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.

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.

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.

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.

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.

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.

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.

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.