Homemade Arc Welder using Microwave Oven Transformers (MOT)

Homemade Arc Welder using Microwave oven transformers

Since I was a small boy, I had a knack of technology and science. It was interesting to see people melt and fuse them together using a thin rod and electricity. During high school, I studied and learned the building process through various sources. I also experimented with small transformers for a long time to get good knowledge about them. Finally, after high school, I managed some money and made it. So this is my homemade arc welder build guide for everyone. This is an interesting project and can be very useful for hobbyists.

An Arc/Stick welder is a device that uses an electrical power supply to create a hot arc. It uses a consumable electrode to melt and fuse the metals. This welder is a really cool project, and I highly encourage you to make one. This is an engineer’s and hobbyist’s tool that can create other tools and fix things. Always learn skills from superiors before trying to weld. Most arc welders use a transformer to step down high voltage low current supply from the wall outlet of the house to low voltage and high current.

The low current makes welding safe but suitable enough to create and maintain an arc. On the other hand, high amperage helps to create heat and melt the metal and electrode to fuse them. This is a short circuit process. Shorting out very high voltage with very low amps won’t burn anything. But they will create a dangerous and deadly arc. We can see such high voltage arcs in tesla coils and substations. Here in a stick welder, we just make low voltage sustainable arc with high amperage.

Background Knowledge

For welding, you will need a good power supply. That is why we need big transformers. Arc welding machine found in the market have a single big transformer. We can use them if you can find one of suitable size. But, here we are using Microwave Oven Transformers (MOT) due to a few critical reasons. First is that MOTs are compact, the second thing is their secondary winding is big, and the third is easy availability. The big transformers can give the required high power output. Our goal is to get low voltage around 40 Volts but very high amperage. The basic formula is (Watts= Voltage x Current). For an ideal transformer, (Input Power=Output Power). However, there are some losses in the winding, plates, eddy current, heat, etc.

The reduced voltage is enough to maintain a good arc. At the same time, high amperage is useful for creating heat and penetrating the metal. One might ask why only Microwave Oven Transformers (MOT).  We have tried everything from TV and Inverter transformers. But there is nothing like good MOT. They have a big core, a less primary winding, big secondary window, and great power that is required for us. While building, make sure nothing is connected to mains, and there is a good measure for protection. There are capacitors in almost all electronic devices, so make sure to short them first to discharge them.

Preparing the Transformers

First, we need at least two MOTs to salvage. Find any old microwave oven at your disposal. You can find it in your local classified, scrap yard, and repair shop. Do ask some friends and people for help. In our country, most people don’t use microwave ovens, so it is hard to find. For the sake of science, I salvaged my own working oven. For another one, I searched for more than 2 years and found it in an electrical repair shop. It cost me Rs 700 or $7. I bargained many times for 6 months and then finally purchased it.

MOTs with no Secondary windings
MOTs with Secondary Coils removed

I chopped the secondary windings of both MOTs. You can break down the transformers completely by removing their I section from E section. You can use a hacksaw or a grinder to cut the welded joints. I didn’t have any welding tool or Epoxy glue to stick them back. Therefore I chopped off the secondary coil using a chisel and a hammer. The secondary should be removed without harming the primary. The filament winding and the shunt should also be knocked off. The shunts are two metal plates kept at the window of the transformer to maintain current or to control it. We are not going to need it because we need high amps.

Two Microwave Oven transformers with new secondary winding
Two Microwave Oven transformers with the new secondary windings

Clean all the residue and lose secondary wires before processing forward. If you see and damage to the primary coil, fix it. Do test of primary winding for continuity. If the varnish of copper wire is scraped, cover it with new varnish or other insulating material. Just make sure everything is safe.

Rewiring and Connections

Now we should put secondary wires in the transformers. As we need fewer volts, we are going to install thick wires and have fewer turns. Lesser the turns higher, the amps and fewer the voltages. We need an appropriate size of wire to fit in so as to get suitable current and volts. Many DIY builders use 12 gauge or 10 gauge copper wire. I am using stranded 14 gauge copper wire. It is also known as 7/22 wire, which means 7 stranded wire of 22 gauge. I installed the winding in both transformers and got 38 volts out, which is about right.

Read More: Arc Welding Guide for Starters


Be very careful while placing secondary winding. Don’t disturb or break the primary coils. Check your output voltages before finalizing the primary input. Install the secondary in series so that output volts will add up. I have 220V transformers and the output from walls or mains is also the same. My secondary output is 38 volts at 175 amps while short-circuiting. If the final output voltage is less then expected, it means MOTs are canceling each other. Just switch primary terminals of anyone of the MOT to fix this.

Do cover every terminal and loose connection properly. Make the wire connection are tight to prevent loose connections and sparks. Don’t touch any terminals with the bare hands. Moreover, don’t take both ends of the circuit at the same time. Use a clamp and suitable stick holder to start your welding.

Finishing and Advice

I also built a casing to make it compact and portable. I used some jumper cables to feed the stick holder. You can use any pliers to hold the stick, but a stick holder makes it easy to weld. I also used some old thick copper wires for the ground with a battery clamp. I bought 25 mm jumper wires that cost me $1.5/m. But it was good enough to handle a good amount of current.

While making case and wiring, make provision to take out the output from a single MOT as well. This can give you another option to use low power devices. Some people use that low voltage output to make carbon rod metal melter. We have also made a small metal melter or spot welder using one MOT. Always be very careful while touching wires. Make sure the area is well ventilated. Wear all protections and always use a mask and eye protection.



Recommended For You

About the Author: Abhishek Chand

Structural Engineer by profession. Computer and smartphone enthusiast. Tech-savvy blogger and former Gadget reviewer of AP1 HD TV and TechnoNepal.


  1. Which type of transformer can be used stepup or stepdown transformer and which volt transformer I need to buy

  2. I don’t have a 12 gauges wire,only have 4 gauges wire,can it work when I put together the 4 gauge wire.

    1. That is quite thick. It may be hard to put many rounds of wire in the secondary winding with 4AWG. Try to put a bit thinner cable so that you can get around 15 turns and 15 volts.

Leave a Reply

Your email address will not be published. Required fields are marked *