.
    I find it very interesting that a power generator would not produce such a surge to blow the varistor. Surges generally come from the electric grid when there is a flashover.

    • Waleed Rishmawi

      August 17, 2020 at 6:13 pm

      Henrique: you are most welcome. the power generator is what caused the varistor to blow. it actually protected the machine from any further damage.have a blessed day

  6. ivan

    August 17, 2020 at 3:04 am

    Thanks sir

    • Waleed Rishmawi

      August 17, 2020 at 6:13 pm

      Ivan: you are most welcome. have a blessed day

  7. Ulises Aguilar pazzan i

    August 18, 2020 at 3:02 am

    good job

  8. Robert Lamparter

    August 19, 2020 at 10:27 am

    I really enjoy reading your reports on repair of welding machines. What causes generators to kill welders?

    • Neil

      December 13, 2021 at 4:19 am

      Great post, Gives me hope.
      How did you determine the parameters of the Varistor? Were the numbers still visible?
      I have a similar problem on a plasma cutter. Blown varistor, presumably saved the rest of components. Miller welders told me its a 580V 410J but didn't say anything else like ac dc or vrm. Could I use a slightly different varistor perchance?
      Any help or comment is greatly appreciated


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