Building a Spot Welding Machine From a Microwave Oven Transformer

In this project I am making a DIY spot welding machine to be used for building battery packs with 18650 lithium ion cells. I also have a professional spot welder, model Sunkko 737G which is around $100 but I can happily say that my DIY spot welder out-performs the professional spot welder by outputting higher currents and being able to solder pure nickel strips to the batteries. There were a few obstacles in the process and a few things I've learned so I hope this tutorial will guide you if you decide to build one yourself.

The video describes the entire build so I recommend watching the video first to get an overview of the project, the problems I encountered and how I solved them. Then you can come back and read the following steps for more detailed explanation.

At the center of this project lies the microwave oven transformer. You can usually get this for free from your local recycling center or if you have an old microwave oven. I think the model I used was from a 800-900W microwave oven. With a higher power model, you will have more space on the transformer for re-winding the secondary. If you go for a lower power model, the space might be insufficient to fit the required turns of the heavy gauge copper wire.

Here are some links for the others parts used in this project

• 25sq mm copper wire: Link1, Link2.
• 25sq mm crimp spade connectors: Link1, Link2.
• spot welding pen: Link1, Link2.
• spot welding pen electrodes: Link1, Link2.
• spot welding controller board: Link1, Link2.
• pedal switch: Link1, Link2.
• small ac transformer: Link1, Link2.
• pure nickel strip: Link1.

I couldn't find 25sq mm copper wire locally so I used the next thing available which was 16 sq mm. Surprisingly the spot welding machine works fine even with 16sq mm wire but if you can, get the 25 sq mm wire for better results.

First, remove the original secondary winding

The first thing you need to do it is to identify the secondary winding of the transformer and then remove it. To identify the secondary winding, look for the part that has more turns and thinner wire. In my case the secondary was wrapped in that yellow paper and to remove it I had to saw one side and the hammer it out the other side.

Next, insert custom secondary winding

Using the heavy gauge wire, wind the secondary, make sure each turn is tight so you can fit at least 3-4 turns on the secondary. I couldn't find 25sq mm wire locally so I used 16sq mm which allowed me to easily wing 4 turns in the secondary.

Depending if you chose to make your own electrodes or if you chose to use a ready made welding pen you might want to terminate the wires differently. I went the DIY route and used some copper lug terminals crimped on the ends of the wires. This allowed me to create some kind of mounting point to install my DIY electrodes.

I made the electrodes from 3mm solid copper wire used for electrical wiring. I sharpened the tips of the electrodes using a dremel tool and bend them into shape at the other end so i could install them between two washers and screw. Although this solution works I would advise getting the ready made welding point and the special spot welding electrodes. Those work better because they use alumina copper which is a better alloy for this application. Also the mounting method is better than anything I could build on my own, but maybe you have better skills.

Before continuing with this project let me take the time to warn you that we are dealing with high voltage ac here, there is a serious risk of shock and death if you make a mistake. Do not build this project if you don’t know how these things work.

Wiring is pretty simple as you can see from the wiring diagram attached to this step but I will outline a few things you need to be aware of. Don't skip using an ac transformer for powering the control board, it needs to be ac because that's where it does the zero-cross detection.

Wire everything behind a 10A ceramic fuse, install it in a nice holder that will offer some protection in case something goes horribly wrong.

I would recommend also adding some form of thermal cut-off and a heatsink for the triac but please be aware that once connected to the triac the heatsink might be at live voltages.

Make sure the wiring is nice and tidy, heat shrink everything, do not leave any exposed wires.

After you've wired everything, double check the wiring,to make sure it's correct. Only then proceed with plugging in the system. If everything is right, nothing will blow-up and the control board will light-up. Turn the knobs all the way to the left (minimum) in preparation for the first test. Make sure the electrodes are not shorted together and trigger the pedal switch for a test weld, you should hear a small noise as well as see a LED indication. Once again if everything is right, nothing will blow up.

You are now ready for the first weld test, once again make sure the knobs are turned all the way to the left for minimum settings, grab a piece of nickel strip, place it over the battery tab, position the electrodes and hold them firmly pushed against the surface while triggering the pedal switch. You should have your first weld. If you feel there wasn't enough power you can try again after increasing the knobs slightly. In my case I'm getting plenty of power with just the minimum settings and increasing the power melts right through the nickel strip.

Another thing you might want to keep an eye for, is the temperature of transformer or the triac. If you use this continuously, the triac might get hot, this can be fixed by adding a decent sized heatsink to the triac, once again keep in mind that might be at live voltage.

Also regarding the transformer, it might get hot if used continuously, so it’s recommended to add a thermal fuse right on the transformer that would cut the connection if a certain temperature is exceeded or even better use one of those thermal contacts, that reset themselves once the temperature is reduced.

So there you go it is possible to build a DIY spot welder and if you take care of all the issues I presented it could work just as well or even better than commercially available machines. It was certainly fun to build this project and I learned a couple of things on the way.

There is a blog post on the subject if you would like to send me some feedback you can do so in the comments and you cal also checkout my Youtube channel for more awesome projects: Voltlog Youtube Channel.