Spot Welder using two microwave oven transformers (part 1)

There are many ways to holding two pieces of metal together, welding is one of the oldest and most reliable. Making a strong weld is an art by itself, and requires proper equipment and lots of experience. Experience requires training, but training is boring as hell if you don’t have any equipment to practice with. There are many types of welders available: Arc, Tig, Mig, Spot…

Picture showing a commercial spot welder unit – from Wikipedia
As I have very short experience, I decided to start with the simplest and somehow most predictable model: the spot welder. The first minute of the following video will explain you what am I tallking about:

Yeah I know, the freaking video is plenty of Puns! but What can I do?

To make our homemade spot welder we’ll reuse two old Microwave Oven transformers. The modification is very simple but requires intensive physical work, so get a six pack of cold beer and lots of patience.

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The ingredients

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I stupidly deleted the step-by-step pictures, so I’ll do my best to describe the disassembly process with the surviving photos.

There was a jungle of wires and cool things inside the microwave oven (I scrapped them before taking the picture, sorry).  The Box on the center of the picture is the transformer, the most important part for our project. But Wait! DO NOT TOUCH ANYTHING YET!

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Hight Voltage Discharge

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The microwave oven (even unplugged from the wall outlet) stores a lethal amount of energy. Locate the rounded box (called Capacitor) and discharge it using two flat screwdrivers connected with a thick wire. THE SCREWDRIVER GRIP MUST BE PROPERLY ISULATED. You will hear a popping sound when the capacitor discharges.

Now when the danger is gone, you can happily scrap everything: The electronic timer, switches, cooling fan and wires are great, but the most awesome part is the rotating tray motor. Turns slowly, and it’s  very powerful. I’ll post a little project using that motor later

Hell yeah

Obviously, don’t forget to take the transformer! it’s the basic part of our welder
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Transformer Modification

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Ok, now we have the transformer, but we can’t use it for welding yet.  We should replace one of the windings, so instead of poducing 2000 volts and 2 amps the output will be 2 volts and hundreds of amps.

On the picture you can see two MOT’s. On the left is “before” the procedure, “after” is on the right

First step is identifying which winding is useful and which is not. As you can see, there is a “Beige-colored plastic thing” covering one of the windings (either primary of secondary, depending on the model). Carefully cut away a piece of the beigé plastic and look.

Using my ultra-high budget picture editor, I’ve colorized each one of the transformer’s windings. Yellow is the primary, Blue is the secondary (under the plastic thing) and the Red wire is the Tertiary. Secondary can be easily differentiated because its made of a much thiner wire than the primary.

PRIMARY WINDING MUST BE PRESERVED! Do not hit, cut or scratch any part of it

Don’t be stupid like me: WEAR EYE PROTECTION AND DUST MASK! A nasty cloud of copper and insulating material will come out. That thing is still itching on my eyes.

Secondary winding will be the first to die, a saw will work great. Keep the tertiary during the process, as a “shield” to cover the primary.

As you can see on the top of the previous picture, transformer’s base may interfere on the path of the saw. Bend it if you need to, and continue

Another method that works well is using a chisel. But be very careful, one hit on the primary and party’s over

A zoom-in of the “chisel” method. Don’t be tricked by the picture! Wires are much thinner, but there are thousands of them! IT’S HARD AS HELL. Personal advice: If you begin to feel frustrated, stop and get a cold beer.

After cutting a side of the winding (as shown) it’s time to go to step B

And this is step B: using some kind of square rod, hammer the winding to push it out of its place.

Pieces of copper will pop-out from the opposite side. Ok, “pop-out” is an exageration, you must hit really hard to make them move a few milimeters each time. I repeat: If you begin to feel frustrated, stop and get a cold beer.

At the final moments of the battle, a big chunk of copper (still maintaining the rectangular-shape) will come out

This piece came from the other transformer: See how densely packed the damn copper wires are!! seems like a solid copper rod!!

It’s a pretty decent amount. Donate the copper to a less fortunate person so he/she can shell it

Cut the tertiary winding using a pair of pliers. It’s just a few turns of wire, piece of cake.

Tertiary winding is history

Last but not least: There are some metal tabs  resting on top of the primary winding. They are called Shunt, do not remove them! If the shunt fell off during the secondary winding removal put them back in place (never happened to me, but you just never know).

PS: Shunt is not green, I’ve colorized it using my ultra-high budget picture editor

My friend Luis Antonio holding the trophies at the end of the day. Remember to clean the remaining dust and debris with a soft brush.

I know there are hundreds of homemade spot welder tutorials on the net. But as I’ve suffered some delays and frustration thanks to pooly explained step by steps, I did my best to spare you that annoyances.
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Electrical Work

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First step is welding a pair of wires on the primary winding terminals. Connect each transformer to the wall outlet, one at the time. If a soft hum is heard, everything is working. If no hum is heard, or the circuit breaker trips and the lights go out, discard all of the following reasons:

1. The connections between the transformer and mains are loose. [Check them]

2. Too much current, the wall outlet was already overloaded. [Find another wall outlet]

2. The primary winding is shorted or open. Maybe the transformer was wrecked before you bought it [No solution]

I was upset because NOBODY on the net knew how much current does a Microwave oven transformer draw under no load.

Answer: A microwave oven transformer draws 3.8 to 4 amps without load. 4A x 120V = 460 watt. Damn!

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How to test the connections

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As you already suppose, we’ll replace the missing second winding with something: a single turn of heavy wire. However, bending #4 AWG wire is a nightmare, so its better to use a tinner wire for testing.

My mains are 120V.  A single turn on your MOT will provide you 5 volts. Two turns 13V. Three turns 21V. We only need 2 volt or so, one turn is a good approximation.

Ok, I’ve connected my two transformers in series. That’s how it shouldn’t be done.

Now in parallel. 5 volts output: Perfect!

Now imagine this situation: Transformers work well each one alone, but when you connect them in parallel the result is zero volts. Why????

Your transformers are “cancelling” each other. No problem, very easy to fix: Take one of the MOTs (no matter wich one) and just reverse its mains connection.

When you are happy with the results, mark the mains connection polarity on the MOT body (Fase in spanish means Phase, or Live).

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Using the real wire

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Time to replace the crappy test wire with the real one. Most tutorials recommend #4 AWG wire as minimum. Getting that gauge may be easy on North American hardware stores, but here is really hard. My solution is using a nice 3×8 AWG + 10 wire, a gift from the power grid maintenance workers.

Not as good as a “solid” #4 AWG, but I hope it works

You can see the three #8AWG wires and the fourth #10AWG

Don’t forget to get some copper tubbing for the jaws!!

To be continued…

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