Semi-automatic spot welder 2,6V 1kA

This welder, thanks to the limit switch, after lowering the upper electrode arm, automatically enables the welding process in a safe and proper way. First, it waits 1 second so that the user have time to clamp electrodes on welded material, and then turns on the weld current for time in the range of 0 – 4 seconds, which is set by the potentiometer. This allows both hands to be free, and there’s no need to use the foot switch.

Transformer comes from a Technics SU-V670 amplifier. Core cross-sectional area is 23.5 cm ². It seems small, but it has a considerable power for such size. Even before rewinding, short-circuit secondary winding, caused to knock out the B10 fuse. Now, the secondary winding measures almost 2 meters of LGY 35mm ² link (10mm thick with insulation). Window dimensions 46.5 mm x 9.5 mm allowed to fit four turns of wire. At 660mV at turn, 4 turns gave 2.6 V, with 950-1000A of short circuit current. This transformer in compare to a microwave oven transformer, has the advantage that it can be permanently supplied from mains, hence there was no need for installing a cooling fan, because all of it can warm up, is the heat from the electrodes and the secondary cable. Numerous holes in the casing provide passive cooling.

The transformer and electronics circuit casing is made of the ATX power supply casing components, welded together. You could say that the welder weld itself together :) Of course at the beginning was no mounting arms or timer, so welding was done by hand and without the time control, but it came out pretty well. At the ends of the secondary cables, a 35mm ² copper ring connectors are soldered. The arms are aluminum 2cm x 2cm angle profiles, they play a very important role in the dissipation of heat from the electrodes – this allows you to perform more welds at a given time. For the electrodes, i have used DY 10mm ² copper wires (approximately 3.5 mm in diameter). They are squeezed between the arms and connectors with 10mm bolt, but run through a 3.5 mm hole made in this bolts. This direct connection provides a very good electrical contact and heat dissipation. Also, without any problem, the electrodes can be attached at any angle, at any length to get into various corners of welded components. Method of attachment prevents the electrodes shift even under strong pressure. Arms working length is 24cm.

Limit switch is activated by a finger attached to the upper arm of the electrode, which starts the timer. Timer is a simple circuit based on 4 NAND gates contained in the structure of 4093 logic chip – LINK. The first timer is set at constant 1 second. After this time, unlocks the second timer, which has a potentiometer for adjusting the time of welding, in the RC circuit. Output of the last gate enables two NPN transistors (its output is only 0.5 mA) – why not one PNP? Because as soon as the button is pressed, the output of the last gate has a momentary voltage drop to 8V, and captured by the transistor, has been passed on further. Such phenomenon does not exist when NPN is used, and due to the fact that it inverts the signal, i had to apply one more NPN to invert it again to its original state. The rest also wasn’t bug free, the timer worked flawlessly in the CircuitMaker simulation, but in reality there have been problems and I had to make a small changes in the circuit, which in turn, did not work in the simulation. I insisted to make it without any uC and i did, also, uC can be very unstable in such harsh conditions (1kA inductance). CircuitMaker simulation file and a working circuit schematic are in the attachment. Further, the latest NPN transistor switches zero-cross detection optotriac MOC3041, and the triac BT138 which is closing the primary circuit. Timer is continuously fed from a small 10V transformer placed in the corner of the casing, with 14V after rectification. I thought that BT138 (12A) is too weak for this job, but i made a lot of testing welds and its still alive. But please use stronger one if you have.

Please wear at least a safety glasses when working with such devices. Even if the current is switched on and off when the electrodes are pressed to the material, there is always a chance that a piece of molten metal will spatter in some direction, and it is only a matter of time when it will choose our eye as his direction. Another thing – in this case, the casing is made of a metal and has been grounded along with the transformer and electrode arms – so if any mains inside touch to the casing, home installation protection will go on and user wont be electrocuted. Triac with such power requires a small heatsink, and if the case is metal, you should carefully consider its mounting. In this case i have applied a double insulation. First, triac isolated from the heatsink with a silicon pad, then the heatsink is attached to the casing with a plastic spacers. Please remember to use only a grounded outlet for your own safety, as you never know when something bad can happen.

Welder easily handles with sheets 2 x 0.75 mm – weld can’t be ripped apart, instead, sheet is ripped right next to the weld. It can’t do 2 x 1mm, weld is braking when ripped, but it can be used for welding some light duty parts. Perhaps if the electrodes were thicker and pointed at the ends, it could weld thicker sheets. Current electrodes are deformed with a strong pressure and time above 3 seconds, and they heat up red with 2 x 1mm sheets with 4 seconds. If i get some thicker electrodes, about 5mm, i’ll enlarge the mountings hole, and these will be used. I didn’t installed any handgrip so far because it is unnecessary at the moment, and it will make impossible to reach some tight corners of the welded material. Aluminum arms quite well conduct heat away from the electrodes, so welding can be done without a lot of worries about overheating or melted wire insulation, and they get cold pretty fast so breaks in work are not too long – the transformer doesn’t heat itself.


DOWNLOAD – Timer schematic in eagle and pdf; CircuitMaker 6.2c simulation file


  1. Stonakis says:

    Greate work thanks for making it public, nice diy solotion for home brew constructions

  2. Super cool! I am totally going to make this. Thanks a lot!

  3. peacefool says:

    Please, kickstarter it for those,
    who’s afraid of tinkering with welding stuff!))

    Shouldn’t cost a bazillion)

  4. lts says:

    Were can i find 4039N part? cant find anything on google

    • manekinen says:

      It is 4093 not 4039. CD4093 exactly. It is a common 4 nand gate logic chip, you should get this anywhere.

  5. Michael says:

    Will this kill me?

    • manekinen says:

      Read about safe voltage, for example

      Its like a car battery, it has a hundreds of amps and only 12V – you can even take a bath with it (don’t take it literally).

      Here, 2,6V is safe as it can be. It doesn’t matter if it have 1000 Amps since voltage is low.

      Of course you can get burn from hot electrodes, but if you are not careful, you can even get burn while making coffee :)

  6. Radu Motisan says:

    A very good tool, extremely useful for soldering .

    Can it work with thicker metal sheets? what is the limit?

    Can you show a video on how does it perform on soldering wires?

    Congrats on your construction!

    • manekinen says:

      Please read description, there is even a picture of 0,75 an 1 mm sheets. It won’t do 2 x 1mm sheets for now, but i think that the electrodes are the problem – i try to change them to something thicker and pointed at the ends, and 1mm sheets shouldn’t be a problem :)

      Can you say something more about welding wires? I don’t think its possible to weld copper wires (or anything copper) with copper electrodes because they have a very big conductivity.

  7. Radu Motisan says:

    But steel wires should be ok, right?

    • manekinen says:

      I don’t know man, i never tried this. Can you show me some example where that kind of connection is needed?

    • Scott says:

      Welding steel wires absolutely works and is very useful. Look at shopping carts, the racks in your oven and other appliances, wire mesh fencing, concrete reinforcing mesh…
      Love the design and plan to build one!
      One typo, the English text on the video says 1 to 4 minutes, should be 1 to 4 seconds.

    • manekinen says:

      Hello, text bug is fixed with YT annotation or whatever is this called. It pops on the screen in the right time, if you have enabled annotations in your player.

      Yes you are right those are a good examples of welding steel wires :)

  8. Eric says:

    Great build ! I also love the fact that you also build the case with your spot welder :)

  9. BruceJ says:

    You might consider getting some replacement tips from a commercial spot welder, such as … it’ll probably give you better performance.

  10. Silviu says:

    Would be possible to use two carbide bits (used/broken 3.17mm drill bits) as tips?

  11. Buzz says:

    Why put up files in formats that require special programs to open?

    • manekinen says:

      What do you mean by “special programs”? Do winrar and pdf reader are some kind of special programs?

  12. Maniek says:

    Rozumiem, dla bezpieczeństwa lepiej używać zabezpieczenia różnicowo-prądowego.

  13. Mechano says:

    Czym lutowałeś tak grube przewody? Da się jeszcze jakaś lutownicą kolbową albo HA? Czy to już trzeba palnikiem?

    • manekinen says:

      To powinno być pomocne. Z tym że ja robiłem to nad palnikiem zwykłej kuchenki. Najpierw nanieś trochę topnika na przewód, potem wrzuć do konektora kawałek cyny i nagrzej go dosyć porządnie nad płomieniem trzymając w szczypcach. Po tym jednym pewnym ruchem wkładasz przewód do konektora, powinien wejść ładnie do końca (zrób próby “na sucho”). Jeśli nie wejdzie tak jak trzeba, to późniejsze dogrzewanie go nad płomieniem zniszczy izolację a tego nie chcemy. Przy palniku takim jak na filmiku taki problem nie występuje bo konektor można błyskawicznie dogrzać i nie trzeba kombinować, nad palnikiem kuchennym jest trudniej. Żadna lutownica HA czy kolbowa się nie nadaje – próbowałem – zbyt mała moc.

  14. John says:

    Hi there, I use english only. I have MOT in stock and want to build my spot welder. I like the design you made semi-auto but how well does it handle with large MOT? My welding cable is 4/0 and possibly wrap two turn in the MOT. I download of schematic but not sure I understand the drawing.

  15. garlinski says:

    Witam, wykonałem płytkę do zgrzewarki, ale po kilku próbach zwiera mi (kolejnego już) triaka.

    Transformator od mikrofali 800W, wtórne 3zw. LGY25mm2, 2.8V~
    Triak BTA16-600B
    Układ gasikowy 100R/2W + 100nF/630V

    Układ obciążony żarówką 200W włącza się i wyłącza prawidłowo.

    • garlinski says:

      Zwiększyłem C w układzie gasikowym do 200nF i chyba jest dobrze, teraz grzeje aż miło. Pozdrawiam.

  16. Danijel says:

    Can someone explain to me how it connects X2-2 and X2-1, X1-1, X1-2 (where and how it connects to 220 V) and can you do this to 12V. Where to find a transformer 10V 12A??

  17. żółty says:

    Witam. Do autora projektu: Czy mógł byś podać dokładniejsze dane transformatora?Zostało podane uzwojenie wtórne a jakie jest pierwotne? Można liczyć na pomoc?

  18. With your ideas, I have build my own spot welder, see here:

    I tried to weld 2 x 0.75mm sheets, but this was not possible. Maybe it is the wrong type of steel? The current is 1100A.

  19. psiho051 says:

    Destroyed three tiles!! . Did somebody agreed this circuit, and his work on this scheme?????

  20. Elmo says:

    Ooh, I like how you made the swotch to turn the thing on! I have built both am MOT spot welder and a stick welder with three MOTs (doesn’t work properly though, I think the secondary voltage is too low, needs rewinding, but copper is expensive :). I will copy your attachment for the arm and the way you turn the thing on. Nice done!

  21. Miroslav says:

    Hi. How did you measure the secondary current to be 1000 A? 1000 A at 2.6 V would be 2600 W and I’m not sure that this transformer or secondary wire can handle that even for a moment.

  22. Boris says:

    Hi! If I’m not mistaken, the primary voltage is 220 – 240 V?

    • manekinen says:

      Yes, but it doesn’t matter scince you use a transformer (for example MOT) with primary winding made for voltage in your country.

      Or if you asking about pic in comment above, it was 233V or so, and it dropped to 222V after switching shorted transformer on. 14,5A is a lot of ampreage for a home mains.

  23. fayez says:

    what is the name of the rectifier

    • manekinen says:

      I don’t know, any rectifier will do. In this case i used dip4 package rectifier.

  24. Michał says:


    Czy jest szansa abyście wykonali dla mnie podobna zgrzewarka, jaka była by cena? Przydała by mi sie bardzo do mojego hobby.

    Prosze o info.

  25. Maciek1995 says:

    Mam mały pproblem: wykonałem płytkę (już 3 raz) i za każdym razem ten sam problem a mianowicie układ 4093 strasznie się grzeje i po kilku cyklach nie pracuje tak jak powinien. Nie wiecie co może być problemem ?

  26. Kasamiko says:

    I have a big 24V transformer, has a big wire in secondary, #8 or #10. Can I used it without rewinding?


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