Build a Microwave Transformer Homemade Stick/Arc Welder




Introduction: Build a Microwave Transformer Homemade Stick/Arc Welder

About: Hi! I'm Star Simpson! I'm a real me! See more at []. photo by [ Jeff Lieberman] ( stasterisk - my name is Star, and when I was 13 I si…

I had no idea making a DIY welder would be so easy to do. And, it's pretty much FREE!

Additionally, the stick welder you get is definitely better than anycheap commercial welder you can buy.
Why is this homemade thing better than something you can buy? Because when you factor in shipping and labor and the little bit of retail markup - the companies that make typical cheap buzz boxes will skimp on copper as much as possible. Whereas you can use enough copper in this to make something really juicy, and still spend less, to nothing, compared to a store-bought arc welder.

So here's what you need to build a welder:
- Two beat up old microwaves
- Some 10 gauge wire
- Wire nuts

People throw out microwaves all the time, if you keep your eyes on the curbs.
Or, you can get microwaves at the local thrift store for $10 each.
Try the warehouse that processes donations - they have to pay to get rid of tons of broken ones.

Stuff you need for welding:
- Welding helmet ($16 and up)
- Welding rods ($6)
- Vice grip or purpose-built electrode holder ($6 for either)
- C clamp for grounding clamp
- Gloves
- Thick nonflammable (leather) clothing that will cover your arms

Disclaimer: High Voltage ELECTRICITY and lots of CURRENT! Heat, electrocution, and DANGER! You could die and you could go blind.

That said, try this at home!

See this for a lot of welding safety tips

Here are the really good how-tos that this project is informed by:
build a 70 amp welder
the tiny tim welder by tim williams
home made welding machine (via afrigadget)

Dan Hartman's how-to is good for reference, too.

And here's the quickest way to make a DC welder with a bunch of 12 volt batteries.

Step 1: Dissect the Microwaves

Invite your non-hardware oriented pals over to help help dissect your donor appliances.
They'll love it. David Grosof donated one of these microwaves under the condition that we take it apart together.

Good safety tip:
You'll find a gigantic capacitor inside the microwave. It looks like a metal can with two tabs on top.
Short it out to make sure it doesn't have any leftover charge on it, before you poke your hands anywhere near. Just put a screwdriver or something metal you aren't connected to, across the two metal terminals shown here.

Step 2: Prepare the Transformers

Chop and and knock out the secondary (thin wire) windings.
Don't nick or damage the primary windings in any way.

If you do, you could create shorts where two windings conduct to each other, allowing electricity to bypass certain parts of the coil, making effectively a smaller coil, and creating something different than what you expect at the output. Or, you might chop the connection entirely, ruining the primary. So do your best to keep it intact.

Step 3: Get Some 24 Foot Chunks of Ten-guage Wire

We scavenged some heavy wire from an old powerboat the owner was scuttling.
We stripped the outer jacket off and separated the inner conductors to wind new secondaries
on our transformers.

Step 4: Wind the New Transformer Secondaries

We wound 20 turns of 10-guage wire on each transformer. That's just about how much wire would fit into the available space. It took a little over 20 feet of wire each.

tip: draw tally marks on your table to keep track of the number of windings.

How does a transformer work?
The primary winding is an electromagnet connected to alternating current.
The humming magnetic field of the primary induces a current to flow in the secondary winding. If both windings have the same number of turns, the output voltage is the same as the input.
(minus a smidgin due to eddy currents, resistance, etc.)
If the secondary has more turns than the input, its output voltage is higher. That's the type of transformer you started out with.


Our primary has 100 turns and gets connected to 100 volts AC. We're winding 20 turns on the secondary, so we'll get about 20 volts out.

The available POWER STAYS THE SAME regardless of what the output VOLTAGE is.

If the primary is made take 1000 watts (100 volts * 10 amps) out of the wall, we'll be able to take 1000 watts out of the secondary. With 1/5 of the windings, we can draw 50 amps out of the secondary.

That's the cartoon version with play numbers anyway.
Over here in our shed full of reality we've got two of these beasts in series and plan to short the outputs through a welding rod like Jennifer Beals.

Let's just say we're going to pull a whole lot of amps, which is why we need to wind our secondary with such thick wire.

The copper conductor in ten-guage wire happens to be 1/10" (0.1") in diameter.

Here's a table of conductor diameter, guage, and current rating.

Step 5: Schematic

It's a pretty simple circuit.
In fact there's nothing in it except wire!

We'll take two transformers and wind low-voltage secondary windings on them with thick wire.

We'll put the secondaries in series with our welding rod and workpiece.
We'll plug the primaries into the wall.

I really like the way aaawelder put it: "do not include yourself in this circuit"

Step 6: Wire Your Two Transformers Together

Why do we use two transformers?
Just one of these isn't big enough to make a really juicy welder.
If you happen to find a big enough transformer somewhere, feel free to use that.

Here's how to hook up two transformers.
First we wire both primary windings in parallel to the wall cord.
Then we wire the thick secondaries in series so they both"Push and pull" in the same direction.

Step 7: Test

Get out yer voltmeter:

Here's the test to make sure the secondaries are both pushing the same direction.
Our two secondaries in series produce 38volts AC with no load. That seems about right.
If they'd phased wrong it could have been fixed by reversing the wiring to any winding.

Where Tim says "out of phase" in the video, he means "in phase". That is, the center tap should be less than the outer two leads, and if things weren't that way, the transformers would be fighting each other, or phased wrong.

Step 8: Weld

holy cow, it works!

We wanted to add a series inductor to give the unit more "inertia", but it didn't matter!

Here's Tim welding with some of those.

Built your welder, but not sure how to weld? Check out the instructional videos on youtube - search "how to arc weld". They're very good.

Here's Star striking an arc.
It welds great with these thin 1/16" 6013 rods. Even better with 3/32" 6013 rods.

Step 9: Thick Rod Test

Those skinny 1/16" electrodes cost about twice as much as thicker ones.
We wanted to see how our welder works with thicker electrodes.
The next size up is 3/32", but we got a box of 1/8" 6011 electrodes.
When we pulled one out of the box we both said "wow, that's thick".

We fired up our welder and I welded this bead across the diamond plate with 1/8" rod.
The arc was pretty short but it burned in well and felt pretty good once I got used to it.
I had to shove it in a bit more than I'm used to to keep the arc going, but sticking wasn't a problem. I welded a long bead and used up more than half the rod without stopping.
That's the long weld in this photo.

Then I set the "torch" in this plastic tub so it wouldn't short out to anything.
I checked the transformers, and they didn't even get warm!
3/32" rods are less likeley than 1/8" to blow a circuitbreaker though. For your first welds get 3/32" 6013 rods.
6011 rods have thinner flux and make it easier to see what the metal of your weld is doing, but tend to spatter a bit more.

The next picture is for reference, from

Udate 4/16/2008:
This is now my favorite welder. I made new leads for it from a pair of jumper cables. I left one alligator clamp on for a ground clamp, and added a $6 electrode holder. I've taught a bunch of people to weld using it.
The next photo is Ita welding for the first time, making an awning frame. That project was welded with this welder by total beginners using 3/32" 6013 rods. As you can see we have every other kind of welder, but the homemade ones are more fun.

Step 10: Welding Stainless Steel

We needed some brackets for Solara's mizzen mast.
So we went to the welding store and bought some 3/32" "Hobart Smootharc+ 316L - 16" stainless welding rods. They're only 12" long because stainless has high electrical resistance and they get really hot.
After much designing and sketching Victor, Kenny, and I cut, drilled, bent and welded these brackets. Very easy. When it cooled the flux went "tik" and fell off the weld. The dark area around the weld is soot from the flux.The welder could have handled much thicker rods due to stainless' high resistance and low thermal conductivity.

Use a fresh grinding wheel on stainless, or one that you only use on stainless.
You'll get rust if you use any abrasives that have been used on non-stainless steel. Same for the wrong wire brush. It will smear rustable iron on the stainless, and due to galvanic effects it'll rust quick if it gets damp.

Hooray! Where did I get the idea you needed TIG for stainless? Stick welds on stainless are just great!

Step 11: Dimmer Control and Welding Thin Wall Tubing

The welder was too hot for thin-walled tubing frames, I kept melting holes even with the 1/16" 6013 rods. So I plugged the welder into a variac dimmer and turned the power down about 30%.
That gave me very fine control over power. Marc Lander and I did some very nice welds as seen here. After a few we got good enough to do the same welds with 3/32" 6013 rods and no dimmer and not burn holes.
More tricks - I used my left hand to feed a piece of mig welding wire into the weld to add more metal in and soak up the heat. Here's Marc doing that. Any wire is fine for this, coathangers are traditional for muffler work. Sand off the paint first if you don't like fumes.
Stopping to eat lunch helped a lot also. Your welds won't be good when you're shaky and tired.

I got my variac for free, don't buy one for this, they cost as much as a welder.
A solid-state dimmer that's rated for inductive loads does the same thing and costs a lot less.

If you're feeling particularly fancy, you can add in your own scr-based switching circuitry to vary the power, like this guy did.

Step 12: Other Welders

Folks have sent me a few photos and videos of welders they've built off of this instructable. I want you to be able to see them too, so here they are!

Here's a video I got from Paul du Buf, of the Netherlands (nice case, Paul!)


Cheyyne said:
Hey there, here's my welder based on your instructabletion. It outputs 35.5v, because the transformers were a little smaller than yours I think (couldn't wind a single more turn). So far I have managed to lay down gobs of metal on various steel objects in my garage, but I still suckat welding. Luckily I rented a nice welding video from Smartflix that had good reviews, hopefully that'll give me some insight into the process. I did manage to lay down a 1" bead though! The whole thing is going into a tacklebox housing.Props for a great instructable. Thanks for it!

llamafur followed with:
Heres another one, same basic welder, but its housed in a .50 cal ammo can. Looks pretty sweet. Its relay controlled ( two 15 amp HVAC control board relays wired in parallel) , I measured 24 volts ac across the output wires.its also sorta heavy, 30 pounds.
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6 years ago on Introduction

I really fell sorry for your loss Sandra.

This project and others are inherently dangerous.

Only a fool will use a microwave oven transformer and they will put out around 20,000 VOLTS far far more than what is need for welding.

Also another stupid piece of very very bad advice from this poster is to "short out" the capacitor; YOU NEVER EVER SHORT A CAPACITOR. You can and most likely have an explosion. You have to either use a resistance load or say the capacitor is 12v charge light in auto stereo BOOM Cars use a light bulb rated for that.

Capacitors to put it simply is like a battery; where a battery is designed to release its charge SLOWLY a capacitor is design to release most of its charge very fast. Also depending on the capacitor type it can store THOUSANDS OF VOLTS and yo touch the terminals on a charged capacitor IT WILL KILL YOU...

This project and others I have seen are very very dangerous and a little bleep disclaimer won't fix that.

This web site needs to ether filter out these dangerous "projects" of put them in a "restricted" location on this site and to get into it you have to prove you know what you re doing.


Reply 2 months ago

Yes touching the charged capacitor will probably kill you. But no, it isn't charged to 20,000 Volts, but around 2,000 V to 4,000 V. Discharging it with a low resistance object like a screwdriver won't make it explode, however some of the screwdriver and metal tabs on the capacitor will vaporize with a bright flash and bang, but not the sort of explosion that it going to fill you with shrapnel. Personally, I use a chicken stick, a 3' PVC pipe with a heavy copper wire bent into a U the right size to short the cap. I short it about 10-15 times just to make absolutely sure I made good contact so there is no charge left. I haven't encountered a microwave oven that didn't have a high resistance resistor effectively across the capacitor to slowly bleed off the charge when the microwave is unplugged. However, I don't know how long that takes and there is no promise it still works, so I'm not going to bet my life of it being there to save me. I unplug for a day, then attempt to discharge, haven't seen an arc, but again, not going to risk my life on a bet that resistor is there and still works, so I always short high voltage capacitors before messing with them. I just to be a bit OCD, I also store them with bare wire wrapped several times around the terminals so it can't somehow acquire a charge.

I don't think any sane person would suggest welding with the MOT as it comes out of the microwave oven because 2000 volts at around half an amp to maybe 3 amps would be useless for welding and very deadly to the operator. That is why the secondary winding is removed and replaced with a few turns of heavy wire--that way it is only a handful of volts and high current suitable for welding.

While I like to DIY stuff just for the sake of doing it, I don't actually see this as a practical project. A used stick welder can be bought for less unless you just happen to have everything you need laying around. It educational value for those fairly well educated to begin with. And it has value in a climb Everest because it is there sort of way. IMO, both of those justify this article's existence despite the possibility that an unknowledgeable or careless person will misuse the information to catastrophically harm themselves, with a good chance of death.

I grew up before the Internet. I did projects like this without any guidance at all aside from what I had already learned. Looking back at some of the things I did, it is slightly amazing I made it into adulthood. I've been shocked, irradiated with x-rays and gods know what from a particle accelerator, exposed to chemicals, chased by exploding rockets, and even fallen without any style from badly built diy aircraft. I wouldn't trade those experiences for anything because they were part of my continuing love of science and engineering. But, I think if I had had access to even imperfect instruction, I would have had fewer injuries and hard knocks. I also would have had fewer close calls if I had never driven in a car, taken a shower, or left the house. Life is risk and tragically sometimes it ends life.


Reply 6 years ago

you're just kidding right?

A capacitor's function is to maintain the voltage potential as in smooth out the ripple left behind when converting AC to DC a capacitor accumulates no more voltage than its charged with. An inductor on the other hand will - think magnito or choke. An inductor's function is to maintain current. The power supplies I made as a kid used a pi regulator where there was a cap in parallel after the bridge rectifier then an inductor in series followed by another cap in parallel. I played with 15,000 (fifteen thousand) volt transformers making Jacobs ladders. Adding homemade caps using window glass and aluminum foil sandwiched together.

Get it together and do some learning, I did. I ended up with 5 credits in electronics and had offers from HP and Raytheon, in Sunnyvale the summer between my junior and senior year.


Reply 6 years ago on Introduction

Well AnonA2, it appears either you didn't read (or understand) the instructions or you don't know much about transformers. Very clearly Stasterisk describes how to REMOVE the secondary windings and REPLACE them with fewer turns and heavier wire than the primary. The output is now in the order of 35V.


Reply 6 years ago on Introduction

Yes voltages under 50Volts are normally considered safe . If some idiot grabs hold of the mains connections , they will get zapped . The biggest concerns with arc welding is not electrocution , but thermal burns from the inherent heat ( don't grab hold of the metal before it has cooled ) , " sunburn " from welding without your body being properly covered , or eye damage from not wearing the proper shade of eye protection , a # 10 shade is normally what I have used . To people who weld , these are " standard operating procedures " . A novice probably needs a little training .

Cheers , take care , and have a good day !....73


Reply 6 years ago on Introduction

I would advise any person who is not absolutely sure of how to work with AC or DC to ask a professional. They can both kill if attention is not paid to the project. Just take some time to understand Voltage (electricity), frequency Cycles/second), amperage (current), resistance (ohms), etc. It can be a fun project with a little care. Never be afraid to ask as terminology can sometime be frustrating for someone who has never worked with certain projects. No such thing as a dumb question.


Reply 6 years ago on Introduction

There is no problem shorting a microwave cap. True they hold about 2,000 volts but the worse you'll get is a big flash and maybe a bit of black on yr screwdriver. Be careful though. This is not for amateurs. I'm a qualified and trained microwave engineer and was taught vigorously to make sure the caps were shorted out before any attempt was made at repair.


Question 10 months ago on Step 3

I made arc welder with 2 transformers from microwave but can't use it because brake gets off as I start


1 year ago

I think I'll try this to power an AC TIG welder for aluminum!


Question 1 year ago on Step 7

I'm interested in making one of these but the main snag is the cost of the thick copper wire to wind around the transformer. I've tried and it's never cheap. All I can find is standard gauge wire, so I may as well but a second hand arc welder. So I was thinking of alternatives; would slicing copper tubes used in plumbing work? They're relatively easy to find. The idea is to cut it along its length, flatten it, cut it into slices to make it more like wire, wrap the cut pieces of 'wire' in masking or insulation tape.... you get the picture! Would this work? Is the electrical conductivity of copper pipe the same as copper wire...??.etc etc


2 years ago

Hi, great project. I am going to build one. Btw some people say dismantling the magnetron can be dangerous because the red inner part is radioactive. Thanks and Bye!


2 years ago

I tried the same thing but it wouldn’t work. I even doubled the coils. No luck. I gave up and bought one from HF. It worked great!


Question 2 years ago on Introduction

Hello! First of all THANK you for this Indestructible! I am planning on doing this as soon as possible. One question, we’ll maybe two, what would you say this is equivalent specifications to in terms of how the regular retail ones are rated, and would this (or an equally rated welder) be adequate for aluminum?


5 years ago

in uganda we use 240v instead of 110v power mains, would i still need to connect the transformers in series


Reply 2 years ago

I assume your microwaves are also 240 input? If so, then yes output is still series. Use less windings on output and bigger wire.

If microwave is 110/120v on an adapter then you also need to series inputs of two microwave transformers together in addition to outputs.


Reply 5 years ago

It depends on the breakers for the circuits...I assume that with a 240 volt mains, it would be be connected to a 50 amp that means that the mains will dump 240 Volts at 50 240*50 Volt Amps....or 1200 Volt 1.2KVa....if the breaker was a 30 amp breaker off your mains then it would be 240 Volts times 30 Amps to equal 720Volt Amps or 0.72KVa.....

That means that with the same voltage ...bigger breaker is 50 amp would be better than 30 comes the safety part...

In order for the TRANSFORMERS to handle MORE AMPS they have to be bigger (the weight of the metal has to be bigger ....that way it doesn't over heat (the metal in the transformer can distribute the heat better if it is more massive...more surface area to get rid of the heat faster and more volume to take up the created electric flux field....

Also the wires need to be bigger to handle more amps....there are rules of thumb, but if you get 10 gauge wire...and just make sure that it doesnt get much hotter than will burn you, then you should be just make sure to turn it off before it gets that hot....(also remember that just because the surface is not hot enough to burn you doesnt mean that the wires in the inside of the transformer (the ones you cant touch) are not getting hot enough to burn you.

The heat burns through the insulation creating a short so dont destroy your transformer by letting it get to hot...

there is much more too it, but hopefully that helps....

more transformers are better than less....and also if your stick wont start an arc, then it is likely too low of a voltage (so add another transformer in series....and if

the welder sounds more like it is cooking bacon in grease, then that means that your amperage is too low for what you are increase it...(one way to increase it would be by using less wraps of thicker wire...

good luck man.


4 years ago

I made it but transformer verry hot outout voltage is 35.5v please solve my problem only hot transformer


Reply 2 years ago

Did you use 10 gauge wire? Use bigger gauge wire. E.g. 8 or 6.

Check number of turns. Reduce the number of windings.

Heat is caused by too much current. You cannot use this continually to weld. You need to take a break and let it cool for a while.

As stoobers said, you can add a third transformer by following the schematic. Just remove ground clamp or rod clamp and put third xfrmr in series with first two then connect clamp to third xfrmr. (Remove a few windings from all transformers. E.g. make each xfrmr 15 turns.)

See my scribble.

Screenshot_20200115-203138_Samsung Notes.jpg

Reply 4 years ago

Drop the turn on each transformer secondary, & then add a 3rd transformer in series.


Reply 4 years ago

How would you add a third transformer in series? I'm having trouble figuring that out. I know how to add the secondaries, but which leg of the -120V - 0 - +120V do you connect the third transformer to? I can only figure out how to wire in 2 more to run the primaries AND secondaries in parallel, to double the current...
Thanks! This Instructable is awesome!