Homemade DIY Spot Welder using MOT
Powerful homemade spot welder using oven transfomer

Spot welding is a very useful equipment for joining metal sheets and pieces. They are both fun and useful. This handy device can weld thin sheets of metal together with an immense amount of heat. Spot Welder is a simple device that uses high current to produce heat and focus it on metal pieces to join them. Mostly there is the use of high current with low voltage. Our homemade spot welder uses one MOT (Microwave Oven Transformer) from a 1050 W rated oven. It is wound with three rounds of thick copper wires. These get out high amperage and low voltages that we need.

DIY stands for Do It Yourself and this project was done by me in early high school. Spot welders are simple in the working mechanism. All they need is a good source of electricity to produce high current and some mechanism to deliver them in a focused way. Here we are using an MOT for this purpose. We remove the older high voltage secondary coil from the transformer and replace it with a low voltage coil. This means we just wind a few rounds of very thick copper cables.

To achieve less voltage, we just put a few windings of very thick cables in the transformer’s secondary. This causes less voltage and more current production. We know the rule (P=IV), which means if power is the same in a system and if we change the voltage, there is an inverse change in current. Our MOT is rated for a specific maximum load. If we put only a few secondary windings, it can only fetch a few volts but the rest of the energy is in the forms of current. And the current is the element that produces heat.

Building the Welder

First, we suggest you to get a transformer from an oven. They are powerful and appropriate in size, as well. Most importantly, they have a small secondary, and it is easy to replace the secondary coil. Protect the primary coil and chop of the secondary coil, including few rounds of tungsten wire. Also, remove the metallic shunts that are used to control current.

Replace the secondary winding with thick insulated copper wire. We recommend using 5 or 4 AWG wire. This means we are using very thick cables, and we can only fit a few rounds. It is what we need for our work. Try to fit 2 to 3 rounds of cable inside the transformer. This should give us a voltage of around 2-3 Volts but current around 100 Amps. This gives us a very low voltage and very high current. It is required for the heating purpose (H=I2rt). We need high current and low voltage for spot welding, unlike arc welder, which requires a higher voltage.

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Test your device by shorting the two terminals. Measure the current using Ammeter or Clamp Meter. After getting the wiring right, we should build an apparatus for welding. We need a few copper lugs, thick copper needles, or screws. Our apparatus needs to hold down metal pieces and put some pressure and send electricity. We suggest using some wood and screw as support and fulcrum. Our suggestion is using copper because other resistive material can get hot and be dangerous.

Welding Work and Advice

We have explained a few things in our video. The video also shows us melting some metal and playing with high current. In our experiment, we melted a few small metal pieces and some screws. It can be an experiment or a useful tool for the home. The use of spot welder in the industrial and manufacturing sector is very high.

The heat created from the short circuit can melt metals and also join them. Just for the warning, this is for educational purposes, and we are not liable for any risks and damages that might occur. Please make sure you have a good knowledge of electricity, high voltage, and transformers.


The original secondary coil is about 2000 V, which is very dangerous. There is usually a big powerful capacitor in coupling. Always be careful and discharge such big dangerous capacitors carefully because they store charge for a long time. They can be very dangerous because of the high voltage and energy.

You can also modify your setup and use carbon rods to create a melter. Short-circuiting carbon rods build high-temperature bright arcs. There are lots of uses of this kind of project. It is fun and useful in times of need. If you have a home and repair things yourself, then definitely try this kind of projects to improve your skills and collection

If you have any questions and feedback then share it with us. Be very careful while working because it is related to high voltage electrical works. User safety gear, protective gloves, and always have a tester for checking. Check voltages with good digital multi-meter and maintain a good distance from anything flammable.

Originally Published: 2013/11/02



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