Turning an old microwave into a spot welder with Arduino


Have you ever seen a microwave sitting in the trash? Save that machine from the dump and make yourself a serious shop tool.


If you’ve ever thought about discarding your old microwave, you may want to think again. In fact, you can do exactly what Matthew Borgatti has done and transform it into some serious shop equipment. The Maker was able to successfully transform the appliance that he found lying out by the curb into a fully-functional spot welder using nothing more than the old device and a few other components.

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As the Maker explains, spot welders are handy to have around the shop. They can tack together wire for quick brazing, and permanently weld sheet metal for durable enclosures. At the heart of this project lies the countertop gadget’s transformer. He rewound it with thick battery cable, turning the original high voltage and low current supply into a low voltage and high current supply suitable for welding. Along with some copper lugs and cut rods, the power system was just about done.

“This means that resistance is going to become a dominant force everywhere beyond the transformer. This means beefy connections, thick copper cable, and an understanding that even conductive metals like copper have some resistance and that will cause heat no matter how you work the problem. Using a spot welder continuously, especially with thin cables carrying the current out to the electrodes, will create a lot of heat and likely melt your cable’s insulation,” Borgatti explains.

The other integral part of this build the timing circuit that is tasked with controlling the cycle times for the weld. The Maker conveniently based his off of a rotary potentiometer plugged into an Arduino board.

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“The key is to test your weld at different cycle times on the machine with some scraps from the same gauge sheet or wire you’re using for your project and try to break the weld. When you end up with a weld that’s difficult to break but doesn’t create a deep sink in the metal, you’re right on the money,” he notes.

Borgatti took the 20VDC transformer and rectifier from the microwave’s display circuit and soldered it onto an Arduino Proto Shield. He also had a foot pedal from a small sewing machine lying around the house, and decided to connect the foot switch’s female jack in parallel to the on-board switch of the Proto Shield. Aside from that, the Maker soldered an LED to one of the digital pins to serve as a status light.

“I had a spare PowerSwitch Tail hanging around the shop and it was a simple matter to tap the AC lines coming in from it to power the transformer. Putting a voltage divider between the transformer and the Arduino gave me a nice steady 10VDC.”

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Additionally, the Maker designed his own trigger mechanism that uses a cam and is recessed into the welder’s handle. This enables him to make multiple welds quickly. The electronics are all housed inside a laser-cut enclosure made of Delrin, acrylic and off-the-shelf fasteners, while 3D-printed brackets (which can be found on Thingiverse) keep it all together. Borgatti used spade connectors for an illuminated safety switch on the back of the unit, and even managed to salvage the microwave’s original power cord.

Intrigued? You can check out the project in its entirety here, and see it in action below.

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