Ambitious Spot Welder Really Pushes The Amps

On the face of it, a spot welder is a simple device. If you dump enough current through two pieces of metal very quickly, they’ll heat up enough to melt and fuse together. But as with many things, the devil is in the details, and building a proper spot welder can be as much about addressing those details as seeing to the basics.

We haven’t featured anything from our friends over at [Make It Extreme], where they’re as much about building tools as they are about using them to build other things, if not more so. We expect, though, that this sturdy-looking spot welder will show up in a future video, because it really looks the business, and seems to work really well. The electronics are deceptively simple — just rewound microwave oven transformers and a simple timer switch to control the current pulse. What’s neat is that they used a pair of transformers to boost the current considerably — they reckon the current at 1,000 A, making the machine capable of welding stock up to 4 mm thick.

With the electrical end worked out, the rest of the build concentrated on the housing. A key to good-quality spot welds is solid physical pressure between the electrodes, which is provided by a leverage-compounding linkage as well as the long, solid-copper electrodes. We’ve got to say that the sweep of the locking handle looks very ergonomic, and we like the way closing down the handle activates the current pulse. Extra points for the carbon-fiber look on the finished version. The video below shows the build and a demo of what it can do.

Most of the spot welders we see are further down the food chain than this one, specialized as they are for welding battery packs and the like. We do recall one other very professional-looking spot welder, though.

18 thoughts on “Ambitious Spot Welder Really Pushes The Amps

  1. Look for an Amana American made oven. The oldest ones had less efficient magnatrons and so made up for it with a monster transformer that is easy to cut off the HV secondary. The earlier ones in general have larger transformers than the latest. 1500 watts. Lotsa iron core.

        1. Because if its still running now it will, if you take any care of it, still be running many decades from now, which is something no modern appliance can come close to claiming… These old, high quality and repairable objects are pretty hard to argue with, and its so wasteful to junk something you can fix when the garbage modern ones can provide good enough parts but are not really fixable…

          Heck I have freezer older than I am that is still in fine fettle (though its status lights are dead and I’ve not got round to looking at replacement) but the much much newer (but out of warranty) one has already needed some fixing up, and now all the internal rails and plastic are busted and you can hear its about to die for good every time the ambient temperature climbs and the pump tries to function… And despite being high efficiency rated doesn’t seem to be any less of a power hog…

          1. A couple decades ago, “Consumer Reports” warned readers against buying a “High Efficiency” furnace, as the higher repair costs and frequency of repairs was more than any savings incurred.

  2. This is by far the best spot welding video built I’ve ever seen.
    I have been collecting some micro wave oven transformers myself, but never got around to building a spotwelder yet.
    I was wondering whether to put the secondary windings of two of them in series, or in parallel, and this video makes it pretty clear.

    Other mandatory details for a **GOOD & RELIABLE** spot welder (which most builds skimp on) is the timer and a mechanism for applying force after the weld is made (Rule of thumb: Keep the clamp closed until the red spot has disappeared. Premature movement may weaken the weld)

    Those “Fotek” relays directly from China / Ebal / Amazon / etc are mostly fake. If you buy the 25A version you’re lucky if there is a 10A TO220 TRIAC in it. Plenty of youtube video’s about those.

    Complete spot welder timers are however common on Ali / Ebay / China too. They have impressive looking triac’s, but with the current state of dubious electronics from china I’m not sure if they are good value for their low price.

    One thing I would do differently, is to add a foot pedal, or at least some easy way to add a foot pedal as extra accessory. You often need two hands to position parts before spotwelding, and using the foot pedal with light clamping force can even be used as a third hand if needed.

    1. Yeah, cool video!

      The foot switch issue can be solved with a slave/assistant/7 year old daughter – which ever is available at the time. But yes, sometimes they’re not there, so a foot switch would be handy.

      If I were to change anything, it’d be to improve the connection to the cable. The single grub screws might be sufficient, but a super strong clamp or a bolted crimp are probably better. Also, would measure the voltage, current and resistance, probably tweak a bit and plot some graphs. But that’s down to being a nerd and not a welder…

      1. You have to be rather prolific in your breeding to always have a 7 year old daughter, and at that age you probably only get 6 months before you have to remember the extra half, which as we all know rounds up to 8… Sounds rather too expensive, time consuming and noisy to me…

    1. This is what I was thinking! But it looks like he has sanded the outside of the cores, which has likely diminished the effects anyway (which is why he probably needed two stages in the first place!)

  3. I have a smaller commercial (cheap import) spot welder. Pops the breaker every time on a 20A circuit. Had to add a small series resistor. Can’t imagine what kind of wiring he has that lets him run two microwave transformers. Inrush current is huge. Also, 3 feet of one inch copper bar costs twice what I paid for my whole unit. Ouch.

    1. 110v is an USA thing.
      230v in Europe, no problem to run any welder from a residential plug. (technically plugs are rated for 16A so about 3600W but quality socket /plug will definitely take more, especially for short duration, with a 20A circuit breaker it’s about 4400W, way more than enough for stick or tig welding.

      1. Not sure about that. My 200 amp welder and plasma cutter plug into a 50A 220v outlet. Might not need to whole 50A circuit (which is pretty standard for welders in the US) but I wouldn’t say that a 20A breaker is way more than enough. A 200 amp welder is not exactly huge either. Most hobby level MIG, TIG, and stick machines come in at a max 200 amp capacity.

  4. The front plate (steel?) has two separate holes, one for each electrode wire. This will create eddy current heating in the plate, as there’s going to be a large induced magnetic field in the plate at the currents this spot welder if operating at. Running the two wires through a single hole cancels out the field and prevents this. Alternatively, cutting a slot between the two holes will reduce the magnetic field in the steel by adding an air gap and reduce the eddy current loss in that piece of steel. Also, using non-ferrous metal would reduce the magnetic field and reduce eddy current to an insignificant level.

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