How To Build Your Own Spot Welder From A Microwave!

This is an article I originally wrote for my website, thought it would be beneficial for you guys to see how I made my 18650 spot welder, it’s actually pretty easy!


This project is an outline of how to build a resistance spot welder using salvaged parts from an old microwave. I’m using it to weld nickel tabs onto 18650 battery cells but depending on how you position the arms it can be used to weld sheet metal and other metals objects. Lets get welding!


Step 1: Salvage the Microwave

Quick note of caution!

The inside of the microwave is quite dangerous. The large capacitor maybe charged and can deliver a nasty or even fatal shock so ensure you discharge as soon as possible by touching a metal rod such as a screwdriver across the terminals to discharge it.

Okay so open up the microwave casing to reveal the electronic bits. Discharge that capacitor and get to work on removing the parts. You should find the transformer which should look very similar to the one in the photos. Remove the nuts and it should slide out pretty easily. I salvaged a few limit switches that we will use later and some of the cables are handy for power connections.


Step 2: Remove The Secondary Coil


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We are going to rewind the secondary coil of the transformer as we want more amps and less volts. The primary is where the mains power is attached and the secondary has finer wire windings with the red wires attached.

The fewer turns of wire increase the amps but lowers the voltage, and more turns increase volts while reducing the amps. We don’t need the secondary coil so it can be removed either by cutting off or by grinding the weld on the transformer body and sliding it out. Be careful not to damage the primary coil as we will be keeping this.


Step 3: Add The New Winding




The new winding will provide the current needed to weld stuff. By using really thick cable we can reduce the thousands of windings to a couple which will provide loads of amps. The thick cable is needed as the resistance will cause it to heat up and melt the insulation if its too thin. Not good!

The primary coil is put in first followed by the 2 shunts either side and finally the low gauge wire (blue cable) is wrapped a couple of turns. Remember to leave a decent length of cable that will attach to the welding electrodes.


Step 4: Finish The Transformer


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Our super high powered transformer is nearly complete. We just need to weld the top back on to seal it up. You could alternatively use 2 part epoxy to stick it on. Options are good, pick whichever one is easiest for you. :slight_smile:


Step 5: Electrode Terminals



We now need to attach our cable ends to the copper pins which we will use to weld. I machined some copper terminals but you could use some copper clamps from the hardware store. I’ve also attached the CAD file for the electrode clamp that I made. Here’s the Fusion360 link too.


Step 6: Welding Arm



I’m using this to weld nickel tabbing to battery cells so I’ve positioned the two welding electrodes side by side although you can easily mount them opposing like a traditional machine. I designed and laser cut a simple MDF case for the arm which houses the switch for operation and holds the electrode terminals.


Step 7: Package It Up



There is 230v of dangerous electricity coming into the transformer so its important that its covered. This laser cut case should do the job pretty well. Just make sure everything is contained as it will also look much better as well as being safe. Bonus!

As for wiring its pretty simple. Just connect the live and neutral to the transformer primary using the existing spade terminals, I would recommend adding a switch in between one of the power cables to make it easy to turn on and off. I salvaged this one from the microwave.

Thats it we’re done! Have fun with your new spot welder!


Looks like I need a laser now!

Really nice build! How do you control the welding power?

i just want to push the issue again

the stuff you will be messing with to get your parts can have enough stored energy to kill you so please for the love of sithis be careful.

you could fucking die

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Then you should stop riding electric skateboards, and stay away from batteries. If you don’t like the risks stay in your bed :stuck_out_tongue:.

True, this is a bit over the (safety) line for me. I’ll stick to my 18650 sleds for now.

there is a difference between taking risk and doing something that could get you killed.

riding a motorcycle during the first 15 minutes of rain is taking a risk

doing a wheelie is asking to die.


There is more energy in a 18650 than a microwave + soldering a 18650 is WAY more dangerous than spot welding it.

@jack Thanks for sharing this OP! I’ll give this a try in near future :wink: One more question, do You use momentary switch visible on welding arm to turn on current in primary coil?

Thanks everyone for the kind words!

@squad Yeah i just tap it for a quick burst of current which works well but I am planning on making a timer circuit which will make it much more reliable.

Jack :smiley:

awesome build process! :astonished:

Thanks man, your battery build thread started me off on this project!

this is amazing. When using it, what is the equivalent of the foot pedal to trigger the weld on commercial ones?

@JTAG There is usually a rather large capacitor inside the microwave that has quite a lot of energy in it.

Also, i think he is referring to the fact you will be working with mains voltages with this DIY. Soldering on a 18650 is extremely safe compared to this.

Capacitors on microwaves have a bleeding resistor, so they will be empty by the time you open up the microwave. And main voltage is nothing to worry about, lots of safety mechanisms are build into the / a mains installation. There is a lot higher risk of severe personal injury if you handle batteries the wrong way (like soldering a battery).


there’s enough juice in an 18650 to make your fingers tingle a bit when you touch both ends at the same time with the same hand. Also, shorting a single cell is enough to set a nickel strip on fire, like with actual flames burning metal like a candle wick.

So yeah, they’re serious shit for sure lol

You are more likely to be harmed by mains voltage than by an 18650 cell. Even though there are multiple forms of protection for AC mains, there could still be dodgy earths or fuses. 18650’s are extremely durable. I have had a propane torch heat one up for a good 20 seconds before it started to leak electrolyte and then pop open. Shorting them does cause a substantial current flow and a decent amount of heat, but even when directly shorted, they will take some time to start leaking and then to pop.

Soldering on 18650’s decreases their life slightly, but it isn’t very risky at all.

What voltage was the pack at when you felt a tingle? As DC voltage needs to be decently high to feel a tingle through the hand.

3.61 ish. I think maybe my hand was sweaty because i had just removed my gloves which i usually wear when building packs, so that may have had something to do with it. It was the tiniest of tingles though.

Mains have protection against irregular power consumption. Batteries have NO protection and will deliver ALL stored energy when incorrectly used. Voltage doesn’t matter. Don’t stare blind on voltage, energy is a much higher risk.