Spot Welder; Don’t Buy It, Build It

Spot Welder

Spot welders are super handy for making sheet metal enclosures for your projects. The problem is, commercial ones are rather expensive… The good news is, they’re actually really easy to make! This is [Caio Paulucci’s] first submission to Hack a Day, and it was a weekend project him and his father just finished.

A spot welder works by dissipating large amounts of heat in between two electrodes in the material you are bonding. It makes use of a transformer that converts mains voltage to a very low voltage, but high current energy source. The cool thing with this type of welder is it’s perfectly safe to hold onto the electrodes as the voltage is so low, you won’t get electrocuted. By running a super high current (generally >1000A @ ~1-2V) through a small surface area, you can super heat most materials hot enough to weld them together.

They can be made using the transformer from a microwave, some heavy duty welding wire (generally 2/0 or thicker), and a few other odds and ends such as wood, electrodes, and maybe a few nuts and bolts. At the most basic level, you are basically re-wrapping the transformer’s secondary coils to change the ratio to produce a low voltage, high current transformer.

For more detailed instructions on how to build your own, we’ve covered these builds many times before.

84 thoughts on “Spot Welder; Don’t Buy It, Build It

      1. I don’t think that you can assume anything about the integrity of a transformer pulled out of a microwave and wired backwards maybe a diode + zener diode to a relay to break the mains if the voltage gets above some point ?

        1. This transformer is not wired backwards. The primary side (mains voltage) is left alone, and the secondary (the HV output) has been chopped out and replaced with a few turns of 2/0 gauge wire. Have you seen the thickness of the insulation on 2/0 wire? It’s a non-trivial task to accidentally nick it, and even if you did, the core would have to be live to get shocked, but it’s not. I’m all for safety, see some of my posts on the stupid laser tricks that get posted here, but this is really a non-issue. Add a fuse, put the transformer in the box so no one touches the mains terminals and be done with it.

        2. Surely voltage on the output will never be that high to get through your body resistance. You can only get yourself burned – and that was stupid for me, that he had no gloves on.
          Although it would be safer if there was some safety measures on the input (high side) and there are projects like that on the net.

        1. This is such an old myth now that I am *shocked* that it still exists.

          First of all we are talking about AC, so capacitive reactance is just as important as ‘R’.

          I saw a man get a non-fatal electric shock (active to earth) when he had NO DC path to earth!!!

          He placed one hand on a mirror that was a backwash for a bar sink. The glass was clean and dry so there was no DC path to reflective surface on the back of the mirror that was earthed though the sink. The capacitance caused by the surface area of his hand against the glass with the reflective coating on the other side was enough to convey the AC. We had to perform CPR otherwise it would have been fatal.

          Cardiac arrest is a result of ‘power’ not exclusively voltage or current. As most test equipment measures voltage or current it is not often expressed as power.

          If you oversimplify this to voltage then you are ignoring the different resistances of bodies. Every body if different an the only real generalisation you can make is that by average (only) is that females are more conductive than males.

          The most generally accepted ‘specification’ is that as little as 18mA is likely to be fatal across the heart, like a left arm to leg shock. Household safety beakers (earth unbalance) trip at 25mA. Medical grade safety breakers trip at 15mA.

          When shock or electrocution does occur it’s NOT a result of a miscalculation of V, R, I or Xc. It’s a result of the current taking an unexpected path as in my example above and in many cases with high voltage jumping air gaps.

          Microwave Output Transformers are very useful IF you cut out the secondary before you use it for anything!!!! Remember to discharge the cap before removal.

          With a MOT you can make high current power supplies, high current battery chargers, stick welders and as we see here – a spot welder.

          Transformers have excellent isolation, just get rid of the high voltage winding first.

          1. You are obviously correct it’s not the resistance, but the complex impedance that matters, because that is what limits AC current.

            To be honest, I find your story a little hard to believe. Assuming the surface area of a man’s hand is about 0.02m^2 (10*20cm), a glass thickness of 1.5mm, a relative dielectric constant of 10 (range is about 5-10, assuming worst-case), the capacitance works out to approximately 1.25nF, which at 50Hz will have an effective impedance of just over 2.5 mega-ohms. Assuming a line voltage of 230Vrms, this would allow a maximum RMS current of about 90 microamps, assuming there is no other source of impedance in the circuit. The resistance of the human body can easily be much less than 2.5 mega-ohms, so we can probably ignore it in this case.

            If this man indeed had a cardiac arrest, I’d think he must have had a pre-existing heart condition, quite possibly unknown to him. Obviously, I’m not a cardiologist, so I wouldn’t claim it’s impossible, but 90 microamps seems very low to trigger any serious problem.

            On the other hand, it would be VERY foolish to assuming anything under 15 milliamps can’t kill you!

        2. Just more of the same asinine argument.
          Yes, Amps (current) is what kills you.
          But No, you can’t get any current without voltage.
          This is stupid. It’s like saying “Watch out for bullets, they kill you”
          True, but without a gun bullets are harmless.
          Got it?

          1. Tongue in cheek: bullets would also work with a slingshot, rail-gun, hydraulic press, dumping a 1000# pallet of them on you, …

            Agreed on needing enough voltage to overcome resistance. Circumstances will affect the resistance; higher with dry skin, lower when soaked in salt water, lowest when hooked to electrodes implanted in the heart.

  1. “The cool thing with this type of welder is it’s perfectly safe to hold onto the electrodes as the voltage is so low, you won’t get electrocuted.”

    That’s nice. What of the potential ill effects of putting a kiloamp across one’s chest? Life without risk is oatmeal without cinnamon, I agree, but…

    1. If the voltage is below 20 volts? I’ll take 20,000,000,000,000 amps. because it cant do anything. 24 volts is the theshold of conductivity for a human being. our resistance is simply too high.

      skin resistance may vary from 1000 ohms for wet skin to over 500,000 ohms for dry skin

      Look up ohm’s law for more fun facts about electricity!

      1. My skin resistance measures a MINIMUM of 3,000,000 ohms, and that’s wet or dry and squeezing as tight as I can. I attribute it to the fact that I used to like to grab exposed 110v plugs as a kid but I could be wrong.

    1. I think the write-up assumes that since the voltage is low, the likelihood of the electrodes arcing to the user is low. I have to agree with kaidenshi that to say it’s perfectly safe is a fallacy.

      That being said, nice build. Be careful folks.

    2. From your link,
      >Between the ears, for example, the internal resistance (less the skin resistance) is only 100 ohms, while from hand to foot is closer to 500 ohms. The skin resistance may vary from 1000 ohms for wet skin to over 500,000 ohms for dry skin.

      Ohm’s law still applies. Unless your body is a very good conductor, the voltage has to be high enough to push that type of current. The person has to electrodes poke through the skin to get that low internal resistance.

      1. 100 Milliamps is the threshold for a human heart and just below 50Hz is the ideal killer frequency. As the brilliant video shows, if the voltage is low enough then nothing bad will happen.

      2. Thanks! As I read anti Ohm law group, I’ve searched for it (That guy is great :D ) I didn’t know that so many great engineers are gathering here but basics of physics are in danger – what might be normal for microelectronics, but not on macro side :P

    3. The article you linked says that the resistance of wet skin is 1000 ohms. At 2 volts, that’s a monster current of .002 amps, which the page also says is in the ‘threshold of sensation -> mild sensation’ region. Read what you link.

    4. From an electrical standpoint, this is about as dangerous as a AA battery with leads connected. It outputs about the same voltage, and while a AA can’t supply the same current, the resistance of your skin means that this welder won’t give you anywhere near that much current anyway. And yes a AA CAN deliver enough current to kill you.

    5. @kaidenshi Maaaaan…. I read a lot of stupid crap written by fearmongers here, but this is among the worst I’ve seen this week. You really think that 2 volts even if 1000 amps are available would kill you? Then a regular 12 volt car battery that can supply a few thousands of amps would kill you dead instantly if you put your hands on the connectors. I can assure you that won’t be hurt by doing that.

        1. As I said below, my point is that it’s possible. No, it’s not likely, but don’t be an idiot around electricity in the first place and you won’t have to find out. Bottom line, dealing with high current AC requires safety and paying attention, not “hurr hurr it’s low voltage so it’s 100% safe, durr”.

          1. The only danger is when you connect the electrodes and generate a bunch of heat and cause a fire or skin burns. Touching the electrodes is just as dangerous as touching a lithium coin cell.

      1. 12 volts dc would be very unlikely to kill or harm you. Ac voltage is a little more hazardous being you only have to interrupt the signal going from you brain to your hart once to kill you. I was talking about this with my doctor and she said it takes 40 volts AC at a 1/4 amp to do this if your unlucky. I have been hit with 208 at 20 amps with no ill affect up one arm and down the other which is why I was talking to my doctor. The proper thing to do is treat all voltage as if it can kill you.

          1. That was my point, which no one here seemed to grasp; they’d rather just throw juvenile insults my way. High current at low voltage CAN kill, and anyone who grabs the poles to show otherwise is tempting fate. In ideal conditions, you’re safe, but common sense goes a long way.

        1. I doubt you got 208V across you. That means your leg was touching one of the “hot” lines, and then you touched the other one with your arm. What probably happened was you just touched one 110V line and it traveled down your leg to ground and that’s what you felt. And then just because the breaker is a 20 amp breaker, doesn’t mean that’s what hit you. The resistance between your arm and the ground is huge, and you probably got a few milliamps at the most.

        2. Yes treat any voltage or current as a danger. As a retired electronics engineer. I am amazed at how people like to take risks. There is reason we stand by the reasoning of always keep one hand planted in the back pocket of our trousers!!! It’s so that we can’t accidentally connect both hands across a source. Back in the day, I had an instructor talk about common CRT’s and how you could grab one by a neck, sling it over your shoulder perfectly safe. Local emergency services had the instructor arrested and put through a dangerous materials course, then removed his teaching credentials permanently. 1) basic safety dictates such a vacuum tube can implode if the 1/8th glass is fractured and can go through 10 steel! and 2) Second anode voltage and current discharge can stop a heart if the tube still has a charge. True we are talking 40,000V at .01milli-amps form a glass capacitor (Vacuum tube) but and there is always a but, point is this conversation here is inciting a dangerous president. Please people be very very very careful. I lost 2 colleagues who thought they were being careful around electricity.

      2. I hope that, if I’m ever tortured, my torturers have seen the same stupid films I have. A car battery across ANYTHING isn’t even going to tingle. “Oh, OW! That, like really hurts! You mean baddie! Stop torturing me!”.

        You might worry about the crocodile clips though.

  2. No . . . 1-2V will not be enough to push the required current through your heart to kill you. . . or do you handle AA and 9V batteries with care because they could kill you if you touch the terminals?

    1. Kept reading and wondering why nobody had mentioned this yet. Seeing the transformer out in the open makes me shudder since my shop tends to get piled up with all kinds of stuff when I’m in the throws of a major build. Enclosing it will complete the build.

  3. – Current through the heart is what kills you.
    – Getting it there is the job of the voltage.
    – People *HAVE* been killed by the application of 12 Volts directly to the chest near the heart. One such documented event occurred during an experiment. A prisoner volunteered to be the ‘victim’. Death was not expected but he could not be revived.
    ie This is EXCEEDINGLY rare but can happen. [Carrying a 12V car battery bare chested when sweating profusely with terminals against your chest would not be the wisest thing you could try :-( ].
    – 12V hand to hand even with sharp spikes in unlikely to kill you. It conceivably could.
    – 2V hand to hand or hand to foot would be exceedingly hard put to do anything.
    **BUT** take a metal bucket, half fill with saturate salt solution (or seawater) clamp one 2V terminal to the bucket, grasp the other firmly in wet salted hand (right may be best) and step into bucket of water. You will almost certainly experience muscle contractions, possibly severe. You MAY experience full muscle lockup.
    I had a friend simulate this at 12V (with a battery lantern and metal spear while flounder fishing – wading in salt water) – due to a wiring fault he experienced full msucle lockup and had to be de-connected by friends – a frightening experience. Do that across your heart and … :-(.

    Russell McMahon.
    FWIW :

    1. Agree with most of your explanation, but as far as I know, muscle lockup should not occur as you described. Nerves and muscles react to AC. When DC is first applied, you get a brief initial muscle activation, because the sudden rise in voltage/current is seen as AC. After that, as long as the electric flow is maintained, there is no further muscle response, or even any sensation of electric shock.

      This lack of sensation can make DC dangerous in strange ways, because you might believe the current has ceased, when it has not. If the current is high enough, heating will occur; and given the right circumstances, this current can concentrate through internal structures (like blood vessels) that are good conductors. Blood vessels lack temperature sensing nerves, and by the time you begin to feel heat in the outer skin, you may have already suffered some serious internal damage.

      I once received a 340VDC shock. Thought it was just a little capacitor discharge, and since I felt nothing beyond the initial jolt I was sure the cap was now empty, so I didn’t bother to withdraw. But a few seconds later, I notice my forearm getting quite warm! I then pull away, and discover the device was still connected to the mains and powered. Fortunately there was no permanent damage. But this unexpected experience taught me that my understanding of the effects of electricity on the body were incomplete, and I hit the books for a better education.

      I would not take any special precautions around 2VDC. Neither would I put myself in strange situations like standing in buckets of saltwater around it, though. ;)

      1. I’ve never seen anyone try to rectify the current with these homemade MOT welders. So odds are it is AC. As far as you claiming that you cannot feel a DC shock I’m not so sure about that. Because I do have a DC welder and I’ve gotten shocked by it a couple of times now too. I distinctly remember feeling it. Now whether the arc starting circuit inside my welder played a part in that I’m not sure about. I do know it is a shock though. A painful shock.

  4. “…But it’s the current that kills you…. blah blah blah”

    Somebody has to drag out that tired excuse to fear almost every project. So what? Ever hear of Ohm’s law? It’s a pretty important fact to learn when dealing with electronics, not just for safety issues but for understanding pretty much everything electrical.

    I = V/R

    This means the current is ALWAYS equal to the voltage divided by the resistance. Sure, you can get pretty high current in a spot welder. That’s because the resistance of the metal being welded is very close to zero. But…. That current only travels the path through the joint which is being made! The resistance of your body is NOT near zero. Therefore to get a large current through your body you need a large voltage.

    So.. so long as your connection between the mains and the primary is well insulated and you haven’t nicked the insulation inside of the transformer how exactly is this device going to hurt anyone? Just don’t drop it on your foot and you should be fine.

  5. MOT spot welders often fall into the trap of assuming that the weld current will equal the breaker current of the AC circuit times the turns ratio. You have to keep in mind that the resistance as seen by the primary side of the transformer will increase by the square of the turns ratio, and this usually limits the current to much less. This welder looks like it is addressing this issue well, using what looks like 3/0 or 4/0 wire for the secondary.

    The other issue that often is overlooked is transformer core saturation. The iron core of the transformer can only carry so much magnetic flux, after which it overheats and robs the circuit of power. Again, he looks like he is using a pretty beefy transformer. It would be nice if he would put a clamp-on ammeter onto the secondary and actually measure the weld current for various material thicknesses.

    1. I’ve built a similar MOT spot welder and with a clamp meter I was able to get 1000 amps short circuit, and 700-800 amps welding thin sheet metal. This was at about 2 volts open circuit. It drew upwards of 30 amps from the wall socket (120v).

  6. Whenever I hear the phrase “it’s the current that kills”, I immediately think that the one saying it is an idiot who’s just repeating a triviality he heard from his uncle or some bad TV show. The comments here remind me where I did get that association from.

    The whole thing about the dangers of electricity is much more complicated than “OMG the current!” or “OMG the voltage!”.
    Yes, if you manage to get an amp flowing through your body you’re going to lie in a hospital bed or a grave almost certainly. It’s exactly the same with having half a pound of metal in your chest, but for some reason people seem to grasp that certain conditions need to be fulfilled to actually get it into your chest.

    There is a reason the electrical codes of practically all countries allow low two digit voltages in products without any measures against touching them. Common laptop PSUs easily deliver 3 to 7 amps at about 20V. And you would scream if you licked the plug. But nobody who is sane does lick plugs or wires one end to a salt water bucket in a conscious attempt to kill themselves. And unless you do such a stupid thing, things like car batteries are totally safe*, despite being able to deliver hundreds of amps and more power than a wall socket.

    If you really try to estimate current flow through a human things get very complicated. The path of the current is not only crucial for the question of how dangerous a shock would be, but also to estimate the resistance met. Various kinds of tissue have different resistances, they vary from person to person (think e.g. amount of muscles/fat) and also play a role in determining the threshold upon which you can feel a current and also how and how intense secondary damage will be.
    The next interesting thing is that the resistance of a current path through the body does vary with voltage. And of course this is different for AC and DC.

    But as long as commenting is easy and idiots who think that any topic can be summed up in one catchy phrase not exceeding 5 words are plenty, I guess dumb comments here won’t stop.

    * car batteries aren’t really safe. But that they’re filled with nasty acid and poisonous lead is another topic.

    1. You’re not really going to get more power out of a car battery than a wall outlet. Circuit breakers take a little time to trip, and in that moment you can get some impressive amperage out of a wall outlet. I’ve shorted both and the amount of metal removed with line voltage is much more than 12 volt batteries in my experiences.

      Remember kids, remove the negative lead first when removing a car battery.

  7. I’ve seen a bunch of these homebuilt spotwelders, usually built with transformers from old microwave ovens and yeah, they all do sort of work a bit.

    None of these seem to put much effort in the reliability of these things however.

    For a good spotweld it’s necassary to control the amount of power which is put into each weld, the pressure in the melting zone has to be high enough to press your material firmly together and the pieces of matiearila may not move during cooling of the weld.

    A foot peddle can easily be added to increase the pressure ( And will keep both you hands free for positioning yor 2 work pieces).
    It’ also pretty easy to add a timer (100Hz, 1/2 cycle counter) and a beefy solid state relay (use your favourite microcontroller) to adjust the engergy put into a spotweld. Every half decent spot welder i’ve seen has a timer to control the energy inpot to the weld.

    Adding the foot pedal and timer also has another important benefit.
    This way you can apply the clamping force on the spotweld untill the weld has cooled down enough to get out of the plastic region and this greatly reduces the chance of disturbing the hot welding zone.

    Checking you weld:
    If you torqe your to work pieces from each other on a single good spot weld your work piece breaks and not the spotweld. This will leave a hole in one of the 2 pieces.

    Electrical safety:
    Secondary is a “non issue” exept for (double) isolation from the mains voltage. Becasue of the extreme turns ratio secondary voltage is usually < 2V and perfeclty safe to touch. I haven't seen any spot welder with isolaton on the secondary side yet. So just put a box over the primary side.

  8. hand or foot operated I’m sure the the “weld” switch in series with machine’s primary power switch is not about electrical safety, but about better and cleaner welds are the result if the power is applied after the electrodes are clamping the metal together. I have an original Le-jay maual. On of rhe projects is an auto battery powered spot welder. 6 V. because it was published before 12 V. became standard. As I recall even it designed so the electrodes clamped the metal before it switched on the power. In the event one plans to use it a lot having a foot pedal that operated the electrode and automatically turns on the power would free up the hands to position the work. For what little spot welding I ever needed to do the stick arc welder . No not a resistance spot welds, but did what I needed.

  9. “Volenti non fit iniuria” means “to a willing person, injury is not done” really smart, old good maxim, which unfortunately nobody remembers, and nobody cares anymore :(

  10. The Harbor Freight 220v ( made in Armenia) welder measures 1.4v open circuit connected to 240v. I’ve only used it a few times. Biggest problem is the manual toggle switch control doesn’t make for consistent welds. It’s similar to a unit Hobart sold and I’d guess others as well. It’s a serious piece of metal. I watched for a long time and picked it up on sale and consider it an excellent purchase. It just needs a timer and a stand w/ foot pedal clamping.

    A typical junk box build won’t match that, but a computer controlled precision welder is a great use for an old microwave. Make your own vacuum tubes. What would really be great is hacking the old controller to work in 1/100 of a second units. I’ve got a 700 w Samsung w/ a bad capacitor which I plan to use for a small bench top welder.

    I’m pondering a timer. The most likely seems a 555 w/ binary switched resistors to set the time. I considered an msp430 until I saw how many buffer parts I needed.

    1. Just FYI switching off an inductive load with some power transistors or the like could be problematic. I’d suggest a TRIAC with a proper snubber or better yet a pair of SCRs. Of course you’ll also need to figure out what to reference the gate voltage(s) to and how to connect your circuit to the gate(s) while respecting their references.

  11. Let’s just assume that it’s not good welding practice to place the electrodes directly on your body, instead of the material that you’re trying to weld.

    A nice simple home built tool for limited use. I like it. All that I would add, as others have noted is maybe a foot pedal for either clamping, or activation of the weld, and a case for the transformer. After all, if you’re in a metal shop, you don’t need metal shavings and grinding dust getting on your transformer.

  12. “A spot welder works by dissipating large amounts of heat”…
    When you dissipate energy it turns into heat. When you “dissipate heat”, what does it turn into?

  13. I have an old and rather massive spot welder that was built by someone very clever, or it’s a factory made antique, built mostly by handwork rather than machine tools. It has a very large toroidal transformer with three secondary windings. To select which output power is used it has a sliding bar with a notch in the bottom edge. Each output has a ceramic housed, high amp light switch and the toggle goes up into the notch in the bar.

    Clamping is done with a spring loaded hand lever on top of the upper electrode arm and there’s a foot trigger to make the weld. It’s in a housing with a steel frame and louvered sheet metal panels.

    I should put some pics up on Flickr…

  14. You’d think hack-a-day readers would be smart enough to realize people generally don’t die from handling 1 1/2 volt batteries. As far as hazard goes, notice how close he’s holding the parts with his bare hand. They’re not even getting hot enough for him to say, “Ouch.” Cool project.

  15. Building a spot welder. Wow! thats really a great idea. Actually I was looking for a used spot welder. I have completed a welding training program from Weldtechtraining in Toronto. I didn’t have purchased any welding machines and tools. Anyway I will try to make this welder soon. The video was superb. Thanks for the inspiration.

  16. I made a spot welder from an old MOT, I used 2 sets of 4 ga and tied the ends together. I get 1.90 volts AC. I am a little surprised about how long it takes to get a good weld with 1/16 inch steel. It needs to get molten to hold. Also I am careful to let it cool off between each use. The wires get warm both at the new secondary and the mains input to the MOT. I’m keeping an eye out for some solid copper contacts. I used a grounding rod to make contacts, but it is only copper clad and a bit too large at the contact point. Still, it’s awesome to be able to spot weld for free. Also, I mounted the MOT to a board that I attached to the base of an old tackle box, and secured the wires in and out, so it has nice case with a handle to carry it. I also made a great little footswitch that uses a standard light switch to turn it off and on.

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