Homemade Spot Welder




Introduction: Homemade Spot Welder

Anyone can build this simple to use light duty spot welder. I found all the parts I needed in my shop. I looked at some other plans on the Internet but they all seemed to require something I didn't already have.

Step 1: The Transformer (core)

I had three MOTs (Microwave Oven Transformers) collecting dust in a box with a bunch of adapters and transformers. The one was exceptional for the fact that the primary and secondary were separated by a shunt. Placed on it's base the secondary was on top and the primary on the bottom (pictured).

I used a pipe cutting blade (hacksaw blade) on a reciprocating saw to cut the secondary off the transformer core. Near the end of the cut I had to use extreme caution as I did not want to damage the primary coil. The primary and secondary can be identified by the number of turns and the gauge of wire. The secondary has many thousands of turns and uses hair thin wire. The primary is more like 18 gauge.

Once the secondary coil was cut away I used a short metal bar and a hammer to pound the trapped portion from the core. It was wedged in there pretty good. I cleaned out everything except the shunt and the primary.

Step 2: The Transformer (new Secondary)

I lucked out and found a 4 foot piece of grounding wire. It looks like a dozen 10 gauge wires bundled in one insulated coating. The only markings were three triangles. I understand from other sources that 4 gauge is recommended. I can't tell if I followed this exactly or not, but based on how hard it was to wind this monster in there I would hate to think of winding something thicker. It's surprising how stiff a thick wire like this is.

I bent the wire into a long tall U shape and fed the free ends into the transformer core. I then bent and curled and gently maneuvered the free ends back through the opposite sides. I ran out of room in the core before I ran out of length. I could get one more turn out of it if need be.

Transformer theory is a little beyond my ability to write about. There are many sources on the net if you are interested in this subject. The idea here is to have 3 or 4 windings on the new secondary. This will produce somewhere between 3 to 6 volts. I measured 2.5 when I plugged it in for a quick test. The lower the volts, the higher the amps. This is the main reason that such a thick piece of wire is used.

Step 3: Building the Jaws

So once I realized that the new secondary was actually producing an output, I decided to proceed. Up until this point I wasn't sure it was even going to do anything at all.

I used scraps of wood. A long narrow piece as the base. A fat piece mounted on it's side and screwed in from the bottom. A piece of thin plywood to seal in the remaining side. I had some copper pipe scraps that worked out well. In my caution over heat I doubled up, using a 1/2 inch hard copper pipe with a 3/8 inch soft copper tube shoved inside. I drilled some small holes in the pipes and then used drywall screws to assemble the entire thing.

The bottom jaw is fixed and I used a couple screws to do that, one into the base is visible in the picture. The upper jaw swings up or down and I used one screw, coming in from the side, near the back to achieve this. A small spring pushes the upper jaw up and an offset cam is used to push it down.

The bolts are made of copper. I stripped them off an old copper battery connector. I don't know how difficult it is to source copper bolts but there is a specialty fastener place in town that I would likely check first. I sharpened them on the bench grinder so that they came to a bit more of a point.

I tapped the holes, but copper is ductile enough that you can drill a hole slightly smaller than the bolt and just force it in. The bolt will cut the threads for you. I put the bolts in at the very start. I did everything else making sure that the bolts continued to meet when the jaws close. It is essential to keep the jaws electrically isolated from each other everywhere except the tips of the bolts.

Step 4: Assembly

A second piece of narrow plywood was required to extend the base for the electrical components. Being the first time I have attempted to make a spot welder I had to improvise on a few things. I would have chosen a longer piece if I had been able to see that far ahead.

I added an 8 gauge copper wire from a clean source as the copper on the pipes was quite tarnished at least on the outside. I forced the secondary wire into each of the tubes and wrapped the 8 gauge wire around the remaining exposed copper on the secondary wire. A pair of vice grips and channel locks crimped the tubes closed as tight as my hands would allow. Note: the 8 gauge wire runs up to the bolts at the front.

I had read a fair amount about spot welders prior to starting but I could never quite figure out the timer and custom power supplies. I had also heard talk of MOTs having power factor issues as well. I decided to just forge ahead and put a light switch on it. The switch is rated for 15A so I figured I couldn't go wrong. I put a red and black dot on the switch for quick reference. The box also provides a good (safe) place to connect the plug and all the wires.

Step 5: Schematic

This schematic is about as simple as it gets. It's akin to the schematic for a lamp. I include it for completeness.

PLEASE USE CAUTION: This project uses 110V AC. There are sections of the device that can deliver a deadly shock to the operator. One should take all the necassary precautions both when building or operating. If possible wires carrying 110V should never be exposed as casual contact can lead to a shock.
Mar connectors, soldering, junction boxes and electrical tape used properly can save your life.

If you don't know what you are doing or if you have never attempted projects involving 110V please don't learn the hard way. Go to Home Depot and take a wiring work shop or something.

Step 6: Operation and Conclusion

I built this spot welder for two reasons:
1) I always wanted to have a spot welder.
2) I broke the handle off a stainless steel cup measure and I wanted it back on.

I built the jaws so that there was enough room to get it in there without necessarily creating an alternate path for the electricity. It took a little fiddling to get it in there properly but once the cam locks down, the piece stays right where you want it.

I plugged it in and crossed my fingers. Threw the switch and I could hear the transformer hum. Then a small spark and an ever growing red glow between the bolts. The glow starts off red but gets orange the longer you leave it. I count to 10 and turn it off. This seems to create a fairly good bond.

The thickness of the piece changes the time you would require. I tried to join two eighth inch pieces and got nowhere. This is a light duty welder for thin metal. Sheet metal work is about the most you can expect. Heat is not as big of an issue as I thought. Maybe because I used a lot of copper. The bolts and the ends of the jaws are warm after use but not as hot as I would have expected.

For more power I could remove the shunt in between the primary and secondary. I could also add an AC capacitor (~30uF) across the primary connection to adjust the power factor. I'm just not sure why. It works fine for small pieces and I'm satisfied with low power that doesn't shoot sparks everywhere anyways.

Step 7: Safety Measures

I have received some negative press regarding safety (and rightly so). I decided that I should add a box around the transformer. It will provide protection for the operator and avoid any exposed 110V contact completely. Going with the idea that it can built without purchasing anything I started scrounging around for a box. They were throwing out a broken PC power supply at work.

SAFETY NOTICE: PC power supplies contain large capacitors that can store energy for a considerable amount of time (days). I checked with the person that removed the power supply and he told me that it had not been plugged in for quite some time (months). The shock isn't likely to kill you but it will make you wish that you had waited a little longer to open the box. Please use common sense.

I removed the internal parts from the box and put them in the pile of e-junk that is scheduled to go off to the recycle depot. I left the switch and plug to avoid open holes but you can cover it with gaffer tape or electrical tape instead. Don't use the switch in these boxes as they are not rated for 15A. I also left the ground wire that is attached to the box. I carried a third wire to the box and grounded it. This effectively grounds the core of the transformer which is now attached to the bottom of the box.

The box can be modified with tin snips if you take your time. I use pliers to bend back any parts that are twisted after cutting. Electrical tape to cover any sharp edges or protect insulated wires. I tested everything after I was done. It increases the audible hum that the transformer makes when it is on. I had considered adding a light that would indicate it was on, but I don't think that's necessary. I know exactly when it is on.

I also talked to a friend of mine who works with a spot welder at an HVAC factory in town. He said that the boxes they were welding would occasionally short across the tubes instead of the tips. It doesn't harm anything but it prevents the welding. They solved the problem by wrapping the tubes in electrical tape.

I hope that this gives you a fairly good idea of how to make this device even safer to operate than it was originally. Please use caution as this device is not a toy. Serious burns and/or electrical shocks are possible if safety is not your primary concern.



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    154 Discussions

    I want to build a rolled spot welder. It would be to weld two 1mm sheets of steel. If l just add a add some copper wheels on the ends would it be enough? Or do l need to get a different/more powerful transfo?

    6 replies

    I had thought of trying this as well. I think you could get away with copper sheet wrapped around bearings as long as the contact is copper to copper and doesn't go through the bearing. In terms of strength I recommend playing with the shims. Pull them out completely and you'll likely blow the circuit breaker but you might be able to limit the current with a toaster over or something like that. Maybe one or two pieces of shims would keep the current from swamping the circuit. I have also heard of people using a second shorted MOT as an inductive load as current control. 1000W MOT should work with 1mm steel pieces. Thicker than that and I can offer no promises.

    The shorter MOT is a very good idea; indeed you see it on the actual machine. however i would avoid the bearing with wrapped copper; once i needed to have extra long arms (+/- 50 cm) on a spot welder to reach the middle of a sheet and the only thing l had at hand was copper tubes and steel rod. the copper tubes were barely useful, there wasn't enough material to transmit the courant but the steel was truly useless. l would advise to use thick solid copper as much as possible. also as you said the trick might be to have thin edges on the wheel

    That's what I meant by copper to copper only. Even with mine I put thick copper wire inside the copper pipe.

    I found a neat video. My guess is they are pumping a bit more than 1000W on this one. The sharp edge on the wheel might help.


    Adding a small motor to have a nice continuous seem is actually a great idea

    Quick safety question. I have rewrapped the secondary of my mot with 2/0 with only a couple turns so I'm getting 2.5 v out. So at 12 amps in at 120 I have the potential of 575 amps out. So I'm just a tab bit nervous about holding the thing I want to spot weld....Even though I realize at 2.5 v it'd have a pretty hard time of poking me. You guys have had no problems?

    2 replies


    Read about the dangers here. It sounds like you need at least 42 volts unless you are attaching it directly to your heart tissue. I've never felt even the slightest shock from my welder. Burns are the greater danger. Using welding gloves should protect you from both dangers.

    it works by giving short circuit? because I think it would be necessary to place a resistor
    My multimeter is saying is that my primary resistance of 3.5Ohm
    I think it strange

    1 reply

    Sorry, I know it's a late reply, but there might be someone else interested in this as well so here goes, anyone feel free to correct me if I get it a little wrong.

    The workpiece acts as a resistor, it's not a very strong resistor (low ohms in other words) which is good because that allows more amperage to flow through and melt the metal more effectively. In addition, your primary has a different impedance to dc and ac, and with ac it has a different impedance if the secondary is open vs the secondary in contact with metal to be welded. Your multimeter measures resistance by putting an electrical current through the coil.

    Electrical energy is only generated in a conductor when a magnetic field changes within close proximity to said conductor. Since the primary windings act as an electromagnet, that means energy is only transmitted through a transformer when the magnetic field is generated or collapses, this doesn't happen with direct current like in your multimeter, so your multimeter only reads the short circuit resistance or dc resistance of the coil, whereas if you measured the resistance to alternating current, you'd notice that the impedance would change depending upon the resistance and winding of the secondary coil.

    fantastic build.will be making one soon.stupid question,whats the chance of getting a shock when holding the metal parts that your welding ? just a thought before i start .

    4 replies

    Mine runs at 1 volt or less so a shock is unlikely but I take precautions just in case. I wrapped the arms in electrical tape to avoid unintentional short circuits but it protects the operator from contact as well.

    so is it safe holding the metal with bare hands or are gloves better ?

    Gloves are always better. Electricity is only half your worry as the heat moves through metal fairly quickly and you can get a nasty burn before you realize it.

    ok cheers .im waiting for warm weather to get into my shed and start.

    already stripped a microwave down .

    That's so cool! What would you use a spot welder for? My husband-strange as it may sound-has recently taken up welding as a sort of hobby. I bet he'd love to make his own spot welder. How thick was the wire you used for this?


    A monumental lack of electric knowledge displayed here!

    1) The key to understanding transformers is that, excluding losses which can be made rather small, the power on the primary side equals the power on the secondary side. So if 110 V is applied on primary and 10 amps flows then the power IN is 110 X 10 = 1,100 watts. Say now you measure the voltage on the secondary side and you find 5 volts then the current on the secondary side MUST BE (1,100 divided by 5 =) 220 amps. If the gauge of primary and/or secondary coils are insufficient to carry their respective currents then you have a fire situation!

    2) A rule for your welder might be that the "waste" in the wiring should be less than 5% ie 50 watts in the above case. From this and the resistivity of copper the gauges can be estimated. One can take more loss in a welder because by its nature its use at high current is short term.

    3) Whenever the secondary is open circuit the current drops to zero and the power also drops to zero. The primary current is them zero (or close to).

    4) Another useful measure of transformers is the term "turns per volt". Hence if the primary has say 220 turns and 110 volts is applied the turns per volt is 2. This applies equally to the secondary so if you now want say 5 volts on the secondary of that same transformer you need 10 turns on secondary coil..

    5) To estimate the turns per volt just wind a few turns around primary or secondary, measure the voltage in your new temporary coil then divide your number of turns by the voltage measured. Typical for a tiny transformer could be 15 and for say a 2 KV power transformer could be say 4 or lower.

    6) A whole new area is what are known as "current transformers" (essentially no different in principle, just in how they are used) which can be highly dangerous and are best avoided!

    1 reply

    Thanks for sharing your knowledge on transformers and your monumental lack of social graces.