Homemade Arc Welder using Microwave Oven Transformers (MOT)

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.

An Arc/Stick welder is a device which uses an electrical power supply to create an arc. It uses a consumable electrode to melt and fuse the metals together. This welder is really cool and I highly encourage to make one. This is an engineers and hobbyist’s tool that can create other tools. 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.

Background Knowledge

For welding, you will need a good power supply. That is why we need big transformers. Welders found in the market have single big transformers, we can use them if you can find of suitable size. Here we are using Microwave Oven Transformers (MOT) due to many reasons. The big transformers can give the required high power. 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.

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The reduced voltage is enough to maintain a good arc. While high amperage is good for creating heat and penetrating the metal. One might ask why only Microwave Oven Transformers (MOT).  We have tried everything from TV, Inverter transformers there is nothing like 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.

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 ask some friends and people. In our country, most people don’t use 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 shop which repaired them. It cost me Rs 700 or $7.

MOTs with no Secondary windings
MOTs with Secondary Coils removed

I chopped the secondary of both MOTs. You can totally break down the transformers by removing their I section from E section using a hacksaw or a grinder. I didn’t have any welding tool or Epoxy glue to stick them back. Thus I chopped off the secondary using chisel and a hammer. The secondary should be removed without harming the primary. The filament winding and the shunt should 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

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. The lesser the turns higher the amps and fewer the voltages. We need appropriate size of wire to fit in so 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 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 primary input. Install the secondary in series so that output volts will add up. I have 220V transformers and same wall output and my secondary output is 38 volts at 175 amps while short-circuiting. If the final output voltage is less then normal, it means MOTs are canceling each other. Just switch primary terminals of anyone MOT to fix this.

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 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 good amount of current.

While making case and wiring, make provision to take our output from 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.

2 thoughts on “Homemade Arc Welder using Microwave Oven Transformers (MOT)”

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

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